Significant Severe Weather Event Possible: Thursday, August 27, 2020

A widespread and perhaps significant severe weather event is possible across parts of southern New England Thursday. There are uncertainties which exist, however, the ingredients will be there for what could be a widespread damaging wind event. A few tornadoes can't be ruled out either. 

The setup:

At the surface it will be very warm to hot with temperatures pushing into the 80's to perhaps near 90 in spots. Humidity will be moderate-to-high with dewpoints into the upper 60's to 70's. This combined with a plume of steep mid-level lapse rates (perhaps in excess of 6.5 C/KM to 7 C/KM) would result in a moderately unstable airmass characterized by mixed-layer CAPE values approaching 2000-2500 J/KG. 

Aloft, New England will be on the downstream crest of a upper-level substantial ridge which is centered over the heart of the country. This will place southern New England in a northwesterly flow in the mid and upper-levels of the atmosphere and also place southern New England in a zone of unseasonably strong wind shear with the mid-level jet stream on order of 40-60+ knots:

A cold front approaches by late Thursday afternoon. The combination of a moderately unstable airmass ahead of the cold front and strong wind shear aloft will yield the potential for numerous thunderstorms; likely in the form of several line segments which will be capable of producing wind damage...which could be widespread. Given a wind direction of northwest in the mid-levels of the atmosphere, more west to west-southwest in the lower levels of the atmosphere, and more southwest at the surface this will create directional wind shear which is evident on this forecast hodograph at Windsor Locks, CT from one of the computer forecast models for late Thursday afternoon:

This changing of wind direction with height indicates the potential could exist for a few tornadoes, especially if any discrete thunderstorms develop. 

Limiting factors: 

This event is not set in stone and there are a few wild cards which could hinder this event and overall potential;

1) Morning shower and thunderstorm activity. A warm front approaches from the southwest overnight Wednesday and lifts through southern New England Thursday morning. A complex of thunderstorms is expected to develop across southeastern Canada on Wednesday and then dive southeast through NY and parts of southern New England Thursday morning. This could result in extensive cloud debris (yielding a lower temperature forecast and much lower instability) along with subsidence (rising air) behind the complex. Subsidence could help erode cloud debris enhancing potential for maximum instability to be realized but if that subsidence is not able to be overcome (strong forcing mechanism such as a warm front or falling heights) this could suppress thunderstorm activity. This is something we will not have a clear picture of until Thursday morning. 

2) Lack of stronger height falls . Without going into a warm and piece long article to explain this...heights are basically the height above the ground you would need to go to find a particular pressure level. Falling heights over time is indicative of rapid upward vertical motion (excellent for thunderstorm growth) while more neutral or rising heights aren't very supportive for large-scale lift. 

3) Forecast soundings also indicate potential for some subtle capping layers (areas of the atmosphere where temperatures warm with height instead of cool). This can inhibit parcels from accelerating upward which is not good for establishing deep convection.

All in all, the ingredients are there Thursday for what has the potential to be a widespread severe weather event with damaging winds, large hail, and perhaps the risk for a few tornadoes. 

Friday, May 15, 2020 Thunderstorm and Severe Weather Potential

A warm front is expected to lift through southern New England Friday morning which may produce a few showers or a rumble of thunder. As the warm front lifts into central New England dew points south of the front will climb through the 50's and into the lower 60's. As clouds begin to break, strong May sunshine should help boost temperatures through the 70's during the afternoon. The combination of temperatures in the 70's and dew points into the 60's will result in a modestly unstable air mass.

A cold front begins to approach southern New England by early evening. The combination of a modestly unstable air mass, strong wind shear aloft, and forcing from the approaching cold front and shortwave energy yielding strong height falls, will result in the development of rain and thunderstorms in the form of a squall line. This squall line will move southeast through the region during the evening hours and will be capable of producing damaging wind gusts.

Assessing severe weather potential:

The potential is there for a more significant severe weather event (despite the presence of steeper mid-level lapse rates), however, not all the ingredients will align to make this happen. Forecast models stall the warm front across central New England. Just south of the warm front the air mass will be unstable thanks to warm temperatures and humidity along with rather strong wind shear; including the presence of directional wind shear (winds changing direction as you ascend from the surface through the lower atmosphere). In this scenario, the potential would be there for supercell thunderstorms capable of producing tornadoes, however, this potential will be mitigated by the lack of a trigger and rising heights through the afternoon.


