First Accumulating Snowfall of the Season Tomorrow into Friday

We are gearing up for our first accumulating snowfall event of the season. While this won't be a significant event, we are looking at a several-hour period of what will be some extremely heavy snowfall. In addition to snow, we will also have to deal with some sleet, freezing rain, and even some plain rain. Our first winter event of the season will certainly be messy. 

The driver behind this event is a vigorous piece of shortwave energy at 500mb (~18,000' above the ground), surface low pressure moving northward up the eastern seaboard, and an anomalously cold airmass currently in place over our region. The low pressure and an associated warm front moving northward will advect warmer and more moist air northward. As this warmer and more moist air interacts with the anomalously cold air the result will be a band of extremely heavy snowfall developing. We will explore this is in further detail below. 

As stated above, a vigorous piece of shortwave energy will be positioned across the Ohio Valley tomorrow morning and this will be moving right overhead overnight and during the day on Friday:

Temperatures at 850mb (~5,000' above the ground) will be about -5C with surface temperatures down well into the 20's tomorrow morning...plenty cold enough for snow (to save time, the temperature profile throughout the atmosphere is plenty cold for snow to reach the ground and the production of snow). 

With warmer/moist air moving northward into this cold airmass, this will end up resulting in intense warm air advection:

This intense warm air advection moving into an anomalously cold air mass will result in very strong frontogenesis (indicating a strong horizontal temperature gradient):

A bufkit sounding for Waterbury, CT from the 0z run of the NAM from Wednesday evening shows rather intense upward vertical motion within the dendritic snowgrowth zone. While not shown here, there is sufficient moisture within this zone. This indicates and further strengthens the thinking of a very intense band of snowfall:

Now getting into storm specifics...

• Light snow begins to breakout between 2:00 PM and 5:00 PM tomorrow afternoon.
• Given what was described above, we can expect a band of extremely heavy snowfall to work south-north across the state with the heaviest snow occurring between 6:00 PM and 11:00 PM. 
• During this time, snowfall rates may approach 2-3'' per hour at times, but this will be in brief bursts. 
• After this band moves through, warmer air will begin working in within the lowest 10,000' of the atmosphere, not only will snow intensities vastly reduce, but we'll begin to see a mixture of sleet/freezing rain across the interior and rain along immediate shoreline. This will persist into the overnight.

Originally, I was thinking we would see snow on the backside Friday morning across western CT, however, that idea doesn't look as probable. With this said, we'll continue to see rain through midday Friday before the system clears out, however, some mixing of snow/sleet is certainly possible across extreme western CT. Below is my latest thinking:

Reanalysis of Tuesday, October 23rd, 2018 Localized eastern Southern New England Tornado Outbreak

An extremely rare and well un-forecasted localized tornado outbreak occurred across eastern southern New England Tuesday afternoon into early Tuesday evening. The National Weather Service in Norton, MA confirmed two tornadoes; North Providence/Lincoln, RI and another in Norton, MA (both were spawned from the same supercell). Both tornadoes were rated as EF-1’s (the scale ranges from the weakest EF-0 to a violent EF-5). In addition to these two confirmed tornadoes, several waterspouts were also observed (two off the RI coast and two more on Cape Cod). Thunderstorms also produced small hail in Hubbardston, MA and Union, CT. Before we get into the science behind this event I want to take a little bit of time to discuss the waterspouts.

Waterspouts are tornadoes which form over the open water. However, if that body of water is landlocked it is defined as a tornado (an example would be from May 15^th, 2018 when a tornado which formed over a reservoir in Barkhamsted, CT was defined as a tornado as the reservoir was landlocked). There are also two types of waterspouts; fair-weather waterspouts and tornadic waterspouts. Each is defined based on the characteristics in which the phenomena develop. Fair-weather waterspouts are typically not associated with thunderstorm activity. These waterspouts form at the base of a cumulus or a stratocumulus cloud. The two sources involved here are; 1) strong wind shear and 2) the instability is generated due to the temperature gradient between the warmer waters and cooler airmass. With this said these phenomena or typically most common during the fall months. Tornadic waterspouts are formed via the same processes as a tornado would over land. They are generated from supercell thunderstorms and the processes involve strong wind shear and the instability source is airmass characteristics. Tornadic waterspouts may also be tornadoes which form over land and move offshore. In the case of yesterday, we were dealing with tornadic waterspouts which leads to this event being classified as a localized tornado outbreak.

Now we can get into the fun stuff…science and meteorology!

As stated during the open, this event was not forecasted. As we proceed with the post we will look at meteorological data from the day before which will show that the signals were there, however, one would have dug deep into the data to latch onto this potential. Then again, who would think/forecast the possibility of tornadoes when we’re dealing with temperatures mainly in the mid 50’s with dewpoints in the 40’s to around 50 (especially around these parts)? To prevent this post from becoming one of my typical war-and-piece novels, for data I am going to focus on the NAM (North American Mesoscale) forecast model with a focus on the afternoon run (18z or 2:00 PM EDT) as well as the Tuesday morning (12z or 8:00 AM EDT) run of the HRRR (High Resolution Rapid Refresh). The three forecast stations selected will be Chatham, MA (KCHHH), Boston, MA (KBOS) and Providence, RI (KPVD). Unfortunately, I only had HRRR data for Chatham, MA and was not able to get Providence so I went with Boston, MA. Finally, we'll look at a few parameters from the SPC mesoanalysis data that afternoon.

Analyzing the 18z NAM from Monday afternoon, the forecast was for a vigorous shortwave trough and associated shortwave energy to dig into New England during the day on Tuesday. Associated with the shortwave trough was an exceptionally cold (especially for October standards) mid-level airmass characterized by 500mb (about 18,000’ off the ground) temperatures into the lower -20’°C range! This lead to the forecast of very steep 700mb-500mb (~10,000’ to ~18,000’) lapse rates (lapse rate is defined as change of temperature with height) on order of 8 C/KM (indicating that for every altitude increase of 1 km the temperature would decrease nearly 8°C). That is a remarkable decrease of temperature with height and despite surface temperatures which would only warm into the mid 50’s with surface dewpoints into the 40’s to around 50 would eventually yield robust mid-level instability:

Sufficient wind shear was also present on the NAM for Tuesday, however, there was a bit of a disconnect between when the strongest directional shear (change of wind direction with height through the troposphere) would occur and when the highest instability would occur. In retrospect, the NAM ended up underdoing both how long the directional wind shear would be present and the degree of directional wind shear that would end up occurring. This will be shown a bit later. The forecast from the NAM showed 100+ knots of westerly wind over the region at 250mb (~38,000’ off the ground) with an impressive jet streak of 110+ knots just to the southwest of southern New England. This likely placed southern New England in the left front quadrant of the jet streak, enhancing upper-level divergence (this favors enhanced upward vertical motion…rising air). At 500mb the forecast was for 70+ knots of westerly wind across southern New England with a jet streak of 80+ knots west of the region with the region on the nose of this jet streak which would further enhance the degree of upward vertical motion. At 850mb the forecast was for 30+ knots of south-southwesterly wind to develop across RI and southeastern MA. Note how at 250mb and 500mb winds were more west. While I did not show this here, winds at 700mb (~10,000’ above the ground) were more west-southwest. Winds were transitioning from south-southwest at 850mb to west at 500mb and 250mb which yields the directional wind shear (wind changing direction with height):

As far as the forecast for directional shear is concerned, the NAM on Monday was not particularly impressive come late Tuesday afternoon. The below is the 18z NAM bufkit forecast sounding for Providence, RI valid 5:00 PM Tuesday afternoon. There are a few things of note:

1) The predominately unidirectional (same direction) winds with height through the troposphere, however, there is some degree of directional wind shear present mainly in the lowest 1000' of the atmosphere where you have surface winds forecasted to be south-southwest with winds above that becoming more southwesterly.

2) The hodograph is like a "graphically" way of visualizing how winds are behaving with respect to height. When the hodograph is more straight in nature that is suggestive of a unidirectional wind profile. When there is curvature to the hodograph that suggests the presence of directional wind shear. On this forecast hodograph we have a predominately straight look, however, the slight curve suggests that slight change of wind direction with height in the lowest 1000'.

3) The forecast CAPE (Convective Available Potential Energy...a measure buoyancy of air parcels) in the 0-3km layer and 0-6km layer is quite noteworthy. Before proceeding further I want to discuss the significance of 0-3km CAPE and 0-6km CAPE and show exactly why it holds such high importance. There are several different computations of CAPE; surfaced-based CAPE, mixed-layer CAPE, most-unstable CAPE, normalized CAPE, and downdraft CAPE. In the case of this setup, analyzing the forecast for mixed-layer CAPE and surface-based CAPE the forecast from the NAM was for values between 200 J/KG and 300 J/KG. Typically these values aren't really eye opening when it comes to the potential for severe weather, however, when you have such strong wind shear in place (such as we did here) these values can be sufficient but it is very borderline. Without trying to go into crazy detail, each of the CAPE measurements are calculated via different processes. Surface-based CAPE indicates the total amount of energy available for an air parcel at the surface to accelerate upwards to the lifted condensation level (level at which water vapor condense and clouds develop...its the base of the cloud). Mixed-layer CAPE yields the average conditions of the lowest 100mb of the atmosphere and indicates the energy available for air parcels to lift to the level of free convection. 0-3km CAPE and 0-6km CAPE indicate the potential energy available to air parcels between the surface and 10,000' off the ground and surface to 18,000' off the ground respectively. First, we will look at the forecast for surface-based CAPE and mixed-layer CAPE values:

Now we will look at the forecast values of 0-3km and 0-6km CAPE along with what was just discussed regarding the shear:

Forecast CAPE values between 0-3km and 0-6km were to forecast to exceed 100 J/KG which is quite impressive, especially with the shear that was being forecasted.

Now we will move onto Tuesday's morning of the HRRR. We will look at the 12z run for both Chatham, MA and Boston, MA for later that afternoon. What we will see are soundings which are not only very opening in terms of indicating the potential for severe thunderstorms but for tornadoes:

Finally, let's take a look at a few parameters from the SPC mesoanalysis at 4:00 PM that afternoon. Mesoanalysis data showed a narrow area where the supercell composite parameter was between 0.5 and 1 with values of 2 just offshore. These values aren't very eye opening but this is a case where you have to look at more than just a raw number. You also have to sort of know/understand the parameters used within the calculation of the parameter. When taking into account the environment these values actually are a bit more eye opening than you would realize. Significant tornado parameter again wasn't eye opening but when taking into account the environment it was a bit more impressive. Also, note how these parameters were positioned over the area the tornadoes/waterspouts occurred. The most impressive out of all of this...0-3km CAPE/surface vorticity overlay. 0-3km CAPE values exceeding 125 J/KG is highly impressive as it indicated the potential for significant vertical stretching (an ingredient favorable for tornadogenesis...development of a tornado). The surface vorticity value of 4 indicated sufficient vertical rotation was present. Combine this with enhanced likelihood for vertical stretching and the result...tornadoes.

What can we take from all this? While the forecast models (in this case the NAM) hinted at the potential for strong-to-severe thunderstorms, the signal a day out wasn't overly strong but the ingredients were certainly present. Also, in the case of this event, as Tuesday morning progressed the signals became quite alarming as all the ingredients were going to juxtapose with a perfect timing of everything.

This is an event in which forecasters can learn heavily and thankfully there were no injuries. As the event was ongoing forecasters at the National Weather Service and local TV meteorologists did a fantastic job at getting word out and issuing warning.

