01/19/2019 to 01/20/2019 CT Winter Storm

We have been tracking the potential for a rather significant winter storm for several-plus days and as we get closer to the weekend this potential not only still exists but the potential is becoming more of a reality. Before I get into the posts I wanted to post a disclaimer. All images that will be posted (unless otherwise noted) will be from the GFS forecast model. I can't post images from the European model as I don't have permissions. Just because I'm posting GFS graphics does not mean I am leaning toward's that model...what I am posting is to just illustrate. 

There are three key features that will be involved; a thermal boundary, southern stream vorticity, and northern stream vorticity. An area of low pressure (perhaps multiple low pressure circulations) will develop along the thermal boundary and strengthen with aid from vigorous vorticity:

There are substantial differences between the GFS and European forecast models with regards to the evolution of the southern stream energy which will play a substantial role in the eventual outcome of this system. The GFS is much stronger or "amped" than the European forecast model. This yields the surface low pressure to actually track either over CT or perhaps slightly west. This solution would yield a snow, to ice, to rain scenario. The Euro not being as amped is slightly farther southeast with the track of the surface low and would result in a snow to potentially significant icing situation across CT. 

Forecast models indicate some very high precipitable water values (PWATS) pushing into the area and feeding into the storm. PWAT values just south of the area are forecasted to be well over 1''...that is suggestive of very juicy air. A very strong temperature gradient will also exist over southern New England which will help to enhance frontogenesis over the region and support vigorous lift. This is shown with the vigorous 700mb vertical velocity values. Very strong warm-air advection (WAA) will further assist in the lift process:

This grab from the 18z GFS is the bufkit overview for Windsor Locks, CT. This is showing exceptional lift overnight Saturday...despite the most intense lift well below the dendritic snowgrowth zone this still shows -20 to -30 units of omega within the zone. Snowfall rates will approach 2-4'' per hour at times during the height of the snowfall:

We will have to carefully watch the exact track of the surface low as the likelihood exists for snow to changeover to freezing rain, sleet, and potentially rain. While the GFS forecast model is not very aggressive with freezing rain here, the European model is. There will be a tremendous amount of cold air just north of us and if the surface wind direction is more northerly as opposed to southerly or easterly this cold air will drain down the CT River Valley. This would keep surface temperatures well below freezing while temperatures a few thousand feet off the ground warm above-freezing...this would result in freezing rain and the potential for significant freezing rain.

Below is what I am currently anticipating in terms of impacts for CT. 

Suppression Depression or Major Storm Threat?

If you're a snow lover living in southern New England you're probably not all that happy with how winter has started. We have had plentiful cold at times, however, we've also had our share of warmth with very little of the white stuff. As we head into the second weekend of January some opportunity exists to bring some excitement to snow lovers. Some work needs to be done to make this happen, however, speaking from a synoptic stand-point (which will be the premise of this post) this weekend will be extremely favorable for cyclogenesis (development and rapid strengthening of a low pressure system)...it's just a question of where the system tracks. This will be answered over the coming days. I will be using the 18z run of the GFS forecast model from January 8th (as this is the most current run available during the composition of this post) to illustrate the key features we will be focusing on. I will also discuss the latest GFS ensembles which will be used to yield what interest level we should have in this event.

Over the next few days the Pacific-North American (PNA) is expected to become more positive in nature as a ridge builds into the western United States. Let's also note the piece of energy to the east of the ridge axis:

The Arctic Oscillation is also expected to DRASTICALLY drop over the next few days. In the animation below notice the very high pressures which develop over the Arctic. This is in response to the major sudden stratospheric warming event which occurred and led to the split of the polar vortex:

This negative AO coupled with the positive PNA will try and dig a trough down into New England. As this occurs the piece of energy (highlighted in the first image) will be progressing across the country. The northern and southern branch jet streams will be phasing (interacting) over the northeast. This will provide sufficient energy and ingredients for a rather potent low pressure to develop:

The jet stream will also be rather strong and this will lead to a very complex (so to speak) atmosphere this weekend with alot of moving pieces. Forecast models have been consistent in some energy moving through the northern stream which 1) may interact with the southern stream energy and further enhance the strength of the storm and 2) may also suppress it south of New England. There is also a rather strong high pressure to the north which could suppress the system:

The latest ensemble run of the GFS, however, still shows decent potential for a snow event here in southern New England. There is certainly a quite a bit of details to iron out through the week, but for snow lovers there is some hope!

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.