Saturday, December 18, 2021 Wintry Mix

 A fast moving low pressure will bring a wintry mix to much of Connecticut Saturday with the exception being near the shoreline where predominately rain is expected (though some mixing is certainly possible at times). 

While snow is possible at the onset of precipitation, this is looking like predominately a sleet or a light freezing rain event. This will not be a significant storm by any means, however, roads will become slick through the day Saturday, especially untreated surfaces. 

Low pressure moving northeast across the Ohio Valley begins to get shunted south as it moves into the Northeast. This is largely due to an area of high pressure across southeast Canada which will act as a "wall":

The flow around the high pressure system (clockwise flow) will help to feed some colder air from Canada into our region within the lowest 5,000 feet of the atmosphere, keeping temperatures through this portion of the atmosphere very close to freezing at the surface and just a few ticks below freezing as you begin ascending through the lower atmosphere. However, with the low pressure system ultimately tracking just to our north and west, this will promote a more west-southwesterly flow above 5,000 feet. This will bring about very strong warm-air advection. This push of warmer (and higher moisture content air) into colder (and drier air) results in a thump of heavy precipitation blossoming over the region:

So for precipitation type, why are we more likely to see a mixture of sleet and freezing rain as opposed to snow? In order to get dendrite (snowflake production) you need the following:

1. Cold enough temperatures (Colder than -4C) within the cloud for the formation of ice crystals and ice nucleation to occur.

2.  Sufficient upward vertical motion and moisture into the snow growth zone (with snow growth maximized between temperatures of -12C and -18C). 

In the case of Saturday, we are actually looking at relatively warm temperatures within the clouds. In fact, temperatures may be a bit too warm for the formation of sufficient ice crystals (first image below). In addition, the depth of the warmer air in the mid-levels of the atmosphere will also result in a relatively high snow growth zone (second image below). Initially, upward vertical motion may be strong enough to produce some snow (as evident by the green/light blue shadings inside of the black outline box in the second image below, however, as that strong upward vertical motion subsides the chances for snow vastly decreases and we are looking at sleet/freezing rain:

What to Expect:

• Precipitation begins to break out mid-to-late Saturday morning. There will be an initial burst of snow away from the shoreline, but this should quickly transition to some sleet and freezing due to the processes described above...very quickly. 
• The heaviest precipitation rates occur with the initial burst with rates more light when sleet and freezing rain is falling. This continues well into the evening. 
• Overnight, a second burst of heavier precipitation traverses the state, however, this will be predominately rain as warmer air has pushed in. 

Below are my thoughts:

Wednesday, December 8, 2021 Connecticut Snowfall Forecast

 Forecast models have been in general agreement for several days in the potential for accumulating snow Wednesday. While there continues to be uncertainties regarding overall strength and track there is a growing consensus and several signals which indicate an area of low pressure will not only track far enough north to overspread the state with snow, but for the low pressure to strength enough and in time to result in a band of moderate-to-heavy snow traversing at least a part of the state. 

A strong upper-level jet places the region in a very favorable position for enhanced upper-level divergence. This will work to strengthen the low pressure system and support large scale upward vertical motion (favorable for the generation of precipitation):

One of the key players here is the interaction between shortwave energy and the amplification (strengthening) of an upper-level trough as it moves across the Ohio Valley. Should this shortwave trough amplify (as the NAM continues to suggest ) earlier this would increase the degree of large-scale lift across the region along with aid in the faster development of the low pressure. This is a signal which appears to be strengthening:

Forecast models are also in general agreement an 850mb closed low pressure circulation will pop just south of Long Island. This indicates system strengthening and would also place a good portion of Connecticut, Rhode Island, and eastern Massachusetts in a potential path for a band of moderate-to-heavy snow to traverse. This is also highlighted by strong 850mb vertical velocities:

There is still plenty of room for this to trend in a direction which favors less snow, however, the signals are pointing towards we may end up in the direction of accumulating snow. In fact, it is very possible snowfall totals (map below) may have to be adjusted upwards by an inch or two. 

What can we expect Wednesday?

• Snow starts off around daybreak Wednesday (maybe a bit later) across much of the state with snow very light. 
• Snow intensity gradually begins to increase through the morning. 
• The heaviest snow rates occur under where the band of snow traverses and the timing of this is between about 11:00 AM - 3:00 PM Wednesday. Snowfall rates could approach 1'' per hour during this window.
• Snow begins to wind down moving into the late afternoon with some flurries and light snow continuing into the evening. 
• Travel will be slick so give yourself extra time traveling. Travel impacts should not be severe but plan accordingly. 
• Below is my first call snow map, however, keep in mind adjustments are likely Tuesday afternoon.

First Accumulating Snowfall Event of the 2021-2022 Season 11.28-2021 - 11.29.2021?

 Confidence is growing that many across New England (especially interior New England) may see the first accumulating snowfall event of the cool season Sunday, November 28 into Monday, November 29. This is something which has been appearing within computer forecast model ensembles for several days and is gaining traction within the operational guidance. (Note: I will be using GFS forecast model outputs for illustrative purposes. This does not mean I am favoring the GFS model). 

As we move through the Thanksgiving holiday and the upcoming weekend, a pattern change is set to occur across the country and this is hinted at by both the North Atlantic Oscillation (NAO) and the Pacific-North American (PNA). Both oscillations will be going through a phase change with the NAO becoming more negative through the weekend and then more positive and the PNA becoming more positive then becoming less positive. Not only do these changes indicate pattern change, but these changes are typically associated with storminess somewhere across the East Coast:

As a ride pumps up across the West a trough digs and amplifies across the Northeast. A potent piece of shortwave energy associated with the amplifying trough dives southeast across New England...perhaps as far south as Long Island. Due to the strong baroclinicity which exists between the still warmer waters off the coast and much colder air over land in the fall/winter/spring this can favor the rapid development and strengthening of a low pressure (cyclogenesis):

The warm sector of the system may be characterized by temperatures about 5,000 feet above the ground as warm as +2C to +5C while temperatures on the cold side of the storm as cold as -4C to -9C. This would result in a strong baroclinic environment across the region favoring rapid strengthening and very strong lift (upward vertical motion):

Given this is still several days away it is way too early to get into any specifics or details as many questions need to be resolved. Some of these questions include;

1) Where does cyclogenesis occur? 

2) Where does the low pressure track? 

3) How quickly does a low pressure develop and materialize? 

4) Do any mid-level low pressure circulations form, and if so, where do they track?

These questions would determine who sees snow, how much snow may occur, and where the heaviest/highest snow totals occur. Obviously, given the time of year the most favorable areas for plowable snow (>3-4'') would be across the interior (moving away from the coastline). 

This signal has been growing stronger over the past few days and now we work to resolve these (and additional questions) over the next few days. 

Hurricane Henri - What to Expect here in Connecticut

As we move through the remainder of this Saturday, all eyes are on Henri and just how much Henri strengthens as it now moves into a more favorable environment for strengthening. For the past few days Henri has been impacted by very strong wind shear to the north. Henri, however, is moving away from this stronger wind shear and is moving over the very warm waters of the Gulf Stream. As of this writing, Henri was not looking particularly great, but it is slowly trying to strengthen. 

Below, is the latest forecast track with cone of uncertainty from the National Hurricane Center (NHC):

The NHC is forecasting Henri to make landfall across the eastern tip of Long Island as either a strong tropical storm or a low-end category 1 hurricane with the center then passing over Madison, CT with maximum sustained winds of 70-mph with gusts upwards of 85-mph and tropical storm-force winds (my estimation of gusts 40-60 mph) across the majority of the state. However, notice there is still a quite a bit of spread in potential landfall with the cone as far west as western Long Island and as far east as eastern Rhode Island. This is due to the questions regarding exactly how strong Henri gets today and interactions with an upper-level low to our west:

My anticipation is that Henri will make landfall across central or eastern Long Island (more likely eastern Long Island) and then swing northwest into the Connecticut shoreline. I am unsure of strength but if it is as strong as what the NHC is suggesting...we will see big problems statewide. Below are my thoughts on each storm phenomena:

Wind: The latest GFS computer forecast model, with it's forecast track of Henri, forecast winds anywhere from 50-70+ knots about 2,500 feet above the ground east of the Connecticut River as Henri moves into southeast Connecticut. 

It is my estimation this would result in widespread wind gusts of 40-60+ mph across much of the state for a solid 3-5 hours. Wind gusts along the shoreline just east of the center could approach 80-90 mph. 

Power Outages: These winds would cause extremely widespread wind damage, some structural damage, and power outages along the shoreline just east of the track with widespread power outages and tree damage across a large portion of the state. Hardest hit areas should prepare to be without power for upwards of several days.

Storm surge/Coastal Flooding: Forecasts are for 3-5 feet of storm surge along the Connecticut shoreline and this would bring about significant-to-devastating coastal flooding. 

Rainfall/Freshwater Flooding: The heaviest rain and highest totals would occur just west of the track and that places the highest rainfall totals along and west of the Connecticut River. Rainfall of 4-8'' with a narrow swath of 8-12'' is likely. This would promote widespread significant flooding. 

Tornadoes: Isolated tornadoes are possible with embedded thunderstorms, especially just to the east and northeast of the storm track. This would likely be northeast Connecticut into Rhode Island and into southeast Massachusetts. 

Timing: It is very possible we see some bands of torrential downpours and embedded thunderstorms move into the state later tonight (which would carry a isolated tornado risk). Weather conditions begin to rapidly deteriorate moving through late Sunday morning. The worst of the winds happen late morning through late afternoon but the rain/flooding threat should persist well into the evening.  

Let's be very thankful this is not a much stronger storm or that it is not approaching us at a faster speed.

Wednesday, April 21, 2021 Low Topped Squall Line Potential

 A cold front is set to sweep across southern New England early Wednesday evening. Combination of marginal instability and rather strong dynamics and forcing from the front will promote the development of a low-topped squall line ahead of the cold front. While the greatest overall potential for damaging winds and even hail exists across New York and Pennsylvania, localized embedded damaging wind gusts and hail will be possible across southern New England (particularly western and central sections). 

Instability is not the driver here with temperatures predominately in the 60's with dewpoints into the 40's and lower 50's, however, steep mid-level lapse rates (~7 C/KM) will aid in the development of marginal instability characterized by about 500-750 J/KG of mixed-layer CAPE. 

The driver is going to be the strong forcing associated with the cold front and very strong jet stream dynamics which are characterized by 50-60+ knots at 500mb (~18,000 feet above our heads) and 35-40+ knots at 850mb (~5,000 feet above our heads). These shear values are more than enough to support the development of organized convection. 

The line initially develops across western New York and central Pennsylvania and rapidly progresses east through the afternoon and evening. Combination of dry low-level air and strong low-level winds will support the potential for damaging wind gusts on the leading edge of the squall line. Wind damage may be more scattered across New York and Pennsylvania but then become more isolated across New England. Given cold mid-level temperatures characterized by -20C at 500mb (~18,000 feet) some of the more robust convection within the line could even produce some small hail. The line is likely to move across Connecticut between 5:00 PM and 8:00 PM Wednesday evening likely causing some localized damaging wind gusts on the leading edge and even potential for some small hail:

Vigorous Upper-level Low Pressure to Bring Accumulating Snow to Parts of Connecticut Overnight 04/15/21 to 04/16/2021

 A vigorous upper-level low pressure system across New England prompts the developments and strengthening of a surface low pressure right in the vicinity of Connecticut moving through Thursday night. If the processes involved with this system occurred say another 50-75 miles farther south and east..at least the northern part of the state would be looking at a pretty significant winter storm, however, looks like the most significant snows occur across parts of western Massachusetts, southern Vermont, and perhaps around the Worcester Hills in east-central Massachusetts. 

As of Thursday afternoon moderate-to-heavy rain has overspread much of Connecticut. As we move into the overnight and the low pressure system really gets its act together, anomalous cold will transpires within the lowest levels of the atmosphere. In fact, surface temperatures look to even dip as low as the mid-30's and even colder across the hills. As this occurs rain begins to transition to snow across the hills and even parts of northern Connecticut. In fact, the majority of the state could even some some snow mix in with the rain. 

This is a highly complex setup due to the features and processes involved, however, forecast models have been zeroing in on a solution which features a band of heavy snow which becomes established across southern Vermont through the Berkshires of western Massachusetts even extending into extreme northwestern Connecticut. This would result in a several-hour period of moderate-to-heavy snow overnight. Snowfall rates could approach 2'' per hour so despite the warm ground, these rates, combined with it being overnight would result in snow accumulating, though taking a bit longer on paved surfaces. Another band of moderate-to-heavy snow looks to originate across the Worcester Hills of east-central Massachusetts with this band pivoting and clipping parts of northeast Connecticut Friday. 

Given this snow will be wet in nature and very heavy, the potential exists for downed tree limbs and power lines. In fact, scattered-to-numerous power outages could be expected in areas which receive greater than 4'' of snow...power outages could even be on the widespread side. 

This is a very dynamic storm and add in the time of year and you get a very challenging forecast which is ultimately going to come down to nowcasting...meaning we won't really know how the situation unfolds until the processes involved get going. 

What to Expect:

• Moderate-to-heavy rain persists through the overnight
• Around midnight, rain begins to mix with and change over to snow across the northwest and northeast hills with snow becoming moderate-to-heavy through the overnight. 
• The heaviest of the snow across the northwest hills winds down between 6:00 - 8:00 AM Friday morning, however, moderate-to-heavy snow may persist through much of the morning across parts of northeast Connecticut. 
• The majority of the state may see some snow mix in, however, most people will probably be counting cows when this happens. 
• Gusty and cold Friday with temperatures only into the lower 40's with a stiff northwest wind gusting 20-25 mph.

Below is what I am currently expecting in terms of snowfall accumulations:

Burst of Moderate-to-Heavy Snow Thursday Night (March 18, 2021)

 A strong Arctic cold front arriving from the northwest moves across Connecticut early Thursday evening. Meanwhile a low pressure system (responsible for the ongoing tornado outbreak across the deep South) approaches from the southwest Thursday morning. As the low pressure approaches, heavy rain pushes into the state early Thursday afternoon. The combination of the approaching cold front and strong Arctic high pressure behind the front helps to push the approaching surface low south of the state Thursday evening.

By early Thursday evening, the Arctic cold front rapidly pushes through the state resulting in much colder temperatures quickly overspreading the state. Despite the cold front moving through the state, lift will remain rather strong (thanks to the surface low passing south of the state) and ample moisture will be thrown into the colder airmass. This results in rain changing over to a period of moderate-to-heavy snow by late evening Thursday before exiting during the late overnight or early pre-dawn hours Friday. 

Forecast models indicate some pretty strong lift across the state during the early overnight hours Thursday night with forecast soundings showing a thermal profile favorable for snow:

Breakdown of what to expect:

• Heavy rain moves into the state early Thursday afternoon.
• Temperatures get into the lower 40's, but during the evening begin to drop rather quickly with increasing winds as the cold front moves through. Wind gusts of 30-40 mph are possible overnight Thursday 
• Snow begins to transition to rain during the mid-to-late evening Thursday with the changeover occurring more quickly across the northwest hills. 
• Snow end during the late overnight or early pre-dawn hours Friday morning. Some parts of the hills could see as much as 4-5' of snow.
• Majority of the snowfall accumulations occurs on the grassy/colder surfaces, however, roads will become slick, especially as temperatures drop below-freezing. Give yourself extra time for the Friday morning commute.
• Sun comes out through Friday morning, but a cold/breezy day with temperatures only into the 30's.
• Snowfall Forecast: