Tuesday, December 17, 2019 Winter Storm Update

I am becoming a bit more concerned for the possibility of a major to perhaps significant icing event for a large portion of CT (virtually south and east of I-84 to the I-95 corridor). This is going to be an extremely messy and high impact winter storm with significant travel impacts Tuesday.

As precipitation arrives late Monday evening, everyone begins as snow (the immediate shoreline, however, quickly transitions to sleet and then eventually rain. A potentially 3-4 hour window of moderate-to-heavy snow is expected during the overnight as some very strong lift associated with warm-air advection arrives (Keep in mind the heaviest rates will be on the leading edge of the lift):

As this stronger lift moves through temperatures a few thousand feet off the ground will warm above freezing while temperatures near the surface remain below-freezing. This is a recipe for freezing rain. The latest NAM run indicates potential for significant freezing rain accumulation across southern CT. This forecast precipitation for New Haven ,CT reveals a long duration period of freezing rain with moderate-to-freezing rain:

What to Expect:

• Snow breaks out late Monday evening and quickly becoming moderate-to-heavy overnight 
• Snowfall rates between 1-2'' per hour across central/northern CT
• Highest totals near the CT/MA border with less along immediate shoreline
• Snow quickly transitions to sleet then rain along the immediate shoreline although freezing rain is possible here too
• Snow begins to transition to sleet/freezing across interior CT during the early morning
• ***MAJOR ice accretion is possible between the I-84 to I-95 corridor***
• Dangerous travel...travel is NOT recommended 
• Numerous power outages possible-to-likely
• Downed tree limbs and weakened trees
• Precipitation slowly begins to wind down during the late morning and afternoon

Monday, December 16, 2019 - Tuesday, December 17, 2019 Winter Storm

The active December continues as we prepare for what will be a rather messy winter storm which will yield high impact to the Tuesday commute (morning through afternoon). This impending winter storm will feature snow, sleet, freezing rain, and even rain. Snow totals will not be significant, however, there is some concern freezing rain may be.

This upcoming storm is what is known as a southwest flow event (SWFE). A southwest flow event is characterized by southwest winds within the 800-500mb layer (~6,000 feet to 18,000 feet above the ground) which in turn is associated with warm air advection. Hence, precipitation ranging from snow to sleet/freezing rain, and rain. These setups are typically characterized by a very brief period of heavy precipitation (associated with the warm air advection). Depending on the profile of the atmosphere, this can often be a period of snow.

Snow should begin working into the state Monday evening as stronger low-level rising motion associated with a push of warm-air advection arrives:

As warmer air arrives snow will begin to transition and this is where things get interesting. Along the immediate shoreline snow quickly changes to sleet and rain (in fact, along the immediate shore precipitation may even begin as sleet). Farther inland, however, the surface will be much slower to warm. This results in more sleet and perhaps more freezing rain...and this is where it gets interesting. Are we going to see more sleet or more freezing rain?

Forecast soundings from the NAM and GFS at Windsor Locks, CT for Tuesday at noon both suggest freezing rain will still be falling (will just show the NAM do to time constraints):

What to Expect:

• Snow begins breaking out between 5:00 PM and 8:00 PM Monday evening and will quickly become moderate to even heavy. Snow persist for 4-5 hours (much less along the shoreline). 
• Snow begins to transition to a wintry mix during the late evening or early overnight (closer to early evening along the immediate shoreline)
• Sleet and freezing rain persist well through Tuesday morning and into the afternoon before perhaps ending as some rain.
• Significant freezing rain is not expected, however, isolated power outages/tree damage may result and this will impact travel. 

Below is what I am thinking:

My December 11, 2019 CT Snowfall Bust...It was Very Bad

My snowfall forecast for the December 11, 2019 event across CT…terrible…bad…horrific…there aren’t enough negative terms in the dictionary to describe how awful of a forecast that was. However, I am not mad or angry. The beautiful thing with weather is that it is an imperfect science (something that most people might not really understand) and there are going to be times when you’re wrong. All you can do is learn from any mistakes.

In the case of this storm, it’s not necessarily a mistake I made. It was geared more towards the fact that I placed too much emphasis on a red flag I saw (red flag meaning something that may prevent a certain outcome from occurring) and ignored some of the glaring signals which suggested we would get heavy snow. So, why did I do this and what have I learned from it?

