Major Winter Storm to Affect Portions of the Northeast…

But does that include us?  I have two words for you – latitude and elevation.  Both will be the deciding factors as to whether one sees rain, heavy wet snow with little accumulation, or heavy wet snow with tree snapping accumulations.

But before I get into that, just how common is snow on the first of April?  The answer is, not very.  However, it can occur.  The last memorable snow event in this area around April Fools Day was in 1997:

It’s not a big surprise that it’s occurring this year either, as La Nina winter’s are typically “book ends” – in other words, there’s a period of cold and snowy weather (which actually lasted much longer this year intially), followed by relatively traquil and warmer weather (February and the first part of March),  followed another (albeit rather annoying) period of cold and (perhaps) snow.   It’s also no surprise that there will be a big storm in the next 48-72 hours, as the NAO is going from strongly negative to strongly positive in a short period of time:

Whenever the NAO shifts rapidly from one phase to another, specifically when it heads from negative to positive, as shown by research by Archambault et al at SUNY Albany, there is a greater risk for a major east coast storm.

So, given those facts, what’s in store for the next 48 hours?  As I said before, latitude and elevation will play important roles here.  Let’s use the 12z European and the 18z GFS surface maps for illustration:

12z European for early Friday morning:

18z GFS for Friday morning:

Look at the surface temperature line (purple 32 degree line) on both models.  On the European, almost all of eastern PA is above freezing at the surface.  Looking at the GFS, all of southeastern PA is above freezing.  That’s not even taking account the winds at 5000 feet, which will be coming in off the ocean, which by the way, is in the 40’s – not at all favorable for coastal snow – gone are the days where the ocean was 32 degrees back in January. So latitude’s a given – the farther north, and northeast, to a point, one goes, the greater the risk for acumulating, if not heavy, snows.  That’s generally true no matter what calender month we’re in, but is of course is exponentially more important in Spring.  One could argue that this storm will “make it’s own cold” by deepening rapidly, and drawing in colder air from the north and northwest.  There are two problems with this theory however, one being the storm isn’t really forecast to deepen rapidly until it’s northeast of the area, and two, that the airmass to the north and northwest of the storm is not terribly cold, but modified, or stale, cold air.

Elevation will also be a huge factor.   This is something that the surface maps above can’t really show you, but it’s implied, given the rather warm surface temps seen on those maps above. Those below 500 feet will struggle to see any accumulation.  500 to 1500 feet should see moderate accumulations.  And above 1500-2000 feet, well, power outtages are a definite possibility. 

There are other factors which will limit precip amounts, whether it be snow or rain.  First of all, the storm will be a fast mover, just like just about every other storm we’ve seen this year.  No matter what you get, it’s won’t be occurring for very long.  The other major factor will be dry slotting.  If you look at the surface maps above, the area of lighter precipitation over extreme southeast PA and southern NJ on the 18z GFS – under the comma, so to speak – is seen on every model run, except the NAM, which is apparently lost with this storm.  The heaviest precip looks to run along a line from about Hagerstown, MD up through the Lehigh Valley and Poconos, then into the Catskills and Hudson valley of New York.  Southeast of there, dry slotting is a real possibility.   

So given all this, here’s my first call map:

The higher amounts of the spectrum will be at the highest elevations for each area.  The hardest area to forecast goes right through the Lehigh Valley.  Southern areas may see nothing in the the way of accumulations, where those only 20 miles north may see 6 inches.  The biggest winners will be the Poconos where I can easily see a foot falling.

One other thing – the HRRR which one of the short range models, shows some fairly heafty snows overnight tonight, with the accumulated snowfall forecast by tomorrow morning below:

Although I thnk this is WAY overdone, I wouldn’t be surprsied to see a few slushy inches overnight over the northern sections of the Lehigh Valley and especially the Poconos.  Just something to keep an eye on.

Later….

Doc