As the cold front approaches during the evening and the associated line of rain and thunderstorms, instability will begin to weaken due to the loss of daytime heating, however, strong wind shear aloft and just enough instability will keep the potential for scattered pockets of damaging winds and hail possible as the line of thunderstorms progresses southeast through southern New England. Expecting the line to begin moving through between 7:00 PM EDT and midnight EDT. Greatest risk for any damaging winds north and west of the I-84 corridor. There is the low probability for a tornado across southern VT/northwestern MA in the vicinity of the warm front as there will be enough wind shear to yield the potential for an embedded supercell within the line.

Historic May Snowstorm on the Way (Friday, May 8, 2020 - Saturday, May 9, 2020)

For the most part, my blog posts focus on Connecticut, however, due to the historic nature of what is about to unfold as we move into Mother's Day Weekend I am expanding the focus to cover all of New England. (However, my snow map will focus on Connecticut due to time constraints).

An unprecedented air mass (for the month of May) is set to enter New England Friday into Saturday. A piece of the polar vortex is expected to break off from the main vortex and swing through southeastern Canada and move through New England. This will be associated with 500mb  heights as low as 516-518 meters. On average, the 500mb level is about 18,000 feet above the ground (and typically a bit higher as we move closer to the summer solstice), however, the 500mb level will be as low as 16,000 - 17,000 feet above the ground! This will also be associated with temperatures at this level as low as -35C to -40C...these heights and temperatures would be record breaking and perhaps record shattering.

At the surface, a cold front will be moving through the region. As the polar vortex dives in an associated shortwave trough will amplify (strengthen) as it digs through New England. This will promote strengthening jet stream dynamics aloft which will be favorable and conducive for cyclogenesis (strengthening) of a low pressure system along the cold front.

As the low pressure strengthens and with the passage of the cold front, we will see anomalous cold across southeast Canada will advect into New England. This cold will set the stage for what looks to be a historic winter storm for interior New England, however, as far south as Connecticut is not out of the woods. The biggest key will be track of the low pressure. Should this track a bit farther southeast than what forecast models indicate, heavy snow would be likely across much of Connecticut away from the shoreline.

While surface temperatures may remain as high as 33-35F, temperatures just a few thousand feet above the ground will be extremely cold which will yield precipitation to remain as all snow...the only problem is it will be very wet and heavy. For any areas which see more than 3-4'' of snow, concern will be high for widespread power outages (which could last for several days in some places) and tree damage. As of now, the greatest potential for this to happen would be; Northwest Hills of Connecticut, western Massachusetts, a large part of Vermont, New Hampshire, and Maine away from the coast.

This is an extremely complex situation due to the unprecedented and historic nature of this air mass and subsequent storm. Significant changes to this forecast are possible as the details become clearer. Right now, the greatest concern for power outages and tree damage in Connecticut is confined to the Northwest Hills.

Timeline for Connecticut:

• Showers arrive into the state by Friday afternoon with some heavier rain by the evening. 
• During the evening temperatures quickly crash, allowing for rain to transition over to heavy, wet snow in the Northwest Hills with rain changing to snow by late evening and early overnight across much of the state away from the immediate shoreline. 
• Greatest accumulations across the Northwest Hills (4-6'' possible) with highest likelihood for widespread power outages and tree damage. 
• Thundersnow is possible
• Saturday features near record-cold with scattered snow showers, snow squalls, and the potential for thunder/lightning along with grauple and small hail. 
• Below is my current thinking for Connectuct:

Gusty Thunderstorms with Small Hail Possible Tuesday, April 21, 2020.

A vigorous shortwave trough is expected to amplify (strengthen) as it digs through southeast Canada Tuesday:

Associated with the shortwave trough will be a pool of very cold temperatures between about 10,000 feet and 20,000 feet above the ground with temperatures up around 18,000 feet around -25C (-30 diving through southeast Canada!). This will provide steep mid-level lapse rates (temperatures decreasing rapidly with height):

With such a vigorous shortwave we will also see some rather impressive dynamics aloft characterized by the nose of a mid-level jet streak of 80+ knots punching into the region with a low-level jet exceeding 40-50 knots:

Mid-level jet:

Low-level jet:

With temperatures climbing into the 50's with dewpoints into the 40's combined with the steep mid-level lapse rates this will yield a marginally unstable airmass. Combination of a marginally unstable airmass, strong dynamics, and strong forcing from the cold front and shortwave energy will likely be enough to spark off scattered showers and thunderstorms Tuesday afternoon. Some of these will be capable of producing gusty winds and/or small hail. The greatest overall risk looks to be across NY, northern PA, and VT, however, depending on overall timing some of these may sneak into parts of MA and CT before fizzling due to loss of daytime heating. These will be scattered so not everyone sees any thunderstorms or even rain. 

Friday, April 17, 2020 into Saturday, April 18, 2020 Snow Event

Well...welcome to winter I guess. Computer forecast models for the past few days have been rather consistent in a fast moving weather system producing a narrow swath of moderate snow across parts of New England. There have been uncertainties, however, regarding where this band of snow traverses, but computer forecast models seem to be converging on a more likely solution.

A vigorous piece of shortwave energy is expected to amplify (strengthen) as it moves through the Northeast. This will help promote the strengthening of a surface low pressure as it passes just south of southern New England. Forecast models indicate a swath of rather impressive upward vertical motion will traverse southern New England overnight Friday into Saturday morning. This will result in moderate (perhaps even heavy) precipitation...which will likely be snow:

Friday morning's bufkit run of the NAM forecast model at Windsor Locks, CT shows a period Friday night of moderate lift extending into the dendritc snow growth zone with plenty of available moisture (not shown). This indicates there will be a period of moderate-to-heavy snow across the northern part of the state overnight Friday into Saturday morning:

Both the NAM and European computer forecast models indicate a thermal profile across northern CT which will be just supportive of snow. Looking at a forecast sounding for Windsor Locks, CT on the NAM notice the green line (dewpoint) and red line (temperature) are hugging the 32 line. This constant temperature through the lowest 5,000 feet of the atmosphere will result in what's called to an isothermal snow bomb...a several hour period of moderate-to-heavy snow which will be wet and heavy:

What to expect:

• Snow may start out as some rain but will quickly change to snow (away from the shoreline) with precipitation beginning between 7:00 PM and 10:00 PM EDT. 
• Heaviest rates between 12:00 AM and 5:00 AM EDT Saturday morning with snowfall rates 1'' to 1.5'' per hour.
• Snow will be heavy and wet. Due to weakened state of trees, tree limbs, and power poles from the wind storm this may result in isolated-to-scattered power outages. 
• Snow ends by 8:00 AM Saturday morning with some residual snow showers through mid-morning.
• Melting happens also immediately as temperatures warm into the 40's. 

Below, is my snow map:

Wild Monday of Weather Expected Across CT (Monday, April 13, 2020)

A vigorous weather system across the south combined with an eastward advancing cold front will set the state for a wild day of weather here in Connecticut on Monday. Periods of extremely heavy rainfall and even some thunderstorms will traverse the state. A strong pressure gradient combined with potential for thunderstorms will result in a windy day across the state. Due to the high winds and wet ground, the potential exists for scattered to widespread power outages and tree damage.

Forecast models show a sub-990mb low tracking through the Great Lakes by Monday morning with a strong area of high pressure across the western Atlantic setting up a strong pressure gradient across the eastern United States. The stronger the pressure gradient, the stronger the winds and with a strong gradient we'll some high winds Monday:

With such an intense weather system passing to or west and the strong high pressure to our east, winds only a few thousand feet above the ground Monday will become exceptionally strong. In fact, winds about 5,000 feet off the ground may approach or even exceed 100-110 knots...that is extremely impressive:

Thankfully, we will have what's called a temperature inversion in place. Typically, temperatures will decrease as you ascend through the atmosphere. However, due to strong southerly winds coming off the much colder waters of the Atlantic Ocean (south of Long Island) and Long Island Sound itself, this will keep temperatures in the lowest few thousand feet of the atmosphere "cool". What's happening is with the low pressure tracking so far to our west a warm front will approach. However, due to the colder air coming off the waters the push of warmer air in the lowest few thousand feet will come to a halt, however, as you ascent about several thousand feet into the atmosphere where the colder waters will have less of an influence, the push of warmer air will usher in much more quickly. This then forms the temperature inversion as temperatures here will warm faster then a few thousand feet below. This is extremely critical because if not for this temperature inversion, we would see widespread destructive wind damage. However, due to atmospheric mixing and potential for thunderstorms, some of these winds will mix down to the surface and this will create the potential for damaging winds. However, instead of 100-110+ knots (125-135 mph) down at the surface we may see wind gusts of 50-70 mph possible (with sustained winds 25-35 mph).