First off, the major red flag to me was the forecast of dry air and subsidence in the lowest ~7,000 feet of the troposphere. Why are these two significant?

• Presence of dry air – Dry air “eats” away precipitation. Basically, what occurs is the process of evaporation. If you have precipitation falling towards the ground through the troposphere and it’s falling through dry air it can evaporate before reaching the ground. Sometimes radar reflectivity will indicate precipitation is falling, however, it is not reaching the ground. This is called virga.

• Subsidence – Subsidence is sinking motion…air is sinking. For clouds and precipitation to form you need rising motion. Warm and moist air rises, as it does so it encounters a cooler environment, the parcel temperatures then reaches the dewpoint temperature and voila…clouds form and precipitation can form (depending on other factors of course). When air is sinking it also warms…not good for the development of clouds and precipitation.

So, what led me to thinking this dry air and subsidence would be a problem? In the essence of time and space I will just pick one of the forecast models. Forecast models differed with just how much dry air would move into place, however, keep in mind my thinking was this dry air would advect in early and quickly. Below, is a cross-section spanning the state of CT from Tuesday’s morning run of the 3K NAM for 3:00 AM EST Wednesday morning. I have highlighted the presence of rather dry air in the lowest 5,000 feet of the troposphere (light shading greens indicating about 75-85% relative humidity). Note the darker green shadings, however, above this along with the purple shaded areas…those are areas of stronger rising motion:

Let’s then compare this with that evening’s 0z run (just hours before the snow) valid for the same time. Notice a SIGNIFICANT DIFFERENCE in the profile:

What stood out as a strong signal for snow to fall (and heavily at times) and accumulate was intense rising in the mid-levels of the troposphere with the most intense lift occurring between where temperatures were -12°C to -18°C. This layer of temperatures is referred to as the dendritic snow growth zone. Snow growth is maximized between this range of temperatures. For snow growth to be maximized you need the following;

•       Very strong upward vertical motion present within the dendritic snow growth zone (referred to as the “cross hair signature).

•       Significant presence of ice crystals (very high relative humidity with respect to ice).

•       Sufficient moisture (relative humidity values > 80%).

If these conditions are present, not only will you develop dendrites (these are the big snowflakes which accumulate very rapidly) but you will also have heavy snowfall rates.

All forecast models were very consistent with this feature. The 12z 3km NAM run from Tuesday morning at Windsor Locks, CT indicated negative values of omega (which indicates rising motion) in between the -12°C and -18°C temperature range. Referring to the relative humidity cross section above, we know it’s plenty moist within that layer. I’ve highlighted in black this area. I’ve also highlighted the yellow-ish shading in the lower-levels of the troposphere. These are positive values of omega (indicating sinking motion). I saw that as a flag and determined this would offset things:

There was also excellent lift provided by a very strong mid and upper level jet stream and a very strong axis of frontogenesis about 10,000 feet above the ground. Below, is the 700mb frontogenesis forecast from the 12z run of the NAM showing intense frontogenesis over CT for 4:00 AM EST Wednesday morning:

After a review, what I’ve learned is DO NOT IGNORE the cross hair signature. When a cross hair signature is present and there is plenty of available moisture…expect snow and heavy snow. While there was dry air involved, the dry air advection was being fought off possibly by evaporative cooling (the process of evaporation leads to cooling). One way to increase relative humidity is to decrease temperature. Also, as snow was being generated and falling, this helped to moisten up the atmosphere as well. The combination of excellent jet dynamics and frontogenesis helped force rising motion.

I incorrectly assessed how everything would come together and how this evolve. This incorrect assessment lead to a horrific weather forecast. You could easily say, “why would you go against everyone else. If everyone else goes a certain way, just go with them”. That seems logical, but that is not the correct way to go. One of the best assets a forecaster can have is knowledge of past experiences. Forecast models are only guidance and if you just forecast based off what a model is spitting out…you’ll find yourself in trouble. After-all, if forecast models were 100% accurate computers would do this job!

