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.