An example of what a temperature looks like visually is illustrated below. This is a bufkit forecast sounding for Windsor Locks, CT for noon EDT Monday. The green line is dewpoint temperature and the red line is air temperature from the surface (where we live and at the bottom of the sounding) rising through the atmosphere. When the lines slope to the left temperature/dewpoint is decreasing with height. When the lines slope to the right temperature/dewpoint is increasing with height. Looking at the lowest 5,000 feet of the atmosphere we see both dewpoint and temperature are increasing with height. This is our temperature inversion:

This warming of temperatures and an increase of moisture with height will also promote the atmosphere to be a bit unstable above the temperature inversion. Instability is one ingredient needed for thunderstorms. Not only will we see some extremely heavy rainfall on Monday, but there will be multiple opportunities for thunderstorms. 1) An embedded line of thunderstorms which moves through mid-morning through mid-afternoon and 2) A much smaller opportunity for discrete thunderstorms during the late afternoon sneaking into southwest CT:

Outside of any thunderstorms tomorrow, it will be quite windy with winds sustained 20-30 mph and gusting 50-60 mph at times. However, with thunderstorms the potential will exist for winds to gust 65-75 mph. Thunderstorms will increase the potential to mix some of those exceptionally strong winds a few thousand feet off the ground down to the surface. This would certainly lead to the potential for scattered to widespread power outages. We will closely have to monitor the potential for any discrete thunderstorms making their way into southwest CT late afternoon as these would pose an additional risk for severe weather.

So what can we expect Monday?

• Sustained winds 20-30 mph with gusts 50-60 mph
• Torrential rain at times with a line of thunderstorms moving through .
• As these thunderstorms move through localized wind gusts of 65-75 mph are possible. 
• Scattered to widespread power outages and tree damage 
• Pockets of flash flooding 

Saturday, January 18, 2020 CT Snowfall Forecast

I would like to begin by saying my notion of a "major" snowstorm for CT a few days ago was a bit misleading. Probably not the proper term to use for this type of setup, however, at the time I was not discounting the fact that there could potentiality be widespread 6''+ of snow.

Anyways, the forecast for accumulating snow Saturday is on track. This will be a very quick moving system and the majority of the state should even remain all snow throughout the duration of the event (with exception of the southern half of the state).

Normally, I wouldn't be as aggressive in terms of my snowfall forecast for such a setup, however, there are several things I like about this setup which I think will favor a bit more of snow then we would typically see. I will briefly go through these below:

1) Dynamic cooling - As low-level dynamics increase with this system this results in air rapidly rising upwards. This process results in cooling temperatures. When this process occurs rapidly there can be a rapid response in temperature. This will help with better snow ratios (see below) and preventing a changeover from snow to sleet (especially away from the shoreline). The GIF below illustrates how temperatures at 850mb (~5,000 feet above the ground) change during the day Saturday...notice how they cool quite a bit! Typically in this setup they would warm:

2) Favorable snow ratios - Typically in these setups our snowfall ratios are less than stellar...around 8:1 or so. However, given how cold the air mass will be (along with my expectation for some modest-to-decent lift into the dendritic snow growth zone...see below) I am thinking snow ratios may be as good as 15:1. This would indicate fluffy snow and possibly medium-to-large dendrites...the kind of snowflakes which accumulate quickly. Caution should always be advised when assessing snowfall ratios outside of 36-hours and with snow ratios on bufkit, however, the signal has been strong for solid snowfall ratios and given the environment I am inclined to believe they will be as good as 15:1 which is indicated below:

3) Forecast models are hinting at some decent lift penetrating the dendritic snow growth zone. This is also something which I think may trend a bit more impressive over the next 24-36 hours.

What can we expect?

• Snow begins to break out by late Saturday morning
• A 2-4 hour period of moderate to even heavy snow is possible mid-to-late Saturday afternoon
• During this period snowfall rates may approach 1.5'' per hour 
• Low-to-moderate travel impacts Saturday afternoon 
• Mixing with sleet southern half of the state...perhaps even to rain along the immediate shoreline
• Snow exits by late Saturday evening