So it’s off to the next storm and the next bust and see what can be learned from that 😉

Sunday, December 1, 2019 - Monday, December 2, 2019 Winter Storm Update

First significant winter storm of the season is still on track for CT. While we continue to draw closer to the onset of the storm there are still some uncertainties to resolve, however, details are starting to become a bit clearer. This is going to be a long duration winter weather event with two different parts (or aspects to it). While I did go in-depth a bit the other day with the meteorology/science behind the storm, I will do so again and discuss some features in greater depth.

Part 1: Sunday, December 1, 2019:

Sunday starts cold and cold enough at all levels of the atmosphere for precipitation to begin as snow statewide. A vigorous piece of shortwave energy diving east-southeast through the Ohio Valley and associated surface low pressure weakens as it moves east through the Ohio Valley. The track of these features will yield warmer air to move towards CT. This warmer air (warm air advection) will be a focal point as it will help yield a several-hour period of moderate-to-heavy snow overspreading the state beginning late Sunday morning.

As the warmer air begins to work into the lower-levels of the atmosphere (and at the surface along the immediate shoreline) we'll see snow transition to sleet, freezing rain, and rain along the immediate shoreline. The biggest question here is going to be how much sleet vs. freezing rain?

Conducting a sounding analysis across the state of CT we see some differences come Sunday evening between the NAM forecast model (on the left) and the GFS forecast model (on the right). The green shaded color accounts for the spread in dewpoint temperature (green line) and temperature (red line) across the state. The NAM introduces a shallow-elevated warm layer...this would result in a changeover to sleet..eventually the depth the warm layer would increase with warmer air becoming more shallow (closer to the ground) which would indicate freezing rain potential. The GFS, however, keeps colder air in place longer and then quickly transitions much or the state to some rain:

The verdict: I think the GFS is a bit too aggressive with the degree of warmth. While the immediate shoreline (perhaps even 10-15 miles inland) may switch to rain, I don't think we'll see rain into central CT. This is where we'll see a mixture of sleet/freezing rain. Significant issues should not result from sleet or freezing rain as the intensity will be very light and more intermittent. This continues through the overnight into Monday morning.

Snowfall totals on Sunday should generally be 1-3'' along the immediate shoreline and 2-4'' the rest of the state. (I would lean towards the lower side of these ranges as opposed to the higher side).

Part 2 Monday, December 2, 2019: 

For my snow map to verify this will require part 2 panning out and there are still uncertainties with how this transpires. Going back a few paragraphs, I mentioned the surface low pressure moving east through the Ohio Valley and weakening. As this weakens a new surface low pressure begins to develop across PA which slides east...the question is how close to CT this tracks...if it's very close to CT warmer air wins out and the strongest atmospheric lift and heaviest precipitation occurs north of the state, if this is more over Long Island or just south, colder air quickly works back into the state, stronger atmospheric lift resides over the state, and we get heavier precipitation...snow.

The NAM indicates the stronger atmospheric lift develops north of CT through the day Monday while the GFS suggests some of the stronger atmospheric lift does indeed develop down into CT:

Verdict: After analyzing all model guidance, my thinking is we will see some stronger atmospheric lift work into the extreme northern part of the state during the day Monday. This will result in a second period of moderate-to-heavy snow developing through the morning and persisting into the late afternoon before tapering off. Periods of light snow remain possible through the overnight.

Snowfall totals with Monday's snow may add another inch or two across the shoreline (though it's very possible this is more of a mix or even rain) with another 2-4'' across interior CT...it's the extreme northern part of the state which sees the heaviest snow where additional totals of 4-8'' are possible.

Here is a breakdown of what to expect:

• Snow overspreads the state Sunday morning from west-to-east
• Quickly becoming moderate-to-heavy for a few to several hours
• Snow quickly transitions to sleet and then rain along the immediate shoreline and changes to a mixture of sleet/freezing rain inland
• Intermittent periods of sleet/freezing rain persist through the overnight into Monday morning
• Snow begins to re-develop across the state through Monday morning (though may be rain or a mix along the immediate shore)
• Heaviest snow along the CT/MA border where rates may approach or exceed 1'' per hour
• Thundersnow a small possibility 
• Significant travel impacts expected Sunday through Tuesday morning. 

