1/21/2012 - Dusting Things Off at 12:30 PM

Been about six months or so since I last wrote anything, lots of stuff has happened personally which I really can't and/or probably won't go in to.  I was compelled to make a mention of some wintry weather that looks headed our way.  There's still a good amount of complication to it, and things could pan out in a couple of different ways.

The short version is that we've got a mixed bag of wintry precipitation headed our way.  You will note that I said wintry, not snowy or icy.  Wintry, which typically encompasses an all of the above type scenario.

A low pressure system is heading our way.  We already have some pretty good cold air in place, so at least to start it's going to be snow.  As the low gets more organized and brings in some warm air aloft, the upper levels will change to mixed precipitation with the lower levels still cold enough to support things falling to the ground as frozen.

The duration of frozen precipitation depends on how much cold air gets dammed in.  Cold air damming (CAD) for our area is when cold air gets wedged in between the mountains and the coast.  By rights, based on the air mass, it should be warmer but with the interaction of those two air masses, some cold gets wedged in.  As cold air is heavier than warm air, it gets wedged in below, so things that fall as rain freeze on the way down (sleet) or at the surface (freezing rain).

What's going to happen tonight?  Sometime after midnight, things will start as snow.  Depending on your location, you could see as much as an inch or two depending on how long the cold air persists.  Some time in the early morning hours, there will be a transition from southeast to northwest to sleet and freezing rain.  Depending on how the CAD plays out, which is always modeled extremely poorly, there should eventually be a transition to all rain.

Current model guidance has it that it should be all rain by mid-morning for most of the immediate metro area, with places farther out (like Frederick or the WV panhandle) staying mostly frozen for the bulk of the event.  I have a hunch the CAD sticks around longer than modeled and wouldn't be surprised if the early watches and advisories that come out underplay things a little.  Reading the NWS forecast discussions, they seem to be leaning towards warmer temperatures than guidance is currently indicating.  I have my doubts, but I've been wrong before.

The reason I have my doubts is this air mass has already been colder than modeled, which might lend support for a longer burst of snow initially, and more persistent frozen precipitation.  Also, some early indications are that the system may track a little farther south, which would allow the cold air to persist a little longer.

If things change significantly, I'll let you know.  For now, I wouldn't expect more than an inch or two of snow/sleet, and probably a tenth of an inch of ice in the immediate metro area.  That should be enough for an advisory.  If it looks like it could be closer to two tenths of an inch or a quarter inch, that would bump it up to warning criteria.  I also wouldn't be surprised to see west Loudoun and Frederick counties see that much anyway.