Sunday, January 27, 2013

Forecast for Monday, January 28, 2013

A warm front is going to begin to approach our region this evening and continue to move closer on Monday. I still expect a light overrunning precipitation event as the warm air begins to come up from the south, moving over the colder air in place. The dynamics still don’t look terribly impressive... but a light precipitation event involving ice can always be troublesome.

The latest model guidance suggests perhaps a tad bit more snow and sleet as the precipitation begins, which is likely sometime during the morning rush hour. Snow and sleet will likely transition to freezing rain and freezing drizzle late in the morning and early in the afternoon. The Southeastern NJ and Delmarva areas may not see much freezing rain as temperatures here will warm quickly because of being located near the warmer ocean. The freezing rain to liquid rain transition will occur throughout the late morning and afternoon from southeast to northwest. This means the highest icing amounts will be in the Poconos and Lehigh Valley. Right now, we anticipate most of the region receiving a coating to two inches of wet snow and sleet, followed by a trace to two tenths of an inch of ice. That two-tenths of an ice is most likely near Mount Pocono where liquid rain may not occur until early on Tuesday. 

There remain a few factors to consider here when trying to pinpoint the transition from ice to rain. The ground has some snow cover or has been covered by snow this past week and the ground has also been subjected to well below freezing temperatures. Modeling always has a difficult time factoring this in and thus I continue to fear surface temperatures may be at or below freezing for longer than what the models are currently projecting. Also, even if the air temperatures rise to 33 or 34 degrees…the immediate ground surface may lag behind. Finally, that few hour window of snow and sleet may coat the ground further which may also at least briefly make the surface colder. Again, we are talking about light accumulations when this is all said and done. But if this comes in around the morning commute, there could be major traffic headaches. A glaze of ice can create very hazardous traveling conditions. It doesn't necessarily take one-quarter of an inch of ice or the criteria for an ice storm to create problems. There is also a slight chance that if the snow can get going and just happens to band a bit…perhaps there could be a bit more snow than presently forecast on the front end of this event. It has happened before in our region. That high of 36 may very well come late in the day or at night. Both 12z GFS and MOS have 37 degrees for a high temperature at Philadelphia International Airport for Monday and this is along the lines of what I was expecting earlier this weekend. However, I would tend to go a little bit below MOS guidance coming off of this arctic cold and so it does give you pause that maybe the warming on the models today is too much, too fast.

I also still cannot imagine escaping without areas of fog later Monday and Tuesday. A moistening column and warmer air moving over a colder ground seems as though it will be a perfect opportunity for fog formation, some of which could be quite dense. The question at this point is the strength of any inversion and the wind speed at the surface. Right now, it looks southwesterly and light. I also believe drizzle will be occurring during this time frame.

For Tuesday, this question remains...."How warm do we go?"... Now there is beginning to be some divergence with the MOS guidance. The NAM MOS has a high of 46 degrees while the GFS has a high of 53 degrees. I continue to like 49 degrees for Tuesday which is somewhat of a compromise. A stronger southerly flow will help to push the temperature upwards to an extent if the warm front can push through. But I believe it is going to have a hard time as that cold air could be dammed and stubborn. Fog is also going to be a huge factor here and I suspect it will be dense throughout a good deal of the morning…if not most of the day. Yet again, some patchy drizzle is possible. So I am not too inclined to forecast temperatures above 50 degrees at this moment. The models usually have a hard time factoring in the low clouds and typically miss out on the extent of the cold ground surface. So I like the NAM vs. the GFS MOS in this case. As we get closer to this period, the LAMP guidance may be the best way to go here.

Now with Wednesday, we will likely see that warm front finally push through and a pre-frontal warm surge ahead of a strong cold front. The models continue to have a “windy” look to the day, especially as a band of heavy rain accompanies a cold front. The Storm Prediction Center has portions of the region in a risk for severe weather. Thunderstorms may contain very little lightning and thus you may hear little thunder. Wind gusts over 55 or 60 MPH may be common as this cold front approaches. GFS MOS guidance has 67 degrees for Wednesday. This is likely overdone. If the winds can roar for a few hours, 60 degrees or slightly better is a possibility. Again it is hard for me to ignore that just a week ago we had a day of full sunshine and down-sloping flow and the airport could rise to 57 degrees ahead of a cold front. This situation looks cloudier and we may eventually have to also add some patchy, morning fog here as well…although the winds at the surface are much stronger than Monday Night and Tuesday. It is notable that 12z GFS MOS is forecasting 67 degrees, but it remains suspicious for a day in January.

A weak clipper system or two may impact the region for the late weekend or next weekend and these could bring chances for light snow or snow showers. Again, a lot of waffling going on and I don’t think there is any certainty for late in the week or the weekend until the strong front pushes through mid-week. So a huge bust potential remains for the week ahead.