Based on the latest guidance, I have made some adjustments to my snowfall map. I have shifted the 2-4'' zone farther north to incorporate a larger area of southern CT and I have shifted the 8-12'' zone slightly north (closer to the CT/MA border). 

This snowfall map accounts both Sunday and Monday:

Sunday, December 1, 2019 - Monday, December 2, 2019 Snow Potential

Are we looking at our first accumulating snowfall event for the majority of CT as we turn the calendar to December? There continue to be growing signals that this may be a possibility, however, there are still numerous details to iron out moving into the weekend. This is going to be a bit of a messy storm as there will be two different parts to it. This will also be drawn out over the span of a few days. (Note: I use GFS graphics for visual reference, however, analysis and my thoughts are derived from analyzing all forecast models).

They piece of energy which will be responsible for our potential storm Sunday and Monday is entering the southwestern U.S. This energy will also be a player in what will be a significant winter storm/blizzard across the Inter-mountain West, northern Plains, and Inter-mountain West moving into the weekend:

Sunday morning, the piece of energy is forecast to dive southeast through the Ohio Valley. At the surface an area of low pressure (weakening) will be moving towards the east-southeast. As this whole system progresses towards the east-southeast a warm front will lift towards southern New England. As the warm front approaches, warm-air advection will rapidly increase. This warm-air advection will be quite significant as it will result in a period (perhaps several hours) of moderate-to-heavy snow overspreading much of the state. As warmer air begins to arrive (first occurring a few thousand feet off the ground) snow will quickly transition over to some sleet (maybe some freezing rain) and even rain along the immediate shore. This will conclude part one of this system:

Moderate-to-heavy snow overspreads the state Monday morning before transitioning over to a wintry mix and rain along the shore by late afternoon. Several inches of accumulation will be possible:

As the energy dives south of CT Monday morning, a secondary area of low pressure is expected to develop...this is where things get a bit dicey as it's uncertain exactly where this happens and how quickly this happens. Prior to this development, warmer air will be in place, both aloft and at the surface, thanks to the warm-air advection. However, as the secondary low develops and strengthens dynamic cooling will take place and this will allow the lower-levels of the atmosphere to quickly cool with colder air working in at the surface.

My current thinking is this all happens a bit too far north. This will keep the column a bit too warm for snow here in CT (although a better chance would exist in the northwest hills and along the CT/MA border) and we'll see mostly rain or a mix with this second part. Although, this is still several days away and plenty can certainly change.

All in all this system will likely be a major snow producer (8-12''+) somewhere in New England, it's just uncertain as to where this area will be. My early estimate is along and north of the MA Pike.

Sunday, December 1, 2019 Forecast:

• Snow begins working into the state from west-to-east during the morning. 
• Snow quickly picks up in intensity; perhaps falling at rates of 1-1.5'' per hour.
• Snow begins to change over quickly to rain rain along the immediate shoreline first due to the influences of the warmer water. 
• Snow transitions over to a mixture of sleet...perhaps some freezing rain and rain inland as warmer air begins to work in aloft.
• Significant travel impacts expected beginning late Sunday morning into the evening.
• Winds do begin to strengthen overnight as the secondary low develops and strengthens. 
• Major uncertainties exist regarding Monday.

12z GFS bufkit for Windsor Locks, CT Sunday suggests ample upward vertical motion and more than sufficient relative humidity and ice (not shown) for the generation of moderate-to-heavy snow:

Below are my initial thoughts for Sunday ONLY. This DOES NOT INCLUDE WHAT MAY TRANSPIRE MONDAY:

I will have some additional updates late Friday afternoon. 

Anniversary of the October 3, 1979 Windsor Locks, CT F4 Tornado

One of the strongest recorded tornadoes in CT history (perhaps only outdone by the Wallingford tornado of 1878 which is estimated to be F4) occurred at Bradley International Airport on Wednesday, October 3, 1979. The killer tornado was rated as a high-end F4 tornado. Not only are violent tornadoes extremely unusual in CT (the only other tornado to achieve an F4 rating was the Hamden tornado which occurred July 10, 1989. Note: The F-scale did not exist in 1878 so the rating is estimated based off damage reports), but the time of year made this tornado extremely unusual. Below, we will explore the science and meteorology behind the F4 tornado of October 3, 1979.

