Two Shots at Snow: Thursday and Late Weekend

Well, it's certainly cold enough to snow but we'll have to wait a bit for any moisture or possible storms to line themselves up.  Still, the atmosphere is getting busy and there's plenty of energy to fuel storms that do form.  Right now, in addition to some flurries and snow showers on Tuesday that shouldn't amount to much, we have two shots at significant snow in the next 7 days.  The first is Thursday the second one is late in the weekend.  That second storm is the one to watch closely.

But the storm Thursday could make a difference in the track of that second potential storm this weekend.

So let's talk about Thursday first.

A weak storm is on the models for Thursday moving northeastward from the southern U.S. and sliding just to the south of our area.  The models have been fairly consistent with the track of the storm being to our south but a little inconsistent in how they describe the associated precipitation field.

Then the GFS and NAM runs late this afternoon showed a slightly more northern track which placed us in the precipitation area.  According to these model, the atmospheric temperatures would be cold enough to support snow and indications are for about 1-2 inches of snowfall.

Still, the other models don't agree.  They show accumulating snow for southern Virginia and North Carolina only.

I'm wary of trusting one model run - but here's the GFS:

As you can see, the track of the center is predicted by this run of the GFS to be more like along the VA/NC border instead of through central NC where it had been forecast earlier in the week.  It bears watching but will almost certainly be a low-impact storm even if it does take a more northern track.  You can see in the map above that over land, the maximum precipitation is between 0.10 and 0.25 inches which if we assume a 10:1 ratio (the rule of thumb) means about 1-2 inches.

Here's the crazy thing though:  if you love snow (like I do), you should probably hope this doesn't happen.

I can see all of you snow lovers out there now:

"Crazy fool's done lost his freakin' mind!"

Bear with me.  Let's talk about the weekend storm potential first and then we'll come back to this crazy "if you love snow, you may not want it to snow Thursday" idea.

Long-range models have had a fairly large coastal storm for December 18-20 in our area for some time now.  Ironic since, if it does happen, it will be right around one year to the day of the first huge snow in 2009.  A big storm on the anniversary of the largest one-day December snowfall in Washington, DC history.

Anyway, when you're watching long-range models a few weeks out, here's the most important thing to look for until about a week before the predicted event:  is the storm still there with each consecutive model run?

As far as that goes: so far so good.

Now that we're inside of a week away, we also start to look at storm track.  Here's where the models are, to use technical terms, all over the damn place.

For now, we'll take the GFS as a fairly reliable long-range model.  Here's what is shows for Saturday evening:

A developing storm moving from the northern Gulf of Mexico and across the southeastern US, emerging just off the coast of South Carolina.  To the northwest, a solid push of cold air that shows the freezing line to our south.  According to the GFS, still some 120 hours away from this forecast time, whatever is falling Saturday night will be snow.

6 hours later the GFS has the storm intensifying rapidly.  Snow really gets going especially in central VA which looks to be the "bullseye" for this run of the GFS.  Up here in the DC area we have less snow as the storm slides along the North Carolina coast - more eastward than northward.

6 hours more and the low is moving off the coast throwing some precipitation back into our area but the motion of the storm suggests lighter amounts than anything we saw on the 19th of December last year:

The east-northeast motion combined with the absence of any "bombing" or rapid intensification indicates that the GFS is predicting that the jet streams will not phase.  I think I wrote about phasing last year but if not: here's a quick description:

Flow patterns in the upper atmosphere occur in sequences of waves.  These waves can and do interact with one another sometimes in a way that is destructive, sometimes in a way that amplifies the original wave.  When waves combine to form a larger wave, it is called "phasing".  In meteorology, especially in the winter, we look for phasing of the northern and southern branches of the jet stream which tends to amplify existing storms.  It's a tricky thing with respect to timing and it doesn't happen all that often, relatively speaking. 

If the phase doesn't happen, you get a weak storm like the one predicted for Thursday.  If it happens too early, a storm like the one above goes too close to the coast and we end up with rain.  If it happens too late (like in the model runs above), it goes out to sea before it can get really wound up.

All that said: it's a good model run for us (remember "snow = good").  This far out, details like storm track and phasing are several runs away from being decided.  We have the storm on the models still - that's one victory.  The storm moves up the coast - that's another.

Now.  What's bad about the Thursday storm?

Because of a complicated series of atmospheric interactions that have to do with something called the "Polar Vortex", the probability of an out-to sea solution for the December 19th storm (like the models show above) probably increases the further north the Thursday storm tracks.  So if you want the potential for a big storm on Sunday, you want that Thursday storm to be as far to our south as possible.

Lots still to be worked out.  We'll be closer to a solution for the Thursday storm tomorrow.  I'll be keeping an eye on things!

Wouldn't a monster storm on the anniversary of a monster storm be cool?

Just to jog your memory:  12/19/2009: