Friday, February 13, 2015

A different blizzard track

Looking at the National Weather Service's hazards map for today, it's clear where the story is:

Another powerful snowstorm forecast to hit the northeast.  Let's run through a quick overview of what the current model forecasts are doing with this storm.

Here's the GFS sea-level pressure (black contours) and relative humidity at the surface (colors; not as important here...) starting with a 24 hour forecast for tomorrow morning at 12Z.  I get these graphics from Weather Underground's Wundermap utility, which not only lets you zoom in on various areas but also is the only place to get full-resolution ECMWF images for free...

We can see that there's a 1000mb low over northeastern Lake Huron and central Ontario.  Note that this is very different from the first blizzard at the end of January, which moved up the coast from the south.  This sort of storm that comes out of the west-northwest is commonly referred to as a "clipper" system, as they tend to be rather fast moving.

Twelve hours later on Saturday evening, the low is forecast to basically be centered over New Jersey.
It hasn't deepened too much, but the storm is about to move out over the northern Atlantic.  Here's a map of the current sea-surface temperatures off the east coast from NOAA/NESDIS:
You can see right near the coast the temperatures are around 4-5 Celsius (39-41 Fahrenheit) but they rapidly warm into the 15-20 Celsius (59-68 Fahrenheit) range the further you get from the coast.  There's a sharp gradient of temperature there, and that's exactly what a storm like this can feed on to grow.  Storms form as the atmosphere's response to temperature and pressure imbalances.  The atmosphere wants to try and "smooth out" the temperature as much as possible.  Therefore, when a developing storm meets an region like this (a "baroclinic zone" if we want to have a technical term), it tends to deepen quickly, and that's exactly what we see.  Here's the forecast for Sunday morning at 12 Z:
An explosion of contours!  The low has deepend to below 976 mb as it feed on the baroclinic zone off the coast.  In just 12 hours the low has deepened nearly 24 mb---clearly meeting the 24mb in 24 hours deepening rate we use for "bombogenesis".  This deepening tightens the pressure gradients and will increase the winds.  Here's the GFS wind forecast for Sunday afternoon:
Strong, 50 kt. winds forecast over Cape Cod and along the new England coast.   The winds along New England and the Mid-Atlantic states at this point have strong northerly-northwesterly components (offshore winds) and they will be bringing in much colder air at the low levels.  The flow above, however, is wrapping around the low from the east over Nova Scotia and into New England, bringing moister, oceanic air over this low-level cold air and setting the stage for snow.  Here's the GFS precipitation forecast for Sunday afternoon:
Heavy precipitation along the New England coast.  The ECMWF forecast has slightly less precipitation at this time, but the position of the low is very similar:

All in all, it will be another blizzard for the New England coast...

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