Meteorological Set-up:

The synoptic pattern was characterized by a large ridge across the western United States with a trough digging into the eastern United States. The graphic below displays 500mb (~18,000 feet above our heads) height anomalies across the United States on Tuesday, October 2, 1979. The yellows and reds indicate above-average heights; indicative of a ridge while the blues and purples indicate below-average heights; indicative of a trough:

In the days preceding October 3, 1979, a ridge was building across the western United States with a trough configured across the east. This troughing was only enhanced by a rather significant trough digging into the northern Plains on the morning of October 3. Within the initial trough was a vigorous piece of shortwave energy which dove southeast from the northern Plains and into the southern-tier of the Ohio Valley before lifting northeast towards New England:

At the surface, a warm front was pushing north through Long Island Sound towards CT. As the warm front lifted north of CT during the early afternoon the airmass became warm and humid while the warm front result in rather strong low-level directional wind shear (an important ingredient needed for tornadoes). This image below (courtesy of The Weather Bureau) shows the surface set-up from 7:00 AM EDT October 3, 1979. At this time, the warm front was positioned south of CT with temperatures across the state only in the 50’s:

An area of low pressure at the surface developed and passed just west of CT with an occluded front just northwest of CT, warm front placed just north of the state, and a cold front moving into the state from NY. This placed CT in what is known as the triple point. The triple point is where the occluded front, warm front, and cold front intersect. While not always a certainty, this area can be favorable for severe thunderstorms; including tornadoes as an overlap of instability and wind shear is most likely to occur within this area. Below is an illustration of the triple point from meteorologist Jeff Haby (Image courtesy of meteorologist Haby – theweatherprediction.com):

During the early-to-mid afternoon, there reports of sun breaking out across northern CT which result in an increase in temperature along with instability. During the time a supercell thunderstorm was south of Long Island, heading north towards the CT Valley. There were reports of a funnel cloud as this moved over Long Island. 

As this storm moved north it moved through the CT River Valley. Due to the configuration of the valley, low-level wind shear was likely enhanced and strengthened with surface winds more southeast while winds a few thousand feet off the ground were more from the south-southwest. As this supercell moved into northern CT, it tapped into the stronger instability likely resulting in a strengthening mesocyclone which ultimately produced a violent tornado.

The tornado formed about 3:00 PM EDT Wednesday, October 3, 1979 causing significant destruction to the airfield, business, and residential buildings. Three people were unfortunately killed with several hundred more injured making this the deadliest tornado in CT since the 1878 Wallingford tornado. At one point, this tornado was the 6^th most costly U.S. tornado on record, causing $200 million in 1979 U.S. Dollars ($690 million in 2019 U.S. Dollars).

Some CT tornado facts:

·       Between 1950-2018 CT has had 105 confirmed tornadoes (yielding an average of about 1.5 per year).

·       The two most active years on record are 2018 and 1973 – both with 8 confirmed tornadoes.

·       Of the 8 confirmed tornadoes in 2018, two occurred during the month of October. These were the first two confirmed tornadoes during the month of October in CT since the 1979 tornado.

·       The most confirmed tornadoes in one day was 5; occurring on July 21, 2010.

·       CT has had two confirmed F4 tornadoes (October 3, 1979 and July 10, 1989). Note: The F-scale rating did not exist in 1878, but that was likely at least an F4 tornado. 

Monday, May 20, 2019 New England Convective Potential

During the day Monday a cold front will be pushing towards New England. Out ahead of the cold front it will be warm and on the humid side with temperatures pushing into the 70's and 80's with dewpoints into the 60's which will yield a modestly unstable airmass.

T'storm coverage may not be very widespread on Monday (perhaps a greater coverage across central and northern New England) given the main shortwave energy tracks just north of the U.S./Canadian border:

Wind shear will be more than supportive for some t'storm develop with a 50-60+ knot mid-level jet punching through New England and a low-level jet of 35-45 knots over the region. This will contribute to both strong vertical and effective shear which will favor the potential for organized and sustained updrafts:

Some forecast soundings also indicate the potential for a little bit of directional wind shear in the lowest 5,000 feet of the atmosphere (winds changing direction with height). Circled in red on the bottom right shows winds slightly changing direction with height and this is shown via the curved look to the hodograph on the top left. Something to watch as Monday progresses as this could increase the potential for some supercell-type structures:

Due to weak mid-level lapse rates (temperatures not really decreasing sharply with height between 10,000 and 20,000 feet) this will limit the degree of destabilization that will take place. However, the combination of temps into the 70's and 80's with lower 60's dews will yield anywhere from 1000-1500 J/KG of MLCAPE which is more than enough for some t'storms to develop given the wind shear aloft.

What can we expect? With the strongest overall forcing (thanks to the shortwave tracking north) remaining north, lack of steeper mid-level lapse rates, and greater instability, t'storms will be more scattered in nature. Given the wind shear parameters though, a few t'storms may become strong-to-severe posing a threat for hail (perhaps as big as 1'' to 1.25'') and damaging wind gusts. If the degree of directional wind shear becomes a bit stronger than advertised an isolated brief tornado can't be ruled out either.

We are not looking at a widespread severe weather event (or even widespread t'storms) but more scattered in nature with a few becoming strong-to-severe. Timeframe will be from 2:00 PM until 8:00 PM. Activity quickly diminishes and weakens as the sun sets.

Anniversary of the Tuesday, May 15, 2018 CT Tornado/Severe Weather Outbreak

Tuesday, May 15, 2018 featured one of the largest severe weather outbreaks in CT's recorded history. 4 confirmed tornadoes and hundreds of large hail and damaging wind reports made for a wild Tuesday afternoon. While severe t'storms are not uncommon across the state, significant severe and widespread significant severe weather are a bit more uncommon, especially during the month of May. What lead to such a large and destructive event? We will explore the science and more below.

The catalyst behind the potential for this event was that of the elevated mixed-layer. The elevated mixed-layer (EML) is the biggest contributor in severe weather outbreaks which occur across the southern Plains. Without the EML, the degree and severity of severe weather events in the south are likely lessened.

What is the elevated mixed-layer? In the United States, the EML can originate across the desert Southwest, Rocky Mountains, or across Mexico. Across these regions air can become quite warm and quite dry. Certain weather conditions (such as surface winds) can displace this region of warm and dry air farther east. Keep in mind that east of the above mentioned areas, the elevation becomes much lower. This area of warm and dry air becomes elevated above the surface. We will show this process by looking at two various forecast soundings. The forecast sounding on the left is from Albuquerque, NM valid 6:00 PM MDT Sunday, May 12, 2019. The station elevation is about 5,300 feet above sea-level. The red line denotes the temperature with increasing height while the green line denotes the dewpoint temperature with increasing height. Note how far apart the two lines are. This indicates extremely dry air. the surface temperature (which correlates to about 850mb or 5,000 feet above the ground) here is just shy of +20°C. The forecast sounding on the right is for Corpus Christi, TX from 7:00 AM CDT Monday, May 13, 2019. The station elevation is only 43 feet above sea-level. Notice at just above 850mb the temperature line makes a sharp turn to the right (indicating rapid warming) while the dewpoint line makes a sharp turn left (indicating rapid drying). This is the EML:

The significance (or role) of the EML and severe weather is the EML creates a "lid" or atmospheric cap. Below the EML (depending on the type of season) the airmass can become quite warm/hot and very humid which can lead to extreme instability. The cap, however, prevents convection (showers/t'storms) from developing which increases the degree of instability that can develop. If there is a strong enough source of atmospheric lift, the cap can be broken and violent t'storms can quickly develop.

Now that we have gone over a bit of what the EML is let's discuss the connection between the May 15th, 2018 severe weather outbreak.

The morning of May 15th featured an ominous look and it was become quite apparent we would be dealing with a very active severe weather day. The observed forecast sounding from 8:00 AM EDT at Pittsburgh, PA showed a well in tact EML which would continue to advect into New England through the morning:

The presence of the EML combined with a highly sheared environment, moderate instability, and a potent shortwave/cold front yielded all the necessary ingredients for a severe weather outbreak. A rather potent upper-level jet (300mb or ~30,000 feet above the ground), characterized by an west-southwesterly 80+ knot jet streak passing through northern New England helped to increase large-scale lift/upper-level divergence while a  west-southwesterly 50-60+ knot mid-level jet (500mb or ~18,000 feet above the ground) helped with strong values of vertical shear and potential for updraft rotation.

Embedded within the jet stream was a strong piece of vorticity digging southeast from Canada. The timing of the shortwave energy coincident with the most unstable part of the day and best wind shear:

Perhaps the more impressive aspect of the sheared environment was how winds were changing between what was mentioned above and down at the surface. Winds around 5,000 feet were more out of the southwest with winds near the surface more from the south. This turning of winds with height (along with increasing wind speeds with height) helped to create very large amounts of helicity - a measure of the rotational potential within a t'storm. The higher the values of helicity, the greater the likelihood for a t'storm to acquire rotation, and the higher the potential for tornadoes. Below is a forecast sounding for Windsor Locks, CT from the 12z run of the NAM forecast model the morning of May 15th valid for 4:00 PM that afternoon. Note the rapid changing of wind direction with height and increase in wind speed with height (highlighted with the red box). The blue box shows helicity values just over 400 m2s2 (typically values approaching 125-150 can be enough to start thinking of the potential for an isolated tornado) and the orange box shows the hodograph which is just a plot showing how winds are changing with height. The long and curved look indicate that winds were increasing in speed with height and changing direction with height:

Forecast sounding from the RAP forecast model from 2:00 PM valid for 3:00 PM at Poughkeepsie, NY shows a very volatile environment...more than sufficient for violent t'storms:

The combination of temperatures well into the 80's, dewpoints well into the 60's, and mid-level lapse rates approaching 7.5 C/KM (temperatures decreasing on average of 7.5 C/KM between about 10,000 feet and 18,000 feet above the ground) thanks to the EML helped to create high amounts of instability characterized by mixed-layer CAPE values approaching 2000 J/KG:

With these parameters in place the SPC issued severe weather probabilities you almost never see in this area (particularly with regards to the damaging wind and tornado probabilities). Their afternoon update contained an upgrade to a moderate risk of severe weather (the second highest risk level issued by the SPC) followed by 10% tornado probabilities, 60% damaging wind probabilities, and 30% large hail probabilities. These probabilities the percent likelihood of witnessing such event within 25-miles of any point within the outlined area:

These damaging wind probabilities are the highest percent issued for all of the above severe weather probabilities. In fact, if that area had been "hatched" (this would indicate 10% probability for damaging wind gusts greater than 65 knots within 25-miles of a point) the moderate risk would have been a high risk...the highest risk level issued by the SPC. The only other time a high risk has been issued in the Northeast is May 31, 1998.

When all said and done numerous severe t'storms develop just west of CT and pushed into the state causing significant damage. Below are some radar snap shots:

As the severe t'storms pushed through the state they caused extensive damage. Very large hail (up to the size of baseball's!) damaged cars, windows, and siding in NW CT, destructive winds in the form of microbusts/macrobursts leveled trees across several cities, and a total of 4 tornadoes were confirmed. Some of the hardest hit areas included; Newtown, CT, Bethel, CT, New Fairfield, CT, Danbury, CT, Naugatuck, CT, Oxford, CT, and Hamden, CT.

And finally the map of storm reports:

CT is certainly no stronger to severe t'storms or even tornadoes, however, severe weather outbreaks of this magnitude are rare and we don't see them often. Anytime the presence of an EML may coincide with high amounts of instability, wind shear, and approaching shortwave energy/cold front high-end severe weather is not only possible, but likely.

Winter's Last Hurrah...One final Winter Storm Threat in the Books?

Yes we are entering the second week of April, however, we may have a chance for what should be...or hopefully be our final winter weather event of the season. We don't think of April as a wintry month or a snowy month, but it isn't all that uncommon to get snow events in April (especially across the higher elevations). So what is yielding such potential? We'll discuss below! Disclaimer: The graphics below are from the GFS forecast model and I will use the model graphics to illustrate the ideas.

