Saturday, January 26, 2013

Forecast for Sunday, January 27, 2013

A warm front will begin to approach our region Sunday Evening into Monday which will result in an increase in cloudiness. As the warm air begins to come up from the south, it will move over colder air. This will result in what we call overrunning precipitation. The dynamics don’t look terribly impressive, so this will be a light precipitation event. With the arctic cold that has been locked into place, I sense we are going to have some precipitation type issues. The latest guidance suggests that initially we may see an hour or two of light snow. However, the warmer air quickly invades aloft and this will cause the snowflakes to melt. There may be a shallow layer of freezing temperatures closer to the surface and so the raindrops may refreeze again and sleet may very well occur for a time as well. Then as that shallow layer of colder air becomes scoured out and shrinks in depth, the sleet will stop and then raindrops will make it all the way down to the surface. However, surface temperatures still appear to be at or below freezing by this time. This means there is a very real risk of at least a few hours of freezing rain and freezing drizzle. I think North and West of Philadelphia there is a high likelihood of freezing rain and freezing drizzle. In Western New Jersey, Northern Delaware, and areas north of the Atlantic City Expressway…there is a chance for freezing rain as here it is not as clear cut. Elsewhere, there is a slight chance of sleet and wet snow with brief icing before a quick transition to liquid rain. 

There are a few factors to consider here. The ground is mostly snow covered and has also been subjected to well below freezing temperatures. Modeling always has a difficult time factoring this in and thus I 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. Maybe an inch or two of snow and sleet at most…one to two tenths of an inch of ice with that two-tenths closer to Mount Pocono. We will key in on the timing later on, but if that snow can get going and just happens to band a bit…perhaps there could be a bit more snow than presently forecast. That high of 37 may very well come late in the day or at night.

I 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, how warm do we go? Well, 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. I think these models aren't factoring in the low clouds and are missing out on the extent of the cold ground surface.

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. It will be breezy at times. However, MOS guidance is pointing to 60 degree temperatures for Wednesday. I believe this is overdone. Even with the full sunshine and down-sloping flow last Sunday, we could only 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 stronger than Monday Night and Tuesday. It is notable that MOS is forecasting 62 degrees as of 12z, but suspicious in January. It can happen, but this isn't an ideal setup for that. Again, I think this model is trending too mild at the moment. This strong, dynamic front swings through with not only rain showers…but possibly a few thunderstorms. Thunder is more likely if we do push past 60 degrees. Right now, mid-fifties look the most likely. 

The front slows down with waves of low pressure developing along it for Thursday. Colder air moves in aloft throughout the day and eventually settles at the surface Thursday Night. If it is still precipitating, the rain may mix with or changeover to wet snow and sleet. As usual, the Poconos and the Lehigh Valley are the best candidates for this transition to frozen precipitation. 

Two weak, clipper systems are possibly in the cards. One for Friday and the other later on Saturday into Sunday Morning. Both could offer a round of snow showers or a period of light snow. I think these will be waffling on the models with both precipitation amounts and coverage until the dynamic front swings on through. So nothing is set in stone here just yet.

There is huge bust potential for the week ahead!