Significant Late Season Snowstorm Likely

News of a potential snow storm has begun to spread this afternoon. The time frame would be Tuesday evening through Wednesday evening. As this period is anywhere from 84-100 hours away, lets state what we know so far.

Upper Level Set Up

Remarkably good agreement from the 3 major global ensemble models (in a winter where there has been minimal compatibility amongst the models). Strong upper level wave will develop downstream of a building western US ridge. The trough in the East will dig deep (black circle), becoming negatively tilted thanks to some weak ridging in the Northern Atlantic (purple circle).

A strong and rapidly intensifying surface low will develop somewhere near the 40/70 benchmark just south of Cape Cod. Strong vorticity advection and favorable positioning will allow this storm to bomb out, dropping several millibars in pressure level over a short period.

So lets jump to the facts at this point as I need to leave shortly to host trivia!

What We Know

- A strong storm will develop close to New England on Tuesday night.
- Exact track uncertain but good agreement on a pass close enough to New England for a significant impact.
-Plenty of cold air in place so precipitation will be all snow.
- Rapid intensification may create blizzard conditions, especially for Cape Cod and southeastern Massachusetts.
-Likely a sharp snow total gradient, with highest totals to the south.
- Extreme drop in pressure will create impressive winds. Sustained winds of 30-40 mph with higher gusts in Boston. 40-55 mph winds likely on Cape Cod and southeastern MA.
- As always, the exact track is key. Such a strong, wrapped up storm will have more narrow band of heavy precipitation. The closer or further the storm tracks to us, the chances of higher snow totals increases or decreases.
- There is some silver lining here. After the storm departs, we can expect a nice warm up for next weekend. Temps on Friday and Saturday will be near 60, eating up any snow that falls.

Lots of details to iron out. Stay tuned!

A Move to the Middle. Monday Storm Picture Becoming More Clear

As expected, the operational models are trending further south with the precipitation shield for Monday's event. You'll remember that yesterday, I posted a discussion on how we can use ensemble models to predict an inevitable squeezing of the heaviest snowfall to the south of New England.

In that discussion, I noted the relatively high uncertainty on two main atmospheric features, the position of polar vortex's southern edge and the strength of the upper level storm. Less than 24 hours later, the ensemble average is coming into better agreement, with greater than 70% agreeing on a greater intrusion of the polar vortex, which results in cold and dry air squeezing the storm southward.

The image below represents a "relative measure of predictability" for the forecasted position of the PV on Sunday night. The darker orange shading, highlighted by the circle, implies that 70% of the individual ensemble members agree on the position of the PV.  70% is certainly not perfect, but its a high enough value at 48 hours out to place confidence in a southward trend.

Interestingly enough, we can still see a high uncertainty in the strength and placement of the upper level storm, which suggests low confidence in the actual surface storm track. This uncertainty is of more importance to our friends in the mid-Atlantic, as New England's fate in this storm has all but been decided.

Watch for forecasted snowfall totals around these parts to continue to fall tonight.