On Monday, April 8, a rather potent piece of shortwave energy is projected to enter the Pacific-Northwest. From here it will continue on an eastward trajectory through the Inter-mountain West and northern Plains. Both the Euro and GFS indicate a rather strong area of low pressure will develop in association with this feature. Forecast models show enough cold air across the northern-tier of the country to perhaps yield a significant winter weather event from ID, WY, CO on east through parts of SD, NE, IA, WI, and MN:

Why do we need to watch this here in CT and New England? This is due to the potential evolution of some major teleconnection patterns which not only support a storm, but support the potential for a cold enough airmass and favorable storm track to at least yield the thought of winter weather.

Moving into the second week of April the consensus is for the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) to become extremely negative while the Pacific-North American (PNA) becomes positive. Without going into detail on what all this means (this would make this post exceptionally long) the basic answer is...this combination is one that favors a more southern storm track and also for below-average temperatures to spill into our region:

Looking near mid-week next week we'll look at the 850mb level (about 5,000 feet above the ground). I've highlighted a couple features; temperatures, an 850mb low, and a boundary. Across southeastern Canada is a very anomalously cold airmass characterized by temperatures -10°C to -14°C.If the image was forwarded in time a bit it would show that some of this spills into our area. The 850mb low shows about where the potential storm is positioned. The orange line shows the separation between miler air and colder air and this boundary will be key:

As the system gets closer, computer forecast models indicate that we will see a surge of warmer air move into our region and we would not have to worry about winter weather at all, however, given what we are seeing with the NAO, AO, and PNA it's a bit difficult to believe such warm air makes it into our region...it will be very close...IF THIS TRANSPIRES AS ADVERTISED.

As we move through April chances for snow rapidly dwindle. This is due to the increasing sun angle, warming airmass, and lack of available cold, however, if we are going to get snow in April...this is a look you would want to see.

Not saying this will happen, but if the NAO and AO become as negative as forecasted and we see the spike in the PNA...this could get quite interesting...at least somewhere in New England.

Snow Event Wednesday Morning (04/03/2019)

This potential has been in the cards for the past week and while the odds were never very high, these odds have skyrocketed substantially over the past 24-hours. This isn't going to be a major event for many (accumulations will likely be confined to the hills), but it will have a high impact to the Wednesday morning commute...good news though as temperatures quickly warm into the 50's to near 60 during the afternoon.

A vigorous piece of shortwave energy at 500mb will be associated with a very strong area of low pressure just off the coast. The track and strength of the system will help to maintain a rather unseasonably cold low-level airmass over the region. Surface temperatures will be marginal which will reduce accumulations and likely limit them to the hills, however, there is tremendous upward vertical motion associated with this system and that will increase the likelihood for snow falling even if temperatures are as high as 34-35°F:

Let's break it down:

• Rain will begin to transition over to snow (especially in the hills between 2:00 and 5:00 AM Wednesday morning).
• A 3-5 hour burst of heavy snow is possible where snowfall rates could approach 1.5''/hour. 
• Snow will be wet and sticky. 
• Snow ends by mid-Wednesday morning and then temperatures quickly warm into the 50's to near 60 by the afternoon.

Meh "Winter Weather" Event Saturday, March 9th, 2019 into Sunday, March 10th, 2019

After quite a pleasant Saturday with ample sunshine and temperatures soaring into the 40's (an indication that spring isn't far away :D) we do have a bit of winter weather to deal with moving into Sunday morning, however, this will not be a big deal as temperatures will quickly go above freezing.

A rather potent low pressure system moving northeast into the upper-Midwest producing significant snow across the upper-Midwest and severe weather across parts of the south will push a warm front towards our area. High pressure to our north will try to lock in some colder air in the lower-levels of the atmosphere which is why we'll start out as some wintry precipitation:

Forecast models do show some decent lift moving into the area towards Sunday morning with 15 units of omega within the snow growth zone. Unfortunately, the temperature profile looks more supportive of sleet with a marginal thermal profile with temperatures several thousand feet up not much above-freezing. Surface temperatures tonight will also struggle to get below-freezing across many spots.

Coldest temperatures will be confined to the extreme northern part of the state and some of these areas may yield a coating of sleet before everything transitions over to rain through Sunday morning. All in all...no big deal.

Don't forget to set those clocks ahead before bed tonight!