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.

Weather Insight: A Look Into How Ensemble Models Can Help Forecast Monday Storm

The upcoming storm set to affect New England on Monday is a good opportunity to show how ensemble forecasting can help us clarify a fuzzy situation. Ensembles are a collection of different forecast models, running with different initialized conditions. The average or mean solution of the various ensemble members gives us a better understanding of how the big weather picture is trending.

This past week, the atmosphere over the United States has undergone some rather dramatic changes. The best evidence of this change comes in torrential rains California is receiving after over a year of receiving nearly no rain. Some familiar trends have crept back into the picture as well, namely extremely cold air over the eastern portion of the country.

With such large scale changes, operational models (the ones that run 2 or 4 times a day based in current observations) have had a difficult time handling two large scale features of the atmosphere.
Lets look at an upper level map from the operational Euro. I have circled the two features of importance.

The red circle is the forecasted position of the polar vortex on Monday evening. The yellow circle is the forecasted position and strength of the upper level storm that is the driving force behind the expected storm on Monday. All well and good, but lets take a closer look at the Euro ensemble.

Notice a difference? It's so small that I would be impressed if you did!. The subtle difference is in the placement of the polar vortex. The ensemble suggests a position located further south (seen in the black circle, causing the upper level feature to flatten out. A flatter upper level will result in a suppressed precipitation sheild, located further to the south over Long Island (as displayed by the arrows).

So, considering the ensemble is an average of several different model runs, it is safe to say at this point that the storm is trending further south. As a point of comparison, lets look at the snow storm that affected the area last January 21/22. 

This storm was memorable for the sharp cut off in precipitation amounts, with nothing falling north of the Mass Pike. This storm is not a perfect comparison given that temps were much colder and the orientation of the storm set up was more north to south. Mondays event will be more west to east. Still, given the trend, I would anticipate a precipitation shield being suppressed to the south and more narrow, like the image below.

I hope this wasn't too technical. Just a little insight into one of the methods we can use to forecast a storm!

More Snow Tomorrow, Then Some Relief!

Is it spring yet? I am sure that's what most of us are thinking after our particularly snowy February. Officially speaking, Boston has received 20 inches of snow this month, nearly half of our average seasonal total of 43 inches! In between all of the snow, it has been cold, resulting in some large snow banks that have barely melted. All 17 days this month have had minimum temperatures below 32 degrees.

So should I talk about the mild air we are expecting later this week? Of course not! Before we get to that warm up, we have one more snow event to get through. This will be the 3rd snow event to effect us in less than a week, and all of these storms have been quite dynamic and difficult to forecast.

Tomorrow's storm is much weaker than the previous two, resembling more of an Alberta Clipper than a coastal nor'easter. Nevertheless, we are once again dealing with a potent piece of energy that will redevelop into a secondary low right over our heads. Our air remains fairly primed for moisture growth and as such, some moderate to heavy snow will fall tomorrow afternoon into tomorrow evening.

Quick redevelopment of a storm center is a pain to forecast!

The details:

- Snow starts from West to East sometime between 11am-1pm
- Evening commute will feel the worst impact, especially to the north and west of Boston. Snowfall rates of 1-2 inch per hour are likely in some spots. 
-Southeast flow at the surface will change the snow to rain/mix on the Cape and in the New Bedford area.
- Areas to the north and west of 95 will experience the heaviest snow, resulting in totals around 4-6 inches
-Boston may mix, especially in the early evening. Expecting 2-4 in the city.
- Quick hitter. All precipitation will be over by 10pm.

This is what the NWS is expecting for total snowfall. Notice the moderate snowfall totals in Middlesex county. As we know from this past weekend, these redeveloping events can be quite fickle, which means there remains a decent chance that these totals change again before the storm starts.

It's a One-Two Punch from Winter

Not even 24 hours after our last winter storm, we are already talking about another one. This system will intensify rapidly in a similar manner to yesterdays storm, but in a much more "traditional" position for a nor'easter.

Boston and much of the south eastern portion of Massachusetts were spared of a significant snowfall yesterday. But mother nature doesn't want us to be left out of the fun too. This storm will be close to an exclusive snow event for the exact same areas that got rain yesterday. Call it karma I guess.

Deja vu? Similar storm set up to yesterday,but with a much better position for NE snow

The details:

-Snow starts in Boston between 4-7 PM, earlier along the south shore and Cape Cod.

-Snow will once again become heavy overnight on Saturday into Sunday morning. Winds really pick up out of the North as the storm right off the coast of Cape Cod. The Cape and the Islands will experience a full fledged blizzard, with heavy snow and winds gusting over 40 mph. Areas of the North Shore and Cape Ann likely to experience blizzard conditions as well.

-No participation type questions with this track. Models quickly came into agreement with an offshore track, favoring an all snow event.

- Tight precipitation gradient, with at least 0.5 inches of liquid for Boston and over an inch of liquid for the Cape but much less west of 495. With this storm remaining all snow, that would result in at least 5 inches for Boston and 10 for the Cape.