2018 has been quite the year across New England in terms of tornadoes. It seems just about every setup capable of spawning a tornado has done so. While tornadoes in New England aren't common, having such an active year such as this is.

Strong-to-Severe Convection Possible Late Afternoon/Evening 09/25/2018

Two windows will exist for thunderstorm development through Wednesday evening and the potential will also exist for a few strong-to-severe thunderstorms. The first window will develop later this afternoon into the early overnight hours as a warm front slowly lifts northward and the second window will be tomorrow evening as a cold front approaches.

A warm front is currently positioned south of Long Island back southwest into NJ. This warm front will continue to slowly lift northward through the afternoon. South of the warm front the airmass is characterized by rich theta-e air/low-level moisture and rather high dewpoints. The combination of rich theta-e air and dewpoints well into the 70's south of the warm front is yielding mixed-layer CAPE values between 500-100 J/KG. A very strong low-level jet is also present with 925mb winds in excess of 40 knots across CT. Forecast models indicate this could strengthen upwards of 45-50 knots through the afternoon.

With the warm front approaching CT and a strong low-level jet in place, strong speed and directional shear is present within the 0-6km layer which is yielding enlarged and curved hodographs. As the warm front lifts northward instability will begin to increase across portions of the state where the warm front lifts through. Forecast models indicate an overlap of the strongest wind shear and instability will occur across CT late afternoon into the early overnight hours. The image below is the forecast hodograph for Waterbury, CT from the 12z run of the NAM (on the left) and the HRRR (on the right) for early this evening. The NAM shows 0-3km helicity values just over 300 m2/s2 with 0-1km helicity values well over 200 m2/s2 (not shown) while the HRRR shows 0-3km helicity values over 400 m2/s2 with 0-1km helicity values just under 400 m2/s2 (not shown). What is also noteworthy is the 0-1km CAPE values just over 100 J/KG:

CAMS (convective allowing forecast models) have been rather consistent and aggressive in developing convection later on this afternoon and given the projected instability parameters and shear there is the possibility that any convection  may develop supercell characteristics. With this said we will have a window of opportunity for an isolated tornado/damaging wind gusts into the early overnight It is also possible that this convection may not contain thunder or lightning.

In addition to the potential for an isolated tornado/damaging wind gusts we will see localized pockets of flash flooding with heavy rain continuing through the day.

 A separate blog post will highlight and discuss the potential for severe weather tomorrow a bit later on. 

Florence Discussion: East Coast Threat?

All eyes are on Tropical Storm Florence which is currently located out in the tropical Atlantic about 935 miles east-southeast of Bermuda (as of this writing). Florence was recently as strong as a powerful category 4 hurricane; however, some dry air and strong wind shear has led to rapid weakening of Florence over the past day. As we enter the weekend, Florence is expected to leave the area of dry air and enter an environment which is less hostile wind shear wise and with this the expectation is for Florence to rapidly strengthen during the weekend. Model Intensity Guidance shows strong support/agreement on this rapid strengthening occurring this weekend with several pieces of guidance suggesting strengthening to a category 3 or perhaps even a category 4 hurricane come the beginning of next week:

The big questions are; does Florence hit the east coast? If so, where does Florence hit? And what will the intensity of Florence be? There are growing indications that Florence may indeed hit the east coast with a potential landfall along the Carolina or VA coast…or at least if not a landfall, impacts occurring across these areas:

There are a few pieces of the puzzle and these pieces will be extremely critical with regards to strength and track of Florence and we will analyze all the information below.

High Pressure

Forecast models are in excellent agreement regarding an amplifying positive PNA (Pacific-North American teleconnection pattern) trough across western Canada and the western United States coast as next week progresses. The response to this amplifying trough is for ridging to begin developing across the eastern United States and western Atlantic. The graphic below is from Friday’s morning run of the GFS ensembles valid for 12z (8:00 AM EDT) Wednesday morning showing 500mb height anomalies:

The key piece here is the high pressure which is outlined. Before we explain why this piece is critical it is important to note that this projected atmospheric pattern configuration combined with the current latitudinal positioning of Florence is NOT FAVORABLE FOR AN EAST COAST LANDFALL. What this high pressure does is block the system from recurving and going well out to sea. In the Northern Hemisphere, the flow around high pressure systems is clockwise. Given the position of the high pressure with Florence tracking south of the high, the steering flow would push Florence into the east coast of the United States as it has nowhere to go with the high pressure to the north. So, this is set in stone, right? Well not necessarily but the indications are growing for this possibility to become a reality. The best thing to do is to further explore this high pressure.

One very interesting aspect to note is the rapid development and strengthening (as well as placement) of this high pressure system early next week along with the projected strengthening of Florence. We will illustrate this by looking at the GFS projected sea-level pressure at 0z Wednesday (8:00 PM EDT) and at 0z Thursday and 500mb height anomalies at 0z Wednesday and 0z Thursday. Notice how the high pressure has strengthened while the sea-level pressure has decreased (indicating a stronger system):

Tropical systems are huge heat machines. They are composed of an incredible amount of moisture with vigorous evaporation and condensation occurring. The process of condensation releases heat and this heat is referred to as latent heat. It’s very tough to illustrate this (well at least for me as I’m not artistic) but try to visualize this…you have a very large system with a tremendous amount of condensation going on...the result is a tremendous amount of heat being released. When you have large and strong tropical systems they can release so much heat that the atmosphere has a response and that response is for rising heights A.K.A ridging. It is very possible that the forecast models are really hitting this process hard which could explain the rapid strengthening of the high pressure/subsequent ridging. The question is…is this correct? This very well could be overdone and even if it’s not where this high pressure develops and where it strengthens is extremely critical. If this happens farther east, there is now an escape route for a rapid re-curvature of Florence…this isn’t to say there wouldn’t be any impacts, but a landfall would become much less likely. If this happens farther west well the system likely goes straight into NC, SC, or VA coast.

There are other questions regarding the ridging as well. There are times where models have a bias with the strength of high pressure systems and the structure of the high pressure systems in the medium range. Perhaps very strong high pressure does develop, but perhaps it doesn’t configure as modeled and there are weaknesses within the high pressure…that is not totally uncommon, and that weakness would act as an escape route.

How this high pressure is handled and how it eventually evolves is extremely critical in the overall result of Florence.

Sea-Surface Temperatures

We can’t talk tropical without discussing good ole sea-surface temperatures (SSTs). I mean after all, it’s the warm waters which fuel these tropical systems and give them the energy they need. There is no question water temperatures are supportive of a hurricane. The minimum threshold for water temperatures is about 80-82°F. Sea-surface temperatures are running much above-average (noted by the SSTA (sea-surface temperature anaomalies) chart on the left in the image below) as water temperatures are into the lower 80’s as far north as the NJ coast (noted by the chart on the right:

Sometimes, however, sea-surface temperatures alone don’t tell the whole story. Especially considering this is just looking at water temperatures at the surface…well what about below the surface? Are these warm waters very shallow in nature or do they penetrate deep? This is extremely important when considering the process of upwelling. The top of the ocean surface is always going to be warmer than water 10…15…20+’ below as the surface of the water is most exposed to the heating from the sun. Upwelling (the rising and mixing of ocean water) brings colder water from below the surface towards the top while the warmer waters from the top go beneath the surface. This results in the colder waters warming and the warmer waters cooling. With tropical systems which can generate big waves the result is for tremendous upwelling…waters from well below the ocean surface which are quite cold mixing to the top. This will quickly result in lowering water temperatures. The map below shows the depth of the 26°C isotherm to be around 50 to perhaps as much as 75 meters off the east coast which isn’t terribly deep. As Florence really ramps up this weekend big waves will begin to generate well ahead of it likely promoting upwelling and the lowering of sea-surface temperatures…enough to lead to any weakening of Florence…that remains to be seen:

This is also important because with a high pressure to the north and ridging across the eastern United States there is nothing to really promote an accelerated forward movement of Florence. Typically, when we see hurricanes near the east coast their forward speed will accelerate (usually enhanced by steering flow around the high and being captured by a trough across the Midwest or Ohio Valley (this is what you usually want to see when talking about a landfalling system). The slower Florence moves, the greater the duration of the upwelling and sitting over a location for too long will also result in cooling water temperatures due to evaporation and rain falling.

Wind Shear

Wind shear is extremely critical to the strengthening and structure of a tropical system. Unlike thunderstorms, tropical systems hate strong wind shear. Strong wind shear disrupts the physics of a hurricane. Currently, Florence is in an area of strong wind shear which has resulted in the weakening mentioned in the opening, however, as you’ll see in the map below Florence will be moving into an area of much less wind shear (Florence is located within the red contours which constitute strong wind shear while off the east coast we see green contours indicating weak wind shear):

The Wild Card…

In baseball they have wild cards and hey…why not in weather? In this case there may be an underrated or overlooked wild card and that is a circulation of clouds and convection in the western Atlantic well off the south east coast.:

This is sure to not only influence Florence but will likely wreak havoc on forecast models and could really yield forecast models to struggle with the development of the atmospheric pattern configuration across the Atlantic. When there is a great amount of convection present forecast models sometimes struggle vastly and this is due to the physics of the models and the complexities of convection.

As it stands there is an increasing likelihood for impacts along the east coast with these impacts more likely to occur across the Carolina or VA coast. There is even a possibility that the mid-Atlantic coast seems impacts but no landfall as Florence begins to quickly re-curve and this is a scenario which I think will unfold. There is an extremely minimal likelihood for any impact (outside of waves and maybe eventual leftover moisture) here across southern New England.

An update will be conducted later in the weekend.  

Severe Weather Outbreak Likely Later Today

Severe weather outbreaks across the Northeast (including New England) are relatively rare although they do happen. It takes a special combination of ingredients to make them happen and today we will have a mixture of these ingredients. While we certainly are no stranger to thunderstorms during the summer months and a few of these thunderstorms due produce damage today is shaping up to be extremely active with widespread damaging winds, hail, and even the possibility for a tornado.

The biggest driver behind today's severe weather potential is that of an elevated mixed-layer (EML). The elevated mixed-layer is a layer of air (typically found between 10,000' and 18,000' above the ground) which in the United States originates across the Inter-mountain west. As air descends down the Rocky Mountains it warms adiabatically (and drys) This feature is extremely critical to the severe weather outbreaks which occur across the Great Plains. Since this feature is associated with a layer of extremely warm and extremely dry air it acts as an atmospheric cap. What it does is it can prevent clouds and showers/thunderstorms forming and it can allow for extreme instability to develop beneath it (especially during situations where surface temperatures are into the 80's and 90's and dewpoints are in the 60's and 70's). Above the cap you have a very sharp temperature decrease with height and this yields a 700mb-500mb lapse rate which can vary between 7.5 C/KM and 9 C/KM. This only fuels the degree of instability. While these features typically don't make it to our parts they sometimes can and if they are timed with a cold front or strong piece of energy moving through the jet stream aloft we can get widespread severe weather events. Some past examples include; May 31st, 1985 tornado outbreak, July 10th, 1989 Hamden, CT F4 tornado, July 15th, 1995 widespread damaging wind event across NY/PA into New England, May 31st, 1998 northeast severe weather/tornado outbreak, and most recently the June 1st, 2011 Springfield tornado.

Observing mesoanalysis data this morning mid level lapse rates (700mb-500mb...about 10,000' to 18,000') just to the west and southwest of New England are as high as 7-7.5 C/KM with these values expected to push into New England during the day:

With surface temperatures away from the coast expected to push into the 80's with dewpoints climbing well into the 60's under the presence of steep lapse rates the atmosphere is expected to become quite unstable this afternoon with surface-based CAPE values exceeding 2500-3000 J/KG, mixed-layer cape values approaching 2500 J/KG, and lifted index values exceeding -5C to -7C. These values all indicate a significant amount of instability.

In addition to strong instability we will have have very strong wind shear aloft. Mesoanalysis data shows 0-6km wind shear values of 40-50 knots across much of the region which will assist in not only strong organization but in the potential for supercell thunderstorms as well:

One thing to watch as well with regards to the wind shear is any directional component (change of wind direction with height) to the winds. This point-and-click forecast sounding from the overnight run of the NAM forecast model for a location in western CT shows the low-levels winds a bit backed with nearly southerly winds at the surface then becoming more southwest as you ascend with height:

The timeframe for thunderstorms today will be between about 3:00 PM and 9:00 PM this evening. A line of strong to severe thunderstorms is expected to develop across southern NY and PA and track towards CT later on. If the instability parameters mentioned above occur given the wind shear we have in place this line of thunderstorms will likely be accompanied by numerous severe thunderstorms capable of producing widespread damaging winds as well as large hail. We also need to watch any discrete thunderstorms as they will have the potential to produce a tornado. This is expected to be a rather active afternoon across the northeast and parts of New England.

Light Snows Monday, April 2nd, 2018

No this is not an April Fool's joke...we are tracking the likelihood for light snows across the Monday morning. While a major storm isn't expected, some light accumulations will occur and given the timing of the snow, this will have an impact on the Monday morning commute.

A cold front moving through the state as of this writing will usher in a cooler (not a particularly cold air mass but cold enough) airmass for overnight Sunday and Monday morning. A weak area of high pressure well to our north will sort of help with reinforcing some cooler air into the state through tomorrow morning. Forecast models all show some shortwave energy moving through a rather progressive zonal flow aloft. This shortwave energy will help aid in the development of a surface area of low pressure which will pass to the south of Long Island. This track will help keep cooler air in place across Connecticut:

While this system will be moving rather quickly, forecast models all agree there will be some very strong upward vertical motion with this system and this could lead to a rather narrow band of moderate to heavy snowfall. The question is where this band sets-up. There is the possibility this could end up just south of Connecticut but there are some indications this could happen across southern Connecticut. This will ultimately be tied to the exact track of the storm. Both the NAM and GFS indicate a period of rather solid 800mb-600mb frontogenesis traversing the entire state tomorrow morning along with a small window of some excellent upward vertical motion intersecting the dendritic snow growth zone. This will yield a brief period of moderate to heavy snow tomorrow morning:

With surface temperatures in the upper 20's and temperatures throughout the lower and mid-troposphere well below-freezing, this combined with the excellent upward vertical motion and high relative humidity within the snowgrowth zone should yield higher than average snowfall ratios. Bufkit soundings indicate snowfall rations perhaps as high as 13:1. This indicates we will be dealing with rather fluffy snow and the kind of snow that can accumulate easily:

So what can we expect?

While temperatures will certainly be cold enough, it may be a bit difficult for snow to initially stick to paved surfaces. This has nothing to do with higher/stronger April Sun angle (the snow is falling during the early morning hours!) but with the fact that the ground will be relatively warm. In order for the snow to stick to paved surfaces it will have to come down moderate to heavy for a good duration. Otherwise we're mainly talking about accumulations on grassy/colder surfaces (such as cars).

• Snow should begin breaking out between 4:00 AM and 6:00 AM Monday morning. 
• Expect light to moderate (and at times even heavy snow) through the morning commute. This will lead to delays and slick travel so be sure to give yourself extra time in the morning. 
• With the warm ground and likely pre-treatment of roadways and paved surfaces, snow will have trouble sticking, however, in any areas which receive a prolonged period of moderate to heavy snowfall snow will begin to stick to paved surfaces. 
• The window for the moderate to heavy snowfall will between 8:00 AM and 11:00 AM. 
• Snow quickly begins to lighten up and taper off after 11:00 AM. 
• With temperatures warming into the 40's during the afternoon whatever accumulates will quickly melt. 
• If we do indeed see a band of heavier snow set-up, this could yield a zone of snowfall totals upwards of 2-4''.
• Below is what I'm thinking:

                            

Accumulating Snow Still on Track

For meteorologists and anyone who forecasts weather these past weeks have been hell. It is situations like this where I always wish the general public had an idea of why forecasting the weather is so difficult and the process involved in constructing weather forecasts. This is sort of the reason why I construct my posts to be so technical and show so many images. My hope is that the reader gets a little insight into the background behind forecasting.

The past 24 hours...even 36 hours has been an absolute nightmare. Forecast models have been all over the place, wavering back-and-forth between solutions and there has been terrible disagreement between forecast models. Typically when we get into the nowcasting aspect of forecasting (typically that's when you're within about 6 to maybe 12 hours out, things become much more clear because you can see how things will unfold as they're happening...well this still isn't really the case with this system. Why is that?

This system was incredibly complex with multiple pieces of energy moving through the atmosphere. The greater the amount of energies at play, the more complex the situation becomes and it becomes more difficult for forecast models to handle them and how they will evolve. Providing a comparison, think of it as solving a puzzle. When you have a puzzle that has 20...30...40 pieces, the puzzle isn't all that difficult to solve, however, when you have a puzzle that has maybe 1500+ pieces, well it becomes much more difficult to solve the puzzle easily because you have more pieces to work with.

