9:55 AM

The 12Z NAM is out and to the left is the QPF output for the storm based on this run.  Now I know many of you in northeastern Pennsylvania, northern New Jersey, and even New York City are screaming right now in frustration.  Is the NAM forecasting a miss for these areas?  Well,not exactly.

What the NAM is showing here are two concerns that I addressed with this storm in this morning’s post.  The first being the development of the 700 MB upper low.  On this run, the 700 MB upper low is slightly and I mean SLIGHTLY further west than in other model guidance.  As a result, the dry slot for this storm drives right up the New Jersey coast and cuts right through Long Island, then slides due east as the 700 MB lifting features slide east towards the new coastal low.  This is a valid and significantly possible outcome, but not something that is written in stone.

The other concern is the development of convective banding of snowfall.  Let me be blunt, someone is going to miss out on this storm compared to their surrounding neighbors.  That location can easily be northeastern Pennsylvania, Philadelphia, or any other location throughout the forecast area.  What the NAM shows is that impact of such intense banding on the snowfall forecast.  Again, this feature will have to be nailed down in the hours up to and even through the development of this storm.  However, the NAM illustrates perfectly the type of resulting snowfall gradient that can develop with this storm.

So for now, I’m not changing the going forecast as a slight shift of the 700 MB features from this run puts the entire region in a QPF zone of over 2 inches.  The best position I would take is prepare for a significant impact and go from there.

 The winter has been like a pendulum for the forecast area.  At one time the Pacific is favorable producing a sustained cold shot via a negative EPO/positive PNA positioning of the ridge, however the Atlantic is out of sorts with little blocking.  At other times, the negative NAO is strong and dominant, but the Pacific is all wrong.  This leads to a storm track that is suppressed or too far east to have much of an impact on the forecast area.  This of course has lead to one very frustrating winter for many snow lovers out there.  So what can we expect for February?

After the first storm of the month, which is basically a near miss for many locations, the pattern will moderate through at least February.  The ridge over the West will collapse and will be replaced by a trough, which will pump up a strong southeast ridge over much of the East coast.  Temperatures will warm to above normal levels by next weekend with a storm track directed towards the Great Lakes.  

However, I think the cold air will return.  There has been a major warming episode in the stratosphere the past few days, which should translate to a return of a -EPO/+PNA pattern from the middle to end of February.  The warming should also support a neutral to slightly negative NAO by the end of the month as well.  As such, I think February will end cold and with significant cold, arctic air waiting in northwestern Canada, this cold blast for the end of the month may be a rather nasty reminder that winter is not over yet.

As for the storm track for the end of the month, the storm track will have two key features.  The first will be the usual clipper type storm track that will bring cold air into the forecast area.  There is a potential for another Gulf Low track as well depending on the interaction of the northern and southern branches of the jet stream.  Precipitation will be near normal with temperatures averaging slightly below normal due to the cold push for the end of the month.

7:23 AM

The basic ideas of the forecast for the period of Sunday through Wednesday is starting to come together slowly but surely.  Let me first say the GFS 00Z and 06Z ideas have little support in developing a strong and defined trough in the West.  As such, I basically through the model guidance out in my overall thinking.  However, the fact the the GFS is over phasing disturbances in this period isn’t exactly surprising.  

To start my forecast this morning, I decided to look at the actual water vapor satellite images for the northern hemisphere (at least on our side of the globe) to see exactly what type of moisture was building over the Pacific and West coast.  I wanted to see the actual movement of this mid level moisture and the impacts at the lower levels in precipitation.  Overall, model guidance had a difficult time picking up the actual precipitation over the West.  Why is this important?  Well, this gives me an idea on how well the data from the South Pacific and Pacific in general is being handled.

Let’s get started with the overall idea here.  The pattern that is unfolding for this period strongly supports the foundation of deep Arctic air over the forecast period Sunday through Wednesday.  If precipitation falls, it will be in the form of snow and/or sleet and I don’t expect any sort of rainfall out of this situation anymore.  The ECMWF is keeping the majority of the precipitation suppressed to the south as strong high pressure appears to dominate the forecast area from the Great Lakes.  Before you scream in frustration, and I know you guys in Philadelphia are right now.  Let’s really look at the data here.  

Note that there is a lot of moisture out over the West that is not being handled very well by the model guidance.  Note that on the GFS, UKMET, Canadian, Ensemble guidance, and GEFS guidance all have a strong thermal gradient from the surface to 700 MB running from North Carolina through southern New York.  We know where the best thermal gradient is, what about the moisture?  The ECMWF is suppressing a good deal of the moisture to the south of the forecast area through much of this period.  The GEFS is a bit further north.  The UKMET appears to be a bit further north, but I have limited data to view the complete evolution on the guidance.  The Canadian is also further north with precipitation threats.  The difference in position of precipitation locations is how each model guidance handles the strength and track of each disturbance moving through the Mid Atlantic.  

For now, in an attempt to give a nod to the ECMWF and Canadian guidance, I am going with a mostly dry and cold Sunday forecast, however I am keeping in snow showers for Sunday evening just to cover the potential here.  I don’t like all that moisture over the West being completely suppressed to the south.  I also think the best threat for precipitation on Sunday night will be south of New York City.  

From there, I am going to put a threat of snow for the forecast area, specifically south of the Hudson Valley and Connecticut through the middle of next week.  The forecast idea at the surface is that where the Arctic front will stall, weak areas of low pressure will capitalize on the deep moisture available from the South Pacific and Gulf of Mexico to produce a persistent band of light to moderate snow over the Mid Atlantic.  Locations just to the north of this stalled out Arctic front are going to see a long period of cloud conditions and waves of precipitation.  The most likely areas to see this unfold look to be from central New Jersey through the Philadelphia metro and points south.  I think New York City has a shot to see the same, but the overall development of the pattern points to higher potential the further south you go.  Don’t be surprised if the amount of precipitation in this period is greater than what guidance is currently suggesting.  

As you can tell, I am very uneasy about this entire set up.  The general synoptic idea is pretty well established, however the details and specific location of precipitation, disturbance, not to mention mesoscale lifting parameters are far from being settled.

I think the best route to take over this period is to keep an eye on radar and water vapor trends, update the forecast, and hang on.  This forecast is likely to change.