In yesterday's post it was discussed that we were watching two pieces of shortwave energy which would phase (combine) across the southeastern United States with the response being the development of a surface low pressure just off the southeastern United States. We have seen this happen as evident by the low pressure southeast of South Carolina.
Now what we are watching (and this was one of the many questions faced with this potential as the week has progressed) is whether or not there would be any interaction with northern stream energy rotating around a developing shortwave trough. Over the week, various computer forecast models differed in how the developing system would interact with this northern stream shortwave trough. Some computer forecast models had the two interact, with the shortwave trough being on the stronger side, thus acting as a "kicker". This allowed for some computer forecast models to show solutions which were more east (less snows for us). Some computer forecast models kept that northern stream shortwave trough a bit weaker, and keeping shortwave energy associated with the storm system much stronger. The result was an interaction (phase) of the northern stream shortwave trough and the storm system which gave solutions of further explosive development of the system which was shown as bombogenesis. It has become apparent over the past day that the later scenario is more likely to verify and this system is going to undergo bombogenesis as it passes to the southeast of New England:
As the interaction occurs the surface low is going to deepen incredibly rapidly and the result will be for blossoming precipitation around the storms center and very heavy precipitation will begin to transform on the northeastern quadrant of the low. Computer forecast models all agree on the surface low deepening down into the 950mb range by early Thursday afternoon southeast of Rhode Island. This is a sign of an incredibly strong coastal storm:
So what can we expect here in CT? This is going to be an extremely challenging forecast and in fact it will be very difficult for any one forecast to completely nail the snowfall amounts. There will be so many mesoscale phenomena happening and occurring and these details just aren't easy forecasted by computer models because they just don't have the resolution to do so. There will be areas which see far more snow than other and there will be areas who see far less...that is always the nature of the beast with the systems. The best thing to do is just explain everything.
1. With the storm becoming so strong (the central pressure will rival that of a category 2 to a category 3 hurricane!) this will have a big impact on the precipitation shield. The further you move away from the center of the storm, precipitation may end up being more band-like in nature as opposed to being a widespread shield of precipitation. For comparison think of bands of precipitation you would see with a tropical storm or a hurricane. If you have precipitation which is more band-like, you'll see two things happen here; 1) Under the bands you'll have vigorous upward vertical motion which will lead to moderate to heavy snows as sufficient moisture is being lifted into the snowgrowth zone along with sufficient upward vertical motion. Snowfall rates here could easily exceed 1'' per hour and may even approach 2'' per hour. However, in between the bands you'll have vigorous downward vertical motion. This is sinking air and this results in subsidence which results in vastly reduced snowfall rates and also poor snowgrowth given lack of stronger moisture and lift being transported into the snowgrowth zone. This alone will make a snowfall forecast very difficult because this could lead to large snowfall gradients across a very small distance.
2) Where the heaviest banding of snow sets up. This is always a tough forecast in any winter storm. When assessing this potential one thing to watch for is where the 700mb low begins to close off and where it tracks. The strongest upward vertical motion occurs on the northwest side of these closed lows (925mb, 850mb, 700mb) but the distance of how far away from the closed circulation can vary. When assessing 700mb vertical velocities (great tool for assessing where the strongest upward vertical motion will be located) there are some differences between the computer forecast models as to where the best upward vertical motion will set up and thus where the heaviest banding will setup. The NAM is a bit west of most models while the GFS is a bit more east and is in line with other models:
Not only is determining where the best banding sets up important for determining where the highest snowfall totals will occur but it's also important for determining where you'll have subsidence sets up. Think of it like this...what goes up, must go down. If you have a zone of rapidly rising air you must have a zone of rapidly descending air. In this later zone this leads to vastly reduced snowfall intensities, snowfall rates, and snowgrowth. Typically this zone will occur on the northwestern side of where the heaviest banding sets up and there is a strong possibility this happens over CT. We can also use bufkit profiles to assess this. NAM and GFS bufkit soundings over Windsor Locks, CT hint at a period of subsidence which would lead to a period of reduced snowfall rates and intensities before the subsidence moves out and a return to moderate to heavy snow occurs:
The NAM and GFS also show an incredibly amount of 800mb-600mb vertically averaged frontogenesis moving across the state with the NAM a bit more west than the GFS. This would be indicative of incredibly heavy snowfall with snowfall rates likely approaching 4-5'' per hour. For time purposes will just show the NAM. The GFS is very similar except just a bit east:
In addition to heavy snows we will also be watching the potential for strong to damaging winds which could not only lead to the potential for power outages and downed trees but could lead to white out and near blizzard conditions at times. Winds could gust upwards of 40-50 mph during the storm and also after the storm.
What to expect:
- Snow begins to breakout between 1:00 AM and 4:00 AM tomorrow morning.
- Snow begins to increase in intensity through the morning hours and will range between moderate and heavy at times.
- The snow forecast will be very difficult given what was just explained above. If precipitation is more bandy this will have a substantial impact on the snowfall totals and accumulations which would be rather difficult to portray on a map. Figuring out where the heaviest banding sets up, and if it even does so in CT will also impact totals, both for higher amounts and lower amounts. If the heaviest banding sets up in CT anyone under the band will experience substantially heavy snowfall rates under the band with rates approaching 4-5'' per hour at times. This combined with strong to damaging winds would yield whiteout to blizzard conditions at times.
- Snow will begin to taper off mid to late afternoon, however, strong winds persist well into the overnight hours.
- Given such uncertainties, unfortunately, this leads to a rather large range in my snowfall forecast:
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