Let's look at some of the inconsistencies the forecast models are still having with this system and what we can expect across Connecticut. We will look at total liquid precipitation accumulation by the NAM forecast model, the GFS forecast model, and the HRRR forecast model through 5:00 AM Thursday morning:

Look at these differences! The HRRR has less than 0.50'' of total precipitation accumulation for the majority of the state while the NAM has generally greater than 0.60'' across the majority of the state and the GFS generally o.30'' to 0.60'' across much of the state with more towards the east. This is just one of the many tools used when forecasting snowfall. On average, 1'' of liquid will equate to about 8-10'' of snowfall. This is an average, however, and can vary based on numerous atmospheric processes including. In the situation with this storm the idea was 1'' of liquid would equate to as much as 10-12'' of snowfall. Seeing these extreme differences in liquid precipitation make this rather challenging to figure out just exactly how much we see. Another issues is the very tight gradient some models, including the European model have in terms of liquid precipitation across the state (with more in the south and a sharp cutoff as you get towards Hartford and points north).

Here is a radar grab from 1:12 PM this afternoon:

 This shows moderate to heavy snows off to our southwest across portions of Pennsylvania, New Jersey and farther south into the mid-Atlantic. While the radar does suggest snow over Connecticut we are dealing with a great deal of dry air within the lower portion of the atmosphere. Until this dry air saturates (becomes more moist) it is going to be very difficult for the snow to reach the ground with the exception of extreme southern Connecticut. The degree of dry air also complicates snowfall forecasts because there are some indications the dry air hangs around for a very long-time across the northern part of the state which would only increase the gradient in snowfall totals from north to south. This bufkit sounding from this morning's run of the NAM forecast model for Windsor Locks, CT at 2:00 PM this afternoon shows the abundance of dry air. Notice how far apart the red line (temperature) and green line (dew point) are? The more spread apart, the drier the air. Forecast models, however, do bring these closer together as we head into the evening indicating moistening of the atmospheric profile:

How is the storm shaping now? The storm is really beginning to take shape and strengthening just off the coast of the mid-Atlantic. This strengthening will continue to occur as the afternoon progresses. This is evident by a tightly closed off circulation developing at 700mb and 500mb just east of Virginia by this evening. As these circulations close off and become more tightly closed (this is indicative by the increased number of "circles" with smaller diameters) this is a tell tale sign of a very strong system:

The big challenge we are still faced with is how far northward can this system push and whether dry air over the northern portion of the state can become more saturated? Forecast models do suggest we will see a band of heavier snow push into the state but not until late afternoon or not even until the evening. From here the question becomes how heavy this band of snow is, how intense the snowfall rates are, how long the band sits, and how far north this band works into Connecticut? So many questions yet to be answered.

Currently the surface low sits east of Delaware where it has been for the past several-plus hours. As the mid-levels further strengthen (which we looked at in the above image) the surface low will continue to strengthen and it will begin to move off to the north and east:

Forecast models still are uncertain with regards to just how much lift we will see in the dendritic snow growth zone and that too complicates snowfall forecasts. Let's again look at Windsor Locks, CT. The 3 km NAM shows a period of extreme upward vertical motion within the dendritic snow growth zone over Windsor Locks, CT later tonight. This would indicate a period of very heavy snowfall. The 12 km NAM is much less robust with the degree of upward vertical motion within the dendritic snow growth zone as is the GFS. Forecast models, however, are more robust across the southern portion of the state:

What should we expect through the rest of the day? Outside of the immediate shoreline we will not see a great deal of snow during the afternoon hours. The air is still very dry and will take some time to saturate and the bulk of the storm is still well off to the south. As we approach late afternoon and early evening, all indications are that the atmospheric column will saturate and as heavier precipitation works into the state we will begin to see snow fall. The heaviest of the snows will occur during the evening hours and lasting into the overnight hours. There is still some uncertainties with regards to how late the snow will persist but there is a possibility it could persist late enough to where the morning commute will be rather messy...messy due to clean-up efforts. Based on the shift with the storm winds should be be as big of an issue as though as we are looking at gusts more in the 35-45 mph range. There could still be some isolated power outages, especially across the immediate shoreline. The worst of this storm will be this evening into overnight. Given everything mentioned above here is the updated snowfall map:

Note: There is a good probability that most locations end up with snowfall totals towards the lower end of these ranges as opposed to the higher end of the ranges. If heavier snowfall works farther northward and persists longer, the higher end of the ranges could be met in more towns and the possibility will exist for some towns to exceed 12'', especially across southern Connecticut:

Accumulating Snows Likely Across Connecticut Wednesday, March 21st, 2018

Our active string of winter weather looks to continue as we are tracking yet the possibility for another accumulating snowfall event on Wednesday. Forecast models have been rather consistent over the past week on this possibility, however, there have been some inconsistencies with the exact track of the storm. Like the last storm, however, it appears that there will be a rather large gradient across the state with regards to snowfall totals which will make constructing a snowfall map rather difficult. Anyone who gets under intense banding will get the most snowfall accumulations while anyone who doesn't get under the banding may not see a whole lot in terms of accumulations. Where the banding sets-up will be key!!

Currently, there is energy moving into the state of California. This energy will continue progressing into the southern Plains over the next few days before strengthening as a developing shortwave trough amplifies across the Gulf Coast states:

There is also a current piece of vigorous shortwave energy over the central Plains with an associated area of surface low pressure. This piece of energy and surface low will continue slowly progressing eastward over the next few days. As the energy across California moves west and amplifies, it will interact with the piece of energy and surface low pressure over the eastern United States. When this process happens the surface low pressure and system as a whole will begin to rapidly strengthen as it works towards the east coast:

One feature to watch which could result in a storm track further south is a vigorous piece of shortwave energy which hangs around across southeastern Canada. While forecast models this morning are suggesting the possibility for a big hit of snow across portions of Connecticut we still have to watch how the forecast models handle this energy in southeastern Canada because it could easily suppress the system further south which would mean less in the way of snow:

Latest forecast models seem to be in rather strong agreement that this system is going to really strengthen just south of Long Island and this is evident by forecast models becoming vertically stacked with the system south of Long Island. We can see this by looking at the 925mb, 850mb, 700mb, and 500mb charts and tightly closed contours (indicating closed off low pressure centers) which develop south of Long Island. This placement of the closed lows is a prime location for a band of extremely heavy snowfall to set-up right over Connecticut:

Forecast models are suggesting excellent snow growth over Connecticut along with sufficient moisture within the snowgrowth zone and excellent upward vertical motion within the snowgrowth zone. The graphic below from this morning's run of the GFS forecast model is showing forecast omega values along with the dendritic snow growth zone. This is showing a deep snowgrowth zone with the -12C to -18C zone extending from about 600mb to 500mb and a period of rather high omega values indicating very strong upward vertical motion. This suggests there would be a period of extremely heavy snowfall, likely within a potent band. We need to watch where this band sets-up as this is where the highest snowfall totals will occur along with the heaviest snowfall rates:

Over the next 24 hours the details should begin to become much more clear and the forecast will be able to become much more fine tuned. Also, there is a very small possibility we could see a period of light to perhaps moderate snow across the extreme southern Connecticut beginning mid-to-late tomorrow afternoon. If this does occur there is a possibility of some spots picking up as much as 1-3'' of snowfall from this. I will have a forecast update tomorrow afternoon.

In addition to the likelihood for accumulating snows, winds could be rather strong too, especially across coastal Connecticut on Wednesday where winds could gust upwards of 50-60 mph. This coupled with fairly wet and heavy snow could lead to scattered power outages and tree damage. This potential needs to be monitored very closely as well.

 Below is what I am thinking...but keep in mind the 8-14'' zone is with regards to the banding. This zone, along with totals will be adjusted and fine tuned once it becomes more clear where the banding will set-up and how intense the banding will be:

Quick Thoughts on the Tuesday, March 2013, 2018 Snowstorm

One of the most unique aspects of weather is forecasting. Forecasting can be challenging, it can be difficult, it can feel nearly impossible at times, it can be stressful, but in the end it is fun! At least for myself, one of the most enjoyable aspects of forecasting (outside of trying to prepare my readers as best as possible for whatever mother nature has in store) is the learning that takes place during a storm and just after a storm. Every single weather event provides us with a different and unique set-up and each of these set-ups usually comes with surprises. For anyone who is a forecaster the one thing that we must understand and accept is that we will be wrong at times. If you can't accept this forecasting is probably not for you. There are two different types of people out there; there is the type who get very emotional and then lash out and criticism when they are wrong and there are people who take being wrong as a learning experience and they go back, re-evaluate the storm and try to find what they miss. For the forecasters in this second category, these are the ones who are on the path of success or on their way towards the path of success. 

This past storm (while still currently underway!) had numerous challenges from day one. As we drew closer to the storm, forecast models indicated we would likely be dealing with a scenario in which we could see two potentially heavy bands of snowfall across Connecticut which could dump a quite a bit of snow over some towns while areas in between these bands could struggle to accumulate much. When you have a scenario such as this, constructing and presenting a snowfall map becomes quite challenging and this is because snowfall totals are not really uniform across a large area and this leads to sharp and large snowfall gradients sometimes in just a span of a few miles! 

In my blog post from yesterday (http://weatherwiz.blogspot.com/2018/03/accumulating-snows-likely-overnight.html) I discussed the possibility of the two band scenario and discussed what impacts this could have. Well this scenario is exactly what unfolded. Below are a couple screen shots from the radar at 9:43 AM, 10:26 AM, and 1:08 PM. I've highlighted the heavy bands of snowfall:

Notice how elongated and how think/narrow they are. The narrowness of these bands is what makes constructing a snowfall map in these cases a challenge because you just don't know where they will set-up and you don't know how long they will sit in a particular area. In cases like this a difference of even 5 miles can mean the difference between like 4-5'' of snow and as much as 12...15...even 20'' of snow!!! I mean how do you reflect that in a snowfall map? 

Also discussed in the post from yesterday was the term subsidence and how anytime you have a heavy band of snow (sue to extreme upward vertical motion) you must have extreme downward motion (think of the all so popular phrase, "what goes up must go down"). It was discussed that you would have subsidence inbetween the two bands. While forecasting how much snow falls in the areas under the bands and how heavy the intensities are (snowfall rates per hour) is tough, forecasting what will happen in the areas of subsidence is another challenge in itself. In areas of subsidence it can still snow...it's typically very light and doesn't accumulate quickly but this helps to shape the very sharp gradients between a few miles. To give an indication on just how crazy some of the gradients are, here is a tweet sent out by award-winning and NBC Connecticut 's chief meteorologist, Ryan Hanrahan:

Just wild stuff! While forecasters struggle with how to visualize this in a forecast, this is why it's very important to listen to what a forecaster has to say. Often times, the forecaster will explain these situations and try to give as much detail as possible because it's just too difficult to provide this detail on a map. 

Perhaps in the future, as our technology improves and we continue to grow a better understanding of how these mesoscale bands work, we will get to a point where forecasters can confidently forecast exactly where a band (or bands) will set-up and snowfall maps can be constructed to point out where these highest totals will occur and what areas will see much less given subsidence. 

Accumulating Snows Likely Overnight into Tomorrow Across Connecticut

We are tracking yet another accumulating snowfall event later tonight into tomorrow and even at this stage there are still some uncertainties with regards to the exact track of the storm. This is just one factor which will determine just exactly how much snow we see across Connecticut and what type of snowfall totals we can expect. Part of the problem with these uncertainties too is in the end this will come down to mesoscale factors (which will be discussed below) and those are extremely difficult to pinpoint sometimes more than 6 hours out...they can even be difficult to do so just a few hours out.

One of, if not, the biggest key with regards to this system is how two pieces of energy will interact with each other. There are some differences within the forecast models on how these pieces of energies not only evolve but how they interact:

Energy #1 continues to dig towards the southeast through the Great Lakes region while energy #2 continues to dig southeast through the Tennessee Valley towards the east coast. Once energy #2 is off the coast of the Carolina's, it will then make a turn towards the northeast and this is when we begin to see the two pieces of energy begin to interact (or phase). This will lead to rapid cyclogenesis (strengthening) of a low pressure system just off the coast. This is where we are faced with some questions with the pieces of energy. There are differences within the forecast models regarding the strength of each of the pieces of energies and also with the handling of energy #2. The NAM forecast model evolution allows for the storm to get tugged in closer to the southern New England coast and also has a much larger precipitation shield.  The result is the potential for higher snowfall totals across much of the state. The GFS and European forecast models, however, are a bit farther southeast and have a much more compact (smaller) precipitation shield resulting in less snowfall potential for much of Connecticut.

This is just one piece of the puzzle, however, and there are other clues in place which I think helps to shape which direction to forecast in terms of snowfall across Connecticut. Unfortunately these clues don't tell us exactly where certain features will setup, just that these certain features will be a big player. These will be addressed below:

Rapid Storm Deepening/Storm Becoming Too Strong

Due to the interaction between the two pieces of energy mentioned above, we are going to see very rapid cyclogeneisis/strengthening of the surface low pressure occur. In fact, the system may become too strong too quickly. This can make snowfall forecasts very difficult because while you'll have some areas hit really hard, you'll have many areas struggling to accumulate. Think of tropical systems...what is one noticeable feature they have? If you answered banded precipitation as you work out and away from the storm's center you're correct! The thinking about this is such sharp pressure gradients help to shape and result in pockets of rapidly rising air...it is under these pockets where you would see the most intense and heaviest snowfall rates. However, because you have these areas of rapidly rising air, you also must have rapidly sinking areas of air. This results in extremely light snowfall rates and intensity. Where these areas set up is extremely difficult, if not impossible to pinpoint this far out. How does this make a snowfall forecast challenging? Let's see Hartford County as an example. Let's say the snowfall forecast for Hartford county was 8-12'' but we had some of these extremely heavy bands setup over parts of Hartford County and some areas were in between these bands. You would have some cities within the county likely fall within the 8-12'' range (and maybe even locally higher amounts) and you would have some cities receive much less than 8''. This just goes to illustrate how these bands can really complicate a forecast.

Frontogenesis/Subsidence 

This can somewhat be tied into the above paragraph, however, not all storms have banded like structure of the precipitation but most of these bigger storms do have areas where strong frontogenesis leads to extremely heavy bands of snowfall...we saw this exact case with this past storm) and outside of this frontogenesis you have subsidence. Frontogenesis is the strengthening of horizontal temperature gradient with height. This leads to very strong upward rising motion and heavy snowfall rates and intensity. Subsidence is the sinking motion of air and that results in much reduced snowfall rates and intensity. The NAM forecast model actually hints at the possibility of two bands of heavy snowfall to set up with subsidence right in between these two bands. The first band would be over western and portions of central Connecticut with the second band right along the Connecticut and Rhode Island border:

The GFS isn't too dissimilar from the NAM in that it also hints at the potential for two bands of extremely heavy snowfall. The exception is the GFS is a bit farther east with the second band and has it kind of setup over portions of far southeastern Massachusetts as opposed to back over the Connecticut/Rhode Island border:

The European forecast model has a nice zone of strong frontogenesis traverse right over the entire state of Connecticut. However, this could also be due to the resolution of the graphics of the source used to view the data and the resolution may not be good enough to sort of pick up what the NAM and GFS advertised. This is something which really makes constructing a snowfall map quite challenging and is a major reason as to why there will likely once again be "in-storm" updates.

Amount of Liquid Precipitation

The other question we face is just how much liquid equivalent is expected to fall across the state with this storm. As a general rule of thumb, 1'' of liquid equates to about 8-10'' of snow. However, this can vastly depend on snowfall ratios (how fluffy the snowfall is). The above, equates to 8:1 to 10:1 snowfall ratios (1 inch of liquid = 8-10'; of snow). If conditions warrant higher snowfall ratios, then 1'' of liquid can equate to as much as 15''+ of snow! In the case of this storm, it does appear ratios have a better chance of exceeding 10:1 and possibly as high as 13-15:1, however, this will largely depend on how great our snowgrowth is (we will explore this later). Forecast models generally are showing anywhere from around 0.50'' of liquid (western part of the state) to as much as almost 1.40'' across the eastern part of the state (NAM is much more pronounced with this. The GFS is generally 0.60'' to 0.70'' across the state while the Euro appears to be about 0.40'' west to near 1'' east:

After analyzing some bufkit soundings across the state we do have great snowgrowth (at least through late morning tomorrow) with sufficient moisture/ice within the snoowgrowth zone and upward vertical motion which indicates that there will be zones of great snowgrowth and fluffy snow flakes which should accumulate rather quickly. However, bufkit soundings also suggest what was mentioned a few paragraphs up and that is subsidence. Within the white box we see blue contours. These blue contours represent sinking motion (subsidence) while the red contours (highlighted in the red box) indicate rising motion. The blue contours within the snowgrowth zone are a strong signal for the potential for subsidence and reduced snowfall rates:

What can we expect?

• Flurries/light snows to begin breaking out between 12:00 AM and 3:00 AM tomorrow morning.
• Light to moderate snows will overspread portions of the state between 5:00 AM and 8:00 AM. 
• Between 8:00 AM and 1:00 PM will be the height of the storm. During this time snowfall intensities will range anywhere from light to perhaps heavy. Who sees heavy snows and light snows will all depend on how the precipitation is structured across the state and whether we are looking mainly at banded precipitation and are dealing with areas of subsidence. In the heaviest bands of snow, snowfall rates could approach 1.5'' per hour. 
• After 1:00 PM to 2:00 PM bufkit soundings show a decaying snowgrowth zone and the storm should be moving off to the north and east so snowfall will begin to wind down. 
• Below is what I am thinking, however, keep in mind this will LIKELY have to be refined once the processes described within this post better become better known and understood.