Tuesday, January 19 - Wednesday, January 20, 2021 Clipper Potential

 After the mid-December snow event, winter has been a zzzzzzzzzz fest in the snowfall department and in the cold department for that matter. While it may not seem like it, the overall pattern hasn't been all that terrible, however, a few nuisances; particularly with the state of the Pacific have prevent us from cashing in on the pattern. We have also seen a more southern stream dominant pattern which well...sometimes works and sometimes doesn't. However, moving into the third week of January, we will be dealing with northern stream energy and these can offer up clipper systems and sometimes these clipper systems produce...so will this one? Let's take a look!

A textbook rex block pattern will evolve across the west coast at the beginning of the week with a cutoff low pressure across the Southwest (or just off the Southwest coast) with high pressure building into the Pacific-Northwest region. This results in the jet stream splitting into two across the western United States with the two pieces finding love again across the eastern United States:

Within this pattern, there are two key features to focus on for the clipper potential. Shortwave energy digging in from southeast Canada within the fast-moving northern stream and shortwave energy across the southwestern United States:

If that shortwave energy across the Southwest remains more off the west coast (or even weakens more rapidly than forecast) this will provide further room for the northern stream energy to amplify as it digs into New England. Should this amplify enough with the main piece of energy passing south of Long Island, this would promote a high likelihood for a rapidly strengthening system with a rapidly blossoming precipitation field. Given there will be sufficient cold air available, this precipitation would be in the form of snow!

Should this northern stream energy amplify enough to strengthen, we could see a light-to-moderate snowfall event unfold Tuesday night into Wednesday morning. Accumulations would certainly be possible. What kinds of accumulations would be looking at? Too early to tell, however, something certainly just getting into plowable territory would be in the cards. 

This is still several days out, however, that potential looms and for those starving for snow, we hold onto any hope we can get in these dire times.

Happy Snow Watching!

Major Blizzard To impact Connecticut: 12/16/2020 - 12/17/2020

 Confidence continues to grow in the likelihood for a significant winter storm with blizzard conditions to impact Connecticut Wednesday, December 16, 2020 into Thursday, December 17, 2020. Significant snowfall accumulations, strong winds, scattered power outages, and severe travel disruptions are all anticipated.

As of Monday morning, the energy responsible for the developing storm which digging into the western United States as part of a long-wave trough. This energy will slide through the southern Plains Tuesday and then begin lifting northward through the Ohio Valley and into the Northeast through Wednesday:

Very impressive upper-level jet dynamics are expected as this system strengthens with the Northeast in a very favorable position for extensive large-scale lift. This will result in the blossoming of moderate-to-heavy precipitation across much of the region:

There are two features which will help to produce a very significant snowstorm across a large area;

1) An area of high pressure just to the north of Maine - This will reinforce colder air into the state and also work to keep a storm track a bit farther south. 

2) A weak negative North Atlantic Oscillation (NAO) with the NAO actually becoming more negative over the next few days. This will setup a 50/50 low or a "block". The block helps to keep a storm track offshore which favors a colder airmass and greater likelihood for snow:

The biggest uncertainties and challenges being faced right now are;

1) The overall track of the surface low pressure.

2) How the low pressure at 850mb (~5,000 feet above the ground) evolves and where that tracks.

3) How the low pressure at 700mb (~10,000 feet above the ground) evolves and where that tracks. 

All 3 of these are keys as these will heavily influence exactly where the band of heaviest snowfall traverses. The heaviest snowfall occurs generally 50-75 miles northwest of where these low pressures track. Forecast models are in general agreement for the surface low to track just south of Long Island. There are disagreements, however, regarding how the 850 an 700 low pressure evolve and where those track. There are some indications the 700mb low could even track over Connecticut, this would then introduce the potential for a dry slot to punch into the state and significantly reduce snowfall totals.

With high pressure to the north and a strengthening low pressure passing to our south and east, this will result in a very tight pressure gradient across the state which means strong winds. Winds will increase out of the Northeast through the day Wednesday to between 15-25 mph by the overnight with gusts 35-45 mph. This will bring about blizzard conditions with extremely low visibility at times. In addition, scattered power outages are possible. 

Despite some of these uncertainties above, forecast models are showing very impressive upward vertical motion into the dendritic snow growth zone with fairly high snow ratios too, however, the NAM forecast model is more impressive than the GFS forecast model. The NAM would indicate extremely heavy snowfall Wednesday night into Thursday morning with snowfall rates upwards of 2'' per hour:

However, what goes up must go down. This will pose another significant challenge regarding snowfall totals. Where there is a band of extremely heavy snow (which indicates intense upward vertical motion) there must be a region of intense downward vertical motion (the atmosphere is always trying to get to a balance). This will result in what is called subsidence and anyone under this subsidence will see snow rates must reduced and this would result in much lower totals for these areas. This is exceptionally difficult to forecast more than a few hours out. 

In the event of this storm, I am leaning more on the aggressive side with snowfall totals and impact. This system will feature very impressive dynamics with large-scale lift. Snow growth should be maximized with higher-than-average ratios given the extent of the lift and how cold the airmass is. Below is what to expect;

• Snow starts to breakout between 6:00 PM and 9:00 PM EST Wednesday evening and quickly becoming moderate-to-heavy moving into the overnight.
• Heaviest snow falls in the window between 11:00 PM EST Wednesday and 5:00 AM EST Thursday. During this window, snowfall rates may approach 2'' per hour.
• Winds increase out of the Northeast through the day Wednesday and peak Wednesday night between 15-25 mph with gusts 35-45. This will bring about blizzard conditions with drifting of snow. 
• High winds and heavy snow may result in scattered power outages.
• Periods of light snow persist into early Thursday afternoon, however, it remains cold and windy with temperatures barely getting above 30.

Below, is my initial snowfall map. As confidence increases regarding the banding potential this map will be updated:

Significant Coastal Storm to Bring Strong Winds, Rain, and Snow to Connecticut Saturday, December 5, 2020

 Looks like Connecticut will JUST miss out on a state wide significant snow event, however, parts of the state could see several-plus inches of snow. An area of low pressure passes just to the south and east of Connecticut Saturday. As it does so, the low pressure will rapidly strengthen. While the entire state will start out as rain, as the low pressure passes to our south and east and strengthens, a process known as dynamic cooling will transpire. Combination of dynamic cooling and heavy precipitation rates will rapidly cool the lower-levels of the atmosphere. This will result in rain changing over to heavy, wet snow across much of the state moving into Saturday afternoon. The northwest and northeast hills will see a changeover occur around mid-morning. 

The forecast track of this low pressure places much of Connecticut on the northwest side of the track. This is a very favorable place for moderate-to-heavy snow. If this whole process occurs say even 2-3 hours faster, then much of Connecticut could be looking at a significant snow storm. It will be very close!

The NAM forecast model shows a potent area of frontogenesis developed and progressing across eastern CT by early afternoon. This will be associated with a band of very heavy snowfall. Snowfall rates could approach 2-3'' per hour under this band. There is even the potential for thunder and lightning as the upward vertical motion will be very intense:

Below is an atmospheric profile for Willimantic, CT. The two red lines running across the image labeled -12 and -18 represent the dendritic snow growth layer. Snow growth is maximized between this temperature range. The color shadings represent atmospheric rising or sinking motion with rising motion denoted by negative values and sinking motion denoted by positive values. Notice the very low negative values penetrating the snow growth zone? This is indicative of very intense lift, however, one thing to note is the dendritic snow growth zone is relatively high (up around 600-550mb). Lift will have to be exceptionally vigorous to transport enough moisture into the snow growth zone to really maximize snowfall production and rates. The red box shows exceptional lift but well below the dendritic snow growth zone. 

While surface temperatures may be warmer than 32°F (perhaps around 34 or so), the depth of this warm layer should be very shallow and given the degree of lift and rapidly cooling lower-levels of the atmosphere, rain should have no problem changing to snow. This will result in snow being wet and very heavy. With the for several-plus inches in spots, scattered-to-numerous power outages would become likely and could persist for a few days. Winds will also become rather strong out of the north-northeast at 15-25 mph with gusts 30-35. This would result in very poor visibility. 

So let's sum this up into what we can expect:

• Rain for everyone Friday night into Saturday morning. 
• The northwest and northeast hills begin to see rain rapidly transition to snow mid-to-late morning with the remainder of the state (especially northern Connecticut) transitioning by late morning and early afternoon. 
• A band of very heavy snow quickly materializes across northern and northeastern Connecticut with snowfall rates 2-3'' per hour possible.
• Thunder and lightning possible. 
• Winds increase out of the north-northeast 15-25 mph with gusts 30-35 by afternoon. 
• Scattered-to-numerous power outages likely.
• Snow winds down by late afternoon. 

Below, is my updated snowfall map. There were some significant adjustments from my previous forecast as the band of heaviest snow will traverse eastern Connecticut where I had western Connecticut earlier.

Significant Weather System to Close Out the Month?...But It Will Be Rain NOT Snow

Signals for a potential storm system to impact the Northeast to close out the month of November have been in place the past several days and as we move closer to the end of the month these signals remain in place with confidence growing. The signals have been the following;

1) A sharp rise in the Pacific-North American (PNA) teleconnection pattern (PNA becoming more positive)

2) A rapid drop in both the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) (AO and NAO becoming less positive)

Without going in great scientific depth regarding these teleconnection's and oscillation's when the PNA becomes increasingly positive, this typically corresponds with a ridge across the western United States with a trough either across the eastern United States. When the AO/NAO become increasingly negative, this typically correlates to a trough across the eastern United States. However, it needs to be noted and clearly stated that there is more to whether or not these are positive or negative, the strength, placement of the height anomalies associated with each, and structure of the anomalies play a significant role in connection to the pattern across the United States.

Moving into the final week of November and into the first of December, forecast models indicate a sharp rise in the PNA as shown below:

At the same time, forecast models indicate a sharp drop in the AO and NAO as shown below:

How can we visualize what this means on the weather pattern? I am going to use the GFS forecast model and the 500mb height anomalies to provide a visualization. Here we see a ridge (orange/red shadings) across the western United States with a trough (darker blue shadings) across the eastern United States: 

So the question becomes, for New England (since my blog typically focuses on New England) why aren't we looking at the chance for snow? Well snow can't be ruled out, especially in the higher elevations of northern New England, but unfortunately snow lovers, this just is not a look for snow. 

Perhaps the biggest reason overall is there is just lack of sufficient cold out ahead of the developing weather system. Looking at around November 27 (a few days before we can expect a storm to materialize) and looking at 850mb temperature anomalies across Canada there just isn't much in the way of cold. Now...if this were say January or February, 850mb temperature anomalies might not be sufficient to look at here. Given those months are getting into the climatological coldest part of the season, even "anomalous warmth" at this level could still be composed of an air mass more than sufficient for snow, however, we are just entering December so above-average temperatures just won't cut it:

Another big factor is where does cyclogenesis take place? Note: It is never a good idea to judge this based on one forecast model, however, for illustrative purposes I will use the same graphic that was posted above the 850 temperature anomaly graphic. While using that for illustrative purposes, analysis of other long-range forecast models plus model ensembles was conducted to generate a forecast:

Based on the analysis, the best likelihood for cyclogenesis would be right around the base of center of the black line indicating trough. This would promote low pressure development well West of New England which favors warmth ahead of the system. This is also shown by the positive height anomalies (red shadings) into New England. 

At this juncture, the period between November 30 - December 2 is likely to see a significant weather system develop. This would include heavy rainfall, gusty winds, and perhaps potential for thunderstorms. Again, some snow can't be ruled out across the higher elevations of northern New England. 

So what happens behind this...well I want to again post the PNA and AO/NAO charts from above to highlight the look just to the right of what I have circled:

Notice how there is "chaos" going on. Does the AO/NO head well negative, does it trend more positive? Does the PNA quickly become less positive? This chaos indicates uncertainty with how the weather pattern will evolve post-storm. While we may face a decent blast of cold air behind this storm, unfortunately, the pattern does not look very supportive to maintain this cold and we may quickly go back into a pattern which favors average-to-above-average temperatures. Those hoping for snow moving into December...good luck. 

