NY NJ PA Weather Forecasts

Snow Storm For This Sunday? Hold Your Horses Everyone!

By the way I have gotten my email account pounded this afternoon, I could already tell that the models were brewing up some mischief for this weekend.  The question is if the models are on to something or if the solution we are seeing is pure fantasy.  

The ECMWF is the main culprit here.  Let me give those who don’t have the ability to see great ECMWF data a description of the situation.  On Sunday morning, the southern branch disturbance has produced a surface low off the Carolina coastline.  Meanwhile, the clipper over the Ohio Valley appears to be transferring energy towards the coastline as well.  Sounds like trouble, right?  Well, the one problem and it is a big one, is the high pressure system position.  First of all, because of the very weak (almost non-existent) blocking over southeastern Canada, there isn’t much convergence over New England.  In fact, the strongest convergence is over the Canadian Maritimes.  So naturally, the surface high pressure system is quickly moving off the northern New England coast.  This is a major strike AGAINST accumulating snowfall over the forecast area on the coastal plain and even for the interior.  

By 00Z Monday or Sunday night for everyone else, an intense surface low is sitting right over southern New Jersey.  The 0°C 850 MB line, which is a nice indicator for a rain/snow line runs from just east of Washington DC to Philadelphia and then cutting across New York City.  It is this image that likely has gotten everyone excited.  The low then races up the Hudson Valley and into New England.  I have to throw some reality into this discussion here.

Remember when I posted that almost everything has to be perfect for a snow storm for this forecast area.  Here is a prime case why we must look beyond the model guidance to make a forecast.  First of all, there is not one mechanism to the north to keep cold air in place.  As such, the track would have to be perfect for snow fall even for the higher elevations of this forecast area, including NE PA, NW NJ, and the Hudson Valley.  What the model is assuming, with no significant support to the north, that cold air will continue to rush towards the coast.  However, the strong WAA ahead of the surface low on Sunday should inhibit the progression of the 850 MB CAA, thus making the position of the 850 MB 0° line suspect.  Either this storm is going to be further east and support a perfect storm track (which is unlikely based on the forecasted position of the trough) or the surface storm track is correct and the 850 MB temperatures should be slightly warmer.  

As such, my thinking is this.  On Sunday, the surface low off the Mid Atlantic coast will intensify, producing RAIN from Washington DC to coastal New Jersey and through the New York City metro.  The interior is more tricky.  I don’t see this storm to be anything but rain for Connecticut.  The storm track right now on most model guidance is to the west, and thus in the warm sector.  

The Hudson Valley is a bit more interesting, but taking an average of the model guidance for this period would suggest a track over or just east of the area, thus I expect a mix of rain, sleet, and snow with a change over to rain and then back to snow on Monday morning as the upper low intensifies.  

Northeastern Pennsylvania is where this forecast gets fun.  The storm track on most guidance is to the east of the region and the average storm track is east as well.  There is support for mid level temperatures to remain below freezing, however I think that the 950 MB to 850 MB levels will warm as the marine influences are stronger this time of year and tend to have more impact than forecasted on guidance.  Given the track of the low and the CAA throughout the atmosphere, which is enhancing the surface low, I think this area has the best chance to see a prolonged mix of snow, sleet, and rain.  I don’t want to state accumulations, as it is way too early for that type of forecast because of the uncertainty of the set up of the thermal dynamics from the surface to 700 MB.  However, this area has the best chance for accumulating snowfall out of the entire forecast area.  

The bulls eye for this storm has been and continues to be western New York.  I’ve been stating this for about a week now and believe that there is little evidence to support any other solution.  What I need to see change in the upper level pattern is the following:

1. Stronger convergence and confluence at the 500 MB level over northern New York or New England, which would place the high pressure in a more favorable position on Saturday night and Sunday.

2.  The trough axis positioned further to the East to support a bench mark track with or without the high.  This is due to the strong nature of the marine influences over the coastal plain.  In other words, the water temperatures are too warm right now and any wind from the east will significantly warm the boundary layer.  

3.  A boundary layer that is colder than what we have in the guidance.  Forecasting a snow storm or frozen precipitation is never a good bet when you need cold air to rush in at 850 MB and work down to the boundary layer to support below freezing temperatures.  It is rare to happen and needs support of strong high pressure to the west or north of the cyclone.

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