NY NJ PA Weather Forecasts

Winter Storm Discussion And Thoughts

 

There is a lot to discuss with this winter storm for Thursday evening through Friday and let me tell you, I had a hard time sleeping with all the thoughts running through my head.  So let’s dive in and get started.

 

First, I think it is important to discuss the trends of the models.  I’m not one for trying to forecast a rain/snow line move than 4 days out.  Guidance tends to have a hard enough time doing so 12 hours out in these type of storms let alone 4 days.  So it’s important to recognize the errors for what they are (GFS cold bias/ ECMWF warm bias) and instead focus on the 500 MB set up.  Now, let me say that this set up is NOT optimal for a winter storm due to the lack of a negative NAO set up.  As such, timing and position of all the features will be key.  What has my interest peaked this morning is the trend of guidance in two areas.  One, the GFS has finally recognized that the southern stream disturbance will remain separate from the northern branch, thus producing the storm that the ECMWF has.  In fact, guidance is strongly leaning towards a closed low solution going into a negative tilted orientation off the East coast, which suggests that the storm will be deepening at a healthy rate.  The second trend is the speed of the cold air, which is not the most impressive.  The GFS is of course the most aggressive while the ECMWF seems to be some what more reasonable as far as timing.  Third, the guidance is closing in on the idea of place a strengthening Polar high to the north and west of the low pressure system, thus producing a north to northeasterly wind pattern along the coastal plain. 

So now that we know where the model guidance is heading, I want to touch on what could go wrong here.  That’s the problem with this forecast.  Is there a threat?  You bet!  However, there is some much that can go wrong, I am not confident of any forecast for this period this far out.  Take for example the 06Z GFS guidance, which is basically the same except the southern branch disturbance goes negative slightly faster thus the more inland solution.  I don’t buy it, however the point is that only a slight change in the guidance can have a drastic change in the forecast, which is never a good place to be for a meteorologist.  Another example is the position of the convergence and confluence at 500 MB.  A slightly shift west or east will have HUGE complications to the forecast.  Finally, there is the question of what to do with the clipper on Friday.  

It is the clipper that has me the most intrigued here.  This coastal low pressure system will not be kicked out east due to the large ridge over the Atlantic.  So I’m not concerned with a wide right solution.  However, the influenced of that clipper will have to be watched over the next 48 hours.  Should the clipper end up being faster than guidance is currently showing, which is certainly possible, then we have a stronger and colder solution on our hands.  Right now that doesn’t appear to be the case, however only a slight change in the speed of the northern branch jet maximum would be needed to make Friday morning a much colder solution.

So with the point laid out that my confidence is low, let me lay out what I am thinking will take place.  

Thursday Morning:   The cold front is to the south of the entire forecast area with strong CAA dominating the forecast area.  The precipitation is to the south of the region.  A disturbance over the Gulf of Mexico will continue to intensify with a low pressure system developing and moving towards the southern Mid Atlantic.  Temperatures over the forecast area will not move much on Thursday with many locations at or below freezing away from the immediate coast.  The immediate coast will still be in the 30’s, but likely above freezing.  

Thursday Evening:  The precipitation will work back into the forecast area as temperatures fall below freezing over much of the forecast area.  Again, there is a question of the thermal gradient along the immediate coast.  At this point from the surface to 850 MB, there will be a very tight thermal gradient from the coastal waters to the interior (northeastern PA, Hudson Valley, etc.).  That is why the slightest change either way in the forecast is so important!  The point here though is that the forecast area will be under the overall influence of CAA from 850 MB to the surface, while WAA will increase from 800 MB to 600 MB.  This type of set up strongly suggest that a sleet event is more likely rather than an all snow event.  Soundings from the GFS over many locations strongly hint of all snow, but we have to consider the cold bias of the GFS in this situation.  

Thursday Night/Friday Morning:  On all guidance, the CAA wins out in changing all the precipitation over to frozen Thursday night and Friday morning.  Whether we have snow or sleet over a certain location is honestly an unknown, however if locations that I think have the best potential for all snow will be just northwest of Philadelphia, west central New Jersey including Trenton, and locations just north and west of New York City.  During this time period, the surface low will be off the New Jersey coast and racing towards the coastal waters of New England.  A developing 700 MB low and a deepening 850 MB will aid in produce strong mid level forcing.  The precipitation I think will be heavier than forecasted in some isolated locations due to strong frontogenesis setting up at 850 MB and 700 MB.  This type of meso-forcing can have significant implications on precipitation amount and type so that’s another issue I’ll have to factor in later, once we get closer to the event.  The mesoscale forcing of this nature COULD over ride the coastal front influence that will have to be examined for the immediate coastal plain as well.  So as you can see, there is a lot to still figure out for the Thursday night/Friday situation.  

After this storm, regardless of the out come, the forecast area will remain under the influence of strong CAA through much of the weekend.  The pattern will relax and then head into a reloading phase, which will support a brief warm period for the beginning of next week.  The guidance is likely over enhancing the ridge over the East, which has been the trend of the guidance this year to over enhance troughs and ridges.  The pattern reload will be short lived as we head towards a cold and possibly stormy pattern for the end of December.

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