-Really going to have to keep an eye on how quickly this storm bombs out. If it happens as some models suggest, some extraordinary snowfall amounts could pile up along the coast.

The latest snowfall forecast from the NWS. I think some of the totals near Plymouth and out on Cape Cod are underdone. I think some of these spots, specifically on the Cape, will see over a foot of snow.

Powerful Storm Headed This Way

A major winter storm will impact all of New England by early tomorrow morning. This storm is already responsible for bringing historic icing to a large area of the southeast, extending from Texas to North Carolina. Before all is said and done, much of the large metro areas including Washington DC, Baltimore, Philadelphia and New York City will see 6-12 inches of snow. This storm may ultimately affect more than 1/3 of the US population.

Water vapor imagery showing massive size of this storm

The forecast for New England is not as clear cut. Once the system reaches the ocean, it will intensify rapidly, resulting in a sharp pressure level drop that we weather nerds call bombogenesis. Problem is, the track of this storm takes it over Cape Cod, which means a significant marine influence will cause precipitation type issues from the onset.

973 mb low is on par with a Cat 2 Hurricane!

This storm presents one of the most difficult situations that a forecaster can face. Small mesoscale factors, often missed by models, will have a tremendous impact on snowfall totals. Having said that, lets get to the details.

Precipitation Type

Massive shield of moisture will spread from south to north early tomorrow morning. Everyone will begin as snow starting between 6-8AM. This thing is juicy when it comes in so expect snow to start piling up quickly.

8AM heavy snow for most

After a few hours of heavy snow, we start to watch the wind direction and those surface temps. With an east-northeast wind, warm marine air will intrude at the surface along the immediate coast by late morning. How far that warmer air moves inland is questionable. Several factors at play. Marginal ocean temp (37 degrees), cold air just above the surface, precipitation intensity, all of these things will play a role in just how warm eastern Mass gets and how quickly.

I feel the changeover along the coast, including downtown Boston, will happen rather quickly, limiting snowfall totals. I believe that cold air will hang on for several more hours right around RT128 before a warm up later on in the afternoon. Areas along I495 and westward will remain all snow for much of this storm.

It is very plausible that metro Boston changes to a period of sleet and ice pellets in the early afternoon. One thing to consider is something we call dynamic cooling. Essentially, if the precipitation is falling hard enough, cold air from above can be dragged downwards into the shallow layer of warm air near the surface. Should this happen, surface temps may be just cold enough in some areas around Boston to avoid a quick changeover.

Finally I should mention the potential for flooding exists over southeastern MA. Upwards of 1.5 inches of moisture could fall on top of a decent sized snowpack. Huge puddles of standing water can create some localized flooding issues.
 

Winds

Winds will become quite strong once this storm bombs out. Sustained winds of 30-40 mph with gusts over 50 mph will develop along the coast. Wind direction will be crucial as well. Even a slight change in orientation to a more northerly direction will affect just where the coastal warm front sets up.


Wild Card!

As if this storm wasn't complicated enough, there remains a wild card towards the end of the storm. By early Friday morning, the center of the storm will be sitting off the Massachusetts coast, moving north east and intensifying. Several models are keying in on an impressive swath of heavy snow redeveloping over the area. Winds will sharply shift to the north, flooding the cold air back in. With wind gusts exceeding 35 mph and very heavy snow, we could see blizzard conditions to end the storm! Yes, we would get a mini blizzard as a finale. Its really something I have not seen before. Usually these back lash precipitation shields dry out but this one needs to be watched as it has the chance to really impact final snowfall numbers, especially along the coast.

Above you'll see the NWS Boston's official snowfall forecast. I think it looks good.  Big questions still remain along the coastal plane and about 10-15 miles inland. Again, some of this could fall on the back end of the storm.

This forecast is extremely volatile and things can change quickly. Hi-resolution models should start giving more answers tonight.

Another Nor'easter This Week

A massive energy change is currently going on in the Pacific, breaking down the immense upper level ridge that has been sitting over Alaska for several weeks. This breakdown will have big impacts on the weather over the Continental US this week.

A piece of energy originating over the west coast of British Columbia will dive south eastward towards the Gulf over the next 72 hours. The upper level feature is expected to become tilted, but not phase with the northern jet stream, which may limit the intensity somewhat. In response to the upper level dynamics, a strong surface wave will form of the SE coast on Wednesday and will move up the East Coast, bringing wintry weather from Atlanta all the way to Maine.

Some initial thoughts at 4 days out:

- Storm track far from certain. Ensembles are showing a westward track trend which would likely mean heavy wet snow for some and a mixed bag for others.

- Precipitation type will be a big question. Cold air won't be as readily available for this storm and with a trend towards a coastal hugger, areas in eastern MA could easily changeover at some point in the storm.

- Timing looks to be late Thursday morning into Friday morning. Will have to wait at least 48 hours to get an idea on storm track before posting potential snowfall numbers. 6+ inches very likely for areas outside of 495 with the coastal areas having the most questions at the moment.

In any event, prepare for some disruption come Thursday.