Storm Update

Here is where we stand:

A strong surface low is currently situated over Tennessee Valley. At the same time, an extremely strong and cold High pressure is situated over southern Quebec. This High pressure has sent 3 reinforcing shots of cold air into New England over the past 48 hours in the form of weak cold fronts. As such, very cold air is settled in all across New England.

 

As the system over the middle of the country moves East, a new surface low will develop off the New Jersey coast, as is typical of New England snow storms.  This new storm have an influx of Gulf and Atlantic moisture, providing plenty of fuel for precipitation. As the storm runs into the Arctic air mass over New England, snow will outbreak across the whole area sometime after 7PM this evening. 

This much we know. The large scale factors are in place for a decent sized snow storm. The finer details are all that is left to sort out. But those finer details can mean everything in a winter storm like this!

Main Issue: How much warm air intrusion is there?

We always here meteorologists say that the "track" of these storms is so important. That is especially true when we have such a large temperature gradient between surface land temps (in the teens/20's) and the ocean (45 degrees). The track of the storm will dictate the surface wind direction. Any wind from the east will use the warmer ocean and try to warm the lower layers of our atmosphere. Its the all important rain/snow line.

So how do we forecast this feature? Well, we really can't! This close to the storm, some of our trusted models won't be able to pick up such a finer atmospheric detail. We can certainly get some guidance by looking at "thermal profiles". Thermal profiles are basically a snapshot of the vertical atmosphere at a particular time and place.

Lets walk through this step by step:

7PM

Light snow has overtaken much of the area. Everywhere will be cold enough to start as snow.

1AM

 

Storm has redeveloped of NJ coast and is moving towards New England. Moderate to heavy snow falling everywhere. These are the EURO projected snowfall totals by 1AM. I think these might be a bit overdone, but a general 2-4 inches will have fallen by this point. Remember, the ground is extremely cold so anything that falls will stick right away. 

4AM-7AM

This will be the deciding time frame for this storm. Up until now, pretty much everyone has stayed snow, except for areas along Cape Cod and the extreme coast. This will be the time frame where we watch both the surface temps and the 850mb (5000 feet up). If the storm tracks further off shore as the euro images above suggest, there will be less of a warm surface layer intrusion. However, some short term models this afternoon are still suggesting a storm path closer to Boston, which would encourage more of an easterly wind direction and warming of the the surface layer.

 

Notice the track of the storm goes right over Cape Cod. This image is projected for 7AM tomorrow morning. Under normal circumstances, this track changes everyone inside of 128 to rain. 

The 850mb temps for the same time show temps above freezing all the way to the NH border. Now, this would of course mean rain for Boston but more of a sleet situation for somewhere like Bedford, MA

 Aforementioned thermal profile for Bedford, MA at 7AM shows surface temps at or near 32, while 850 temps have warmed to above 32. This would be a sleet situation.

So lets just get to the nitty gritty. I've already made this blog post too long so I hope you've made it this far. What do I think is going to happen?

I feel that the cold air in place over all of SNE will be hard to "push" out, even by a warmer storm track. Not to mention, we will continue to be fed cold air at upper levels for the majority of the storm, up until the surface low starts tracking closer to the area. As such, I think most of us stay snow for 80% of the storm and more importantly for 80% of the heaviest precipitation.

Eventually, warm air will win out at the surface, sometime between (4-7AM) but by then, the heaviest precipitation has already fallen. It will amount to just coating of sleet or rain on top of the already fallen snow.

So here is my snowfall map!

Timing will be from 7PM tonight to about 10AM tomorrow morning. Its all gone by tomorrow afternoon. Again, some changeover is likely after 7AM but I feel most of the snow has fallen by this point. Updates on Twitter and Facebook thoughout the day