Significant Coastal Storm Possible End of the Week

Computer forecast models are in very strong agreement for a substantial storm system to impact Connecticut at the end of this week. We are looking at the likelihood for heavy rains, strong to damaging wind gusts (ESPECIALLY along the coast), the potential for significant coastal flooding, and there is even the possibility of snow as well. What is leading to the potential for such a potent event? Substantial changes are occurring within the atmosphere and these changes will be tied to a big sudden stratospheric warming event (SSWE) which just occurred. First off, what is a sudden stratospheric warming event? A sudden stratospheric warming event is defined by a substantial increase in temperatures in the lower stratosphere which can be up to 50°C in just a couple of days! These events can play substantial roles in the evolution of the weather patterns down in the troposphere. 

Using the period from February 18th to February 24th, we see above-average temperature anomalies at 10mb (~100,000 ft above the ground!) along with above-average height anomalies (indicating ridging):

This event is rather significant in that it will lead to something we have not seen since late November...the negative phase of the North Atlantic Oscillation (NAO). For us here in New England, the negative phase of the NAO during the winter and early spring months can be a dominant player in our weather. When the NAO goes from the positive to negative phase it can coincide with shots of very cold weather and it can also be associated with stormy weather, particularly an increased chance for snow. In the end, however, there is much more to than whether the NAO is just positive or negative. The structure of the NAO is extremely important as well but we will not get into that here. For the past few weeks, long-range forecast models have been very aggressive and consistent with the negative phase of the NAO developing. In fact, forecasts are for the NAO to dip to -2 standard deviations below-average:

The negative phase of the NAO is correlated with above-average height anomalies (ridging) over Greenland. We can see this by looking at the forecast 500mb height anomalies for Thursday morning. The red's and purple indicate above-average heights and ridging. The purple shadings indicate much above-average height anomalies which is why the NAO is forecasted to become so negative:

Since about mid-February we have also been in a negative phase of the Pacific-North American teleconnection pattern (PNA). This, along with a pretty positive NAO were big players in the record warmth we saw last week. The PNA is expected to remain fairly negative through the week:

A negative phase of the PNA is typically associated with a trough across the western United States with ridging into the eastern United States. This would usually correlate to below-average temperatures in the west and above-average temperatures in the east. Given the extreme negative development of the NAO, however, this will really help to suppress any ridging come Friday and establish a trough across the northeastern United States on Friday which will be enhanced by ridging across the central portion of the country:

A combination of a negative NAO and negative NAO holds a fairly high correlation to a rather active storm track and has even been associated with some rather potent storms and that looks to be the case once again come the end of the week. So now that we sort of took a quick look into how the evolution of the atmosphere over the past week will lead up to the potential of this event let's looks at how things will evolve this week. 

Currently there is shortwave energy entering the Pacific Northwest. This shortwave energy will continue digging into the western United States and phase with the sub-tropical jet stream. As it does so, the energy will continue progressing northeast through the central Plains before phasing with northern stream energy on Thursday:

The result will be rapid cyclogenesis (strengthening) of a surface low across the Ohio Valley during the day on Thursday:

As this system continues to rapidly strengthen we will see low pressures develop at 925mb (~2500 ft off the ground), 850mb (~5000 ft off the ground), 700mb (~10000 ft off the ground), and 500mb (~18000 ft off the ground). The system is also expected to become vertically stacked rather quickly. A system is vertically stacked when these series of low pressures throughout the troposphere are vertically aligned. This is when a storm system is at it's strongest. In the below image, come Thursday evening the placement of all these low pressure centers is roughly over NE Ohio which indicates the vertically stacked nature of the system:

During this process, a secondary area of low pressure will begin to develop just south of New England as the main energy begins to shift towards the southeast from the Ohio Valley. Typically when we see storms track to our south this time of year we have to keep an eye on the possibility for snows. At this time it appears there will be just too much of an easterly component to the winds and not much of a cold air mass in place ahead of the system for a major snow event. The easterly component to the winds will advect in warmer air in the lowest few thousand feet of the troposphere from the Atlantic Ocean. This isn't to say, however, we won't see snow with this system. But this aspect of the forecast is a bit too complex now. As the system strengthens and the winds aloft begin to strengthen a process called dynamic cooling will occur. Dynamic cooling occurs when very rapid upward vertical motion occurs along with heavy precipitation falling. This will work to cool the atmosphere about several thousand feet up. At this juncture, it does not appear snow will be a big threat but snow could be possible within the hills and we could see rain perhaps end as snow but this potential will continue to be tracked as the week progresses. 

What we will focus on right now is the closed off circulation at 500mb as this will add some complex challenges to the forecast. When systems close off at 500mb soon after systems will occlude and this process begins to shut off the influx of warmer and moist air into the system which is an ingredient necessary for both strengthening and precipitation production. Forecast models indicate that the system will close off at 500mb well west of Connecticut. This could mean less in the way of heavier precipitation here but this is something to watch as the week progresses. 

Regardless, there is the potential for a quite a bit of rainfall as forecast models suggest a very anomalous low-level jet on the order of -3 to -4 standard deviations indicating a rather strong easterly component to the winds. This would supply a great deal of moisture into the state:

At this juncture this is what we are tracking for the state:

• Potential for heavy rainfall (but again how much rain we see may be tied to the evolution of the 500mb low and whether it occludes too early and shuts off inflow of warm/moist air into the system.
• Potential for strong to damaging wind gusts, especially along the immediate shoreline. Very strong winds aloft and a very strong pressure gradient will yield the possibility for strong to damaging wind gusts statewide, however, the biggest threat will be along the shoreline. 
• Coastal flooding could be a fairly big concern. A prolonged period of easterly/northeasterly winds coincident with astronomical high tides and strong winds could make for a moderate-impact coastal flooding event, especially along the southwestern shoreline. 

As the week progresses details will begin to become much more clear but for now the idea of a rather significant storm is certainly in the cards. 

Updated Snowfall Forecast 02/17/2018 into 02/18/2018

No big changes to yesterday's forecast, however, forecast models have become a bit more aggressive with total precipitation accumulation across the state thus have decided to increase snowfall totals for northwestern CT. There are some indications that the immediate shoreline will see warmer air towards the surface work in thanks to a developing southerly flow transporting warmer air off the ocean so this will work to keep snowfall totals down here. However, with this influx of warmer air, this will make for snow to be on the wet side across these areas and this could result in some pockets of power outages. While the atmosphere is rather dry right now, forecast models indicate we will saturate rather quickly and because of this the timing of the onset of the snow may be on the earlier side.

Forecast models continue to indicate a several hour period of very intense upward vertical motion moving over the state and there are strong indications that the maximum degree of frontogenesis will coincide with maximum upward vertical velocities. This coupled with excellent snowgrowth and strong upward vertical motion and moisture within the snowgrowth zone will result in a several hour window of moderate to heavy snowfall. This window should be from 11:00 PM this evening to 4:00 AM tomorrow morning. During this time snowfall rates will approach or perhaps even briefly exceed 1.5'' per hour. It will be during this period where the majority of the snowfall accumulations will occur. The latest run of the European forecast model drops upwards of 0.5'' of liquid equivalent over much of CT during a 6-hour window which suggests very heavy precipitation falling and this precipitation will be in the form of snow. Computer forecast soundings also show modest instability within the 700mb-500mb level which further indicates the likelihood for moderate to heavy snows.

What to expect:

• Snow begins to breakout between 7:00 PM and 9:00 PM. 
• Snow quickly begins to increase in intensity with the heaviest snows falling between 11:00 PM and 4:00 AM. During this time snowfall rates may approach or exceed 1.5'' per hour. 
• While snow will be on the fluffier and lighter side across inland Connecticut, closer to the shore were warmer air will work in, this will allow for snow to be wetter and heavier. This could result in some isolated pockets of power outages.
• There is the possibility for a few towns to receive upwards of 9-10'' of snow, however, due to the very fast movement of this storm those amounts are not expected to be widespread.
• Snow begins to wind down after 4:00 AM and will be out the state by 7:00 AM, however, some leftover flurries may remain across eastern CT. 
• Below is the updated snowfall map:

                      

Quick Hitting Storm to Bring Accumulating Snows 02/17/18-02/18/18

A quick hitting but potent storm system will bring plowable snow to the state of Connecticut overnight Saturday into Sunday morning. The snow is expected to begin mid-to-late evening, however, the snow should be cleared out by 7:00 AM Sunday morning.

A potent piece of shortwave energy propagating through the northern branch of the jet stream will interact with shortwave energy moving through the southern branch of the jet stream. Associated with the southern stream piece of shortwave energy will be an area of surface low pressure. As the shortwave associated with the northern stream continues to amplify (strengthening) and interact with the southern stream shortwave, the result will be for further strengthening of the low pressure system as it begins to pass south of Long Island. One question we still face is how far south of Long Island the system tracks as this will ultimately dictate if the heaviest of the snowfall pushes into Connecticut or not:

While this will be a rather fast moving system, there are numerous indications we will see a period of moderate to heavy snows work into the state in which snowfall rates could approach 1.5'' per hour. Forecast models indicate that a period of intense upward vertical motion will move over the state during the overnight hours on Saturday. It is during this period when we would see the heaviest snowfall intensities and snowfall rates:

Forecast soundings indicate we will have excellent snowgrowth potential with good upward vertical motion within the snowgrowth zone and plenty of moisture. This is how you get the production of dendrites which are the snowflakes which accumulate quickly. This bufkit profile sounding from the latest run of the NAM forecast model for Windsor Locks, CT shows excellent snowgrowth and decent omega within the snowgrowth zone:

While perhaps not a true MAUL (Moist Absolute Unstable Layer) some forecast soundings indicate some fairly unstable layers within the troposphere. This could act to further enhance upward vertical motion and snowfall rates:

Based on the above (along with several other factors) the thinking is for a several hour period of moderate to heavy snows to overspread the state during the overnight hours. With a marginal southerly component to the surface winds, this could lead to some mixing issues across the immediate shoreline which may reduce snowfall totals here. Across the northwestern part of the state totals may be reduced due to being just a bit too far northwest of the heaviest snows. Below is what to expect:

• Snow begins between 7:00 PM and 9:00 PM Saturday evening. 
• Snow quickly begins to increase in intensities with the heaviest snowfall rates occurring between 11:00 PM Saturday and 4:00 AM Sunday. 
• During the peak, snowfall rates could approach 1.5'' per hour. 
• Snowfall begins to wind down by 5:00 AM Sunday and will be cleared out by 7:00 AM Sunday. Some leftover flurries may remain across eastern CT.

                  

Wintry Mess on Wednesday, February 7th, 2018

Our active weather pattern looks to continue and we are tracking quite the mess for Wednesday as an approaching storm system looks to give us accumulating snows with much of the state even transitioning over to sleet, freezing rain, and rain. The northwest hills stand a better likelihood of remaining mainly all snow.  This is a very challenging setup with many uncertainties and those uncertainties will all be looked at in detail below:

Wednesday morning an area of low pressure will approach from the southwest. Initially, temperatures statewide will be cold enough to support all snow, however, with a southerly flow developing and becoming established in the lowest 5000' of the atmosphere this will eventually change snow over to sleet, freezing rain, and even rain for extreme southern CT. One of the biggest questions right now is how long does the cold air remain in place and another big question is do we lock in colder air at the surface which could enhance the potential for a prolonged period of freezing rain and this could be a major concern for some. 

At 500mb (about 18,000' off the ground) a shortwave trough within the northern stream will dig into the upper mid-west on Wednesday morning. This main shortwave energy will lag quite a bit behind the surface low pressure which indicates a "tilted" system. Strong shortwave energy within the southern stream will help with the generation of a surface low pressure across the Gulf coast states/Tennessee Valley:

 With a big disconnect between the main upper energy and surface low pressure, this will work to establish a southwesterly flow in the lower levels of the atmosphere which will promote the advection of warmer air in the lowest 5000' of the atmosphere. Initially the temperatures through the troposphere will be cold enough to support precipitation to begin as snow state wide, however, with warmer air working in aloft, this will promote snow to begin transitioning towards sleet, freezing rain, and even rain for extreme coastal CT:

One of the bigger questions and challenges at this time is what happens with the track of the surface low pressure as this will play a big impact on surface temperatures which will be key in how precipitation transitions. While there is a bit of uncertainty with regards to where the surface low will track, the general idea seems to be for the surface low to either track over CT or track just to the southeast of CT. If the track of the surface low is over CT this will allow for a southerly flow to become established across a good portion of the state which will allow for warmer air off Long Island to to work in at the surface. This would yield a transition from snow to sleet/freezing rain to plain rain for many. If the surface low track is more southeast of CT this would enhance the likelihood for colder air at the surface to remain in place and this would increase the odds for more in the way of sleet and freezing rain for many. One feature we need to watch extremely closely is an area of surface high pressure just to the north of Maine. This feature could work to keep colder locked in place across a good part of the state, especially if the surface low tracks just to the southeast of CT. This would be a strong signal for a prolonged period of freezing rain and this has to be watched very closely:

Another challenging aspect of this system is with regards to how much snow falls before we see snow begin to transition over to sleet, freezing rain, and rain. The potential exists for upwards of several inches of heavy, wet snow to fall across portions of the state (with the highest totals across the northwest hills where they will remain mainly all snow) before the transition process occurs. 12z GFS bufkit data for some locations across the state indicate the potential for a several hour thumping of heavy wet snow before any changeover occurs. The below soundings are from Waterbury, CT and Windham, CT. They show excellent snowgrowth with high values of omega and relative humidity within the snowgrowth zone. This is a strong signal for moderate to heavy snows:

Given the marginal temperatures in the lower portion of the atmosphere, this will make for snow to become very wet and heavy and will also yield snowfall ratios likely on the order of around 8:1 (maybe even slightly less). The snow will not be fluffy by any means and will stick to trees and utility wires. This will pose a major problem for any areas which see a prolonged period of freezing rain. 

What are we looking at right now?

• Snow begins to break out across the state between 7:00 AM and 9:00 AM Wednesday morning. 
• Snowfall rates quickly increase and could approach and exceed 1''/hour at times. Expect snow to be heavy and wet making it difficult to move. 
• A several hour period of moderate to heavy snows can be expected, especially across the northern portion of the state and northwest hills. The northwest hills will likely remain all snow. 
• By early afternoon we begin to see snow transition to sleet/freezing rain across much of the state. The immediate shoreline will see this changeover happen by late morning thus much less snow will be expected here. The shoreline is expected to transition over to all rain, however, a further south and east track of the surface low would mean more in the way of frozen precipitation. 
• There is the potential for a moderate-impact freezing rain event with the possibility for some areas to receive upwards of 0.25'' of freezing rain accretion. If this occurs on top of heavy, wet snow this could yield power outages and some tree damage. This potential needs to be watched EXTREMELY closely. 
• Everything begins to wind down early evening. 
• Travel on Wednesday is not advised and numerous school closings should be expected. Road conditions will deteriorate extremely quickly and travel will become especially dangerous with the onset of freezing rain. 
• Below is what I am expecting right now: