Winter Storm Update

We are hours away from the onset of a winter weather event here in CT. This has been a bit of a forecast challenge this past week, however, for the most part the ideas of what may transpire have been rather consistent...it's just been a question of how they transpire.

Over the past day we've seen our weather forecast models actually trend towards the warmer side...not necessarily because of a farther north storm track, but with how the system strengthens. The more rapid strengthening and farther southwest helps to draw in warmer air from off the Atlantic. Some of our forecast models are even going to the degree of bringing in 40's and 50's into CT...but that's just not going to happen. 

There is currently a lot of dry air in place (dewpoints down into the teens in CT and single digits and even below zero across northern New England). 

As long as the surface winds are more northerly/northeasterly...cold air will continue to drain down the valley into CT. While the drier air may initially hurt when it comes to snow reaching the ground initially, the atmosphere is expected to saturate.

While we won't stay snow for long, it should rip heavy snow for a good 3-4 hours. Snowfall rates during this time will range from 1-3'' per hour. The window for the heaviest snow will be from 10:00 PM Saturday through 2:00 AM Sunday. Forecast models indicate some very intense upward vertical motion moving through:

After about 2:00 AM or so we will begin to see precipitation change. Along the immediate shoreline the changeover will happen earlier...closer to 11:00 PM. The shoreline will go from snow to a mixture of sleet/freezing rain and then over to rain. I don't think there will be significant icing issues along the shore, however, flooding is going to become a concern due to extremely heavy rainfall. A few thunderstorms may be possible too.

My biggest concern area for icing is right along the I-84 corridor. While forecast models do warm temperatures into the mid 30's (again some models much warmer) I am hesitant about this much warming. Establishing a snowpack of several inches could also favor keeping colder temperatures in place. Here is the difference between significant icing issues and not...about 2F. If surface temperatures are closer to 28...29...even 30 we will have MAJOR problems. If surface temperatures are 31...32...33...well we avoid major problems, but we will still have icing to deal with. 

Regardles, as the storm departs very strong cold air advection is going to occur and temperatures are going to drop very quickly. This will result in the likelihood for flash freezing (this is something that tends to get overplayed alot, but this is a situation where this might actually happen). 

Breakdown:

• Light snow/flurries begin between 4:00 PM and 7:00 PM this evening.
• Heaviest snow between 10:00 PM and 2:00 AM. Snowfall rates 1-3'' per hour
• Rain changes to sleet/freezing rain then to rain across the shoreline during the early overnight. 
• Heavy rain across the shoreline through late tomorrow morning leading to flooding concerns.
• Snow transitions over to sleet/freezing rain inland. A narrow zone of significant icing sets up. As much as 0.50'' to 0.75'' of ice possible. Can't rule out total close to 1'' in spots.
• Scattered to widespread power outages for the areas hit hardest by icing.
• Precipitation winds down late Sunday morning.
• Winds may gust 25-35 mph. 
• Temperatures drop VERY quickly through tomorrow afternoon allowing any liquid water to freeze. 

Snow/Ice map:

01/19/2019 to 01/20/2019 CT Winter Storm

We have been tracking the potential for a rather significant winter storm for several-plus days and as we get closer to the weekend this potential not only still exists but the potential is becoming more of a reality. Before I get into the posts I wanted to post a disclaimer. All images that will be posted (unless otherwise noted) will be from the GFS forecast model. I can't post images from the European model as I don't have permissions. Just because I'm posting GFS graphics does not mean I am leaning toward's that model...what I am posting is to just illustrate. 

There are three key features that will be involved; a thermal boundary, southern stream vorticity, and northern stream vorticity. An area of low pressure (perhaps multiple low pressure circulations) will develop along the thermal boundary and strengthen with aid from vigorous vorticity:

There are substantial differences between the GFS and European forecast models with regards to the evolution of the southern stream energy which will play a substantial role in the eventual outcome of this system. The GFS is much stronger or "amped" than the European forecast model. This yields the surface low pressure to actually track either over CT or perhaps slightly west. This solution would yield a snow, to ice, to rain scenario. The Euro not being as amped is slightly farther southeast with the track of the surface low and would result in a snow to potentially significant icing situation across CT. 

Forecast models indicate some very high precipitable water values (PWATS) pushing into the area and feeding into the storm. PWAT values just south of the area are forecasted to be well over 1''...that is suggestive of very juicy air. A very strong temperature gradient will also exist over southern New England which will help to enhance frontogenesis over the region and support vigorous lift. This is shown with the vigorous 700mb vertical velocity values. Very strong warm-air advection (WAA) will further assist in the lift process:

This grab from the 18z GFS is the bufkit overview for Windsor Locks, CT. This is showing exceptional lift overnight Saturday...despite the most intense lift well below the dendritic snowgrowth zone this still shows -20 to -30 units of omega within the zone. Snowfall rates will approach 2-4'' per hour at times during the height of the snowfall:

We will have to carefully watch the exact track of the surface low as the likelihood exists for snow to changeover to freezing rain, sleet, and potentially rain. While the GFS forecast model is not very aggressive with freezing rain here, the European model is. There will be a tremendous amount of cold air just north of us and if the surface wind direction is more northerly as opposed to southerly or easterly this cold air will drain down the CT River Valley. This would keep surface temperatures well below freezing while temperatures a few thousand feet off the ground warm above-freezing...this would result in freezing rain and the potential for significant freezing rain.

Below is what I am currently anticipating in terms of impacts for CT. 

Suppression Depression or Major Storm Threat?

If you're a snow lover living in southern New England you're probably not all that happy with how winter has started. We have had plentiful cold at times, however, we've also had our share of warmth with very little of the white stuff. As we head into the second weekend of January some opportunity exists to bring some excitement to snow lovers. Some work needs to be done to make this happen, however, speaking from a synoptic stand-point (which will be the premise of this post) this weekend will be extremely favorable for cyclogenesis (development and rapid strengthening of a low pressure system)...it's just a question of where the system tracks. This will be answered over the coming days. I will be using the 18z run of the GFS forecast model from January 8th (as this is the most current run available during the composition of this post) to illustrate the key features we will be focusing on. I will also discuss the latest GFS ensembles which will be used to yield what interest level we should have in this event.

Over the next few days the Pacific-North American (PNA) is expected to become more positive in nature as a ridge builds into the western United States. Let's also note the piece of energy to the east of the ridge axis:

The Arctic Oscillation is also expected to DRASTICALLY drop over the next few days. In the animation below notice the very high pressures which develop over the Arctic. This is in response to the major sudden stratospheric warming event which occurred and led to the split of the polar vortex:

This negative AO coupled with the positive PNA will try and dig a trough down into New England. As this occurs the piece of energy (highlighted in the first image) will be progressing across the country. The northern and southern branch jet streams will be phasing (interacting) over the northeast. This will provide sufficient energy and ingredients for a rather potent low pressure to develop:

The jet stream will also be rather strong and this will lead to a very complex (so to speak) atmosphere this weekend with alot of moving pieces. Forecast models have been consistent in some energy moving through the northern stream which 1) may interact with the southern stream energy and further enhance the strength of the storm and 2) may also suppress it south of New England. There is also a rather strong high pressure to the north which could suppress the system:

The latest ensemble run of the GFS, however, still shows decent potential for a snow event here in southern New England. There is certainly a quite a bit of details to iron out through the week, but for snow lovers there is some hope!