Sunday, December 20, 2009

What do I want for Christmas?

Well, a foot of new snow would be nice.

There's chit-chat of some sort of storm moving through the Upper Mississippi Valley this coming week, and, well, it's well placed, but I remain somewhat skeptical. Why? Because until three days ago we didn't know if the East Coast was going to get a lot of snow or not. (If you're following along, they did. If you are trying to fly to the East Coast this weekend, well, you probably are still in Minnesota.) The models there bounced around inordinately, and the GFS and the ECMWF, the two longer-range models, had different solutions until about 48 hours before the storm. Washington, New York and Boston all had scenarios where they'd see two feet and other where they'd see sunny skies over the weekend, and the models only resolved in the last couple of days. So, anything put forth is almost pure speculation and while I wouldn't be too surprised to see a deep snowpack by Friday morning, I also wouldn't be surprised if we just had sun and starts (and Santa Claus) in the skies.

Let's go, as it were, to the video tape. Here are the last two runs of the GFS model, each printing out a foot of snow, or so, from Minnesota. Wisconsin gets the jackpot (as usual, it seems) but the North Shore gets hit, too, which they need up there. Either of these scenarios would be fine with me, but the models have been wobbly enough I'm not getting too excited. Yet.

On the other hand, the ECMWF, which doesn't show QPF, is intriguing because it seems to bring the storm up a bit closer to the Twin Cities. The latest ECMWF model run is further to the west of that storm track and could, presumably, shift the axis of heaviest snow in to Minnesota. The printout of the 12Z ECMWF vs the 12Z GFS shows how the models are still quite different.

This map shows the models, with the ECMWF in orange and the GFS in green, at 7 a.m. on the 25th (120 hours out from the model run). I've included the locations of the center of the low on the 24th (96 hours out) and 26th (144 hours out) for comparison. The GFS takes the low almost due north from Central Arkansas to near Milwaukee, and then retrogrades it (retrograding means that does not follow the normal east-west pattern) to near Watersmeet in the UP. This is the scenario which would dump snow heaviest in a Madison-Wausau corridor. But look at the ECMWF. It starts further west, near Texarkana, and goes due north to near Des Moines, before turning to the east. This would seem to throw the heaviest snow from Omaha to Sioux City to Minneapolis and Birkieland.

And I'm inclined to believe this model for two reasons. First, while all the models have been pretty bad this year, the ECMWF has been slightly better than the GFS. In the recent East Coast storm, the EC started with the coastal storm which eventuated, while the GFS was much further east and came back west with time. The second reason I believe the ECMWF a bit more is that it's generally more likely to have a normal east-west pattern than a retrograde