Friday, October 30, 2020 Snowfall Forecast Update

Heavy rainfall currently across Connecticut giving us some much needed rain and putting a dent in the drought. As we move through Thursday night into Friday morning, much colder air is going to filter into the state. This combined with some pretty intense upward vertical motion will result in rain transitioning over to a period of moderate-to-heavy snow across much of the state. 

By early Friday morning, computer forecast models indicate temperatures above 5,000 feet above the ground dropping to as low as -2C to -3C with temperatures down at the surface not much above freezing:

Forecast models also indicate a band of high vertical velocities traversing the state. This is indicative of rather strong upward vertical motion...favorable for the development of precipitation (in this case snow):

Forecast models also indicate we'll see some strong upward vertical motion into the dendritic snow growth zone:

Given the signals for some rather strong upward vertical motion across the state combined with cold temperatures in the lower-levels of the atmosphere and strong lift into the dendritic snow growth zone, the signals are there for what should be a 3-5 hour window of moderate-to-heavy snow across much of the state...even down to the shoreline! Given this signal, snowfall totals have been increased slightly from Thursday's forecast:

Potential Impacts:

• Rain transitions to snow across the northwest and northeast hills between between 12:00 AM and 3:00 AM Friday morning. The remainder of the state will see rain transition to snow between 4:00 AM and 7:00 AM (perhaps as late as 8:00 AM along the immediate shoreline) 
• Expect a slow morning rush hour commute due to slick roads. Travel should not be significantly impacted, however, give yourself extra time for travel. 
• Breezy during the morning with winds subsiding through the day, however, it will be cold with temperatures only into the 30's with lower 40's along the shoreline. 
• Very cold Friday night with temperatures into the teens and 20's so slick spots will develop. 

Significant Coastal Storm to Bring Rain, Gusty Winds, and...First Snows???

The potential exists for a rather significant coastal storm to impact southern New England heading into the end of the week. Much needed rainfall, gusty winds, and even some snow is possible. There are several pieces involved here and how they interact will determine the strength of the storm and the impacts we see. Let's dig into the science!

#1) Hurricane Zeta which is set to emerge into the southern Gulf of Mexico Tuesday morning. After making landfall late Wednesday in the vicinity of the Louisiana - Mississippi border, the remnants of Zeta  will rapidly progress through the Tennessee Valley, through the mid-Atlantic, and eventually pass just south of Long Island. 

#2) A closed area of low pressure across the southern Plains will eventually become an open wave as it pushes east through the Tennessee Valley. This system is a main player in the rapid speed of Zeta after it makes landfall. 

#3) A shortwave trough across southeast Canada amplifies (strengthens) as it approaches New England. 

#4) A strong area of surface high pressure slides into southeast Canada behind the shortwave trough. 

These features are all shown below using the 12z run of the  GFS from Monday looking ahead to Thursday evening:

The position of the high pressure and potential track of the low pressure (remnants of Zeta) are very critical as there will be quite a bit of cold air in Canada. In fact, 850mb temperature anomalies (temperatures about 5,000 feet off the ground) may be -10 to -20F below-average! 

The clockwise flow around the high pressure could support the transport of anomalously early season cold into portions of southern New England. Meanwhile, the counterclockwise flow around the low pressure could act to reinforce the transport of this low-level cold. 

In addition, an extremely strong jet streak will be placed north of the region with a jet streak in excess of 190 knots!!! The region will also be in a highly favorable quadrant of the jet stream which will favor vigorous upward motion and favor for low pressure strengthening off the coast. These factors also enhance the likelihood for much colder air to be drawn into the region. 

The verdict? Well, it's a bit too early for exact details and specifics, however, the signals and confidence continue to grow towards a significant coastal storm the day before Halloween. Potential impacts include; 

• Heavy rainfall (much needed!) 
• Strong and gusty winds 
• Potential for snow (especially for elevations above 1,000 feet)
• Accumulations are also possible above 1,000 feet. 
• Snow can't even be ruled out across elevations lower than 1,000 feet though the likelihood decreases quite a bit as you near the immediate coastline. 

Bottom line, this needs to be watched as many could see their first flakes of the season; including the potential for accumulating snowfall (with the highest likelihood above 1,000 feet). One final thing to mention...we are heading towards a full moon so costal flooding due to higher than usual high tides is a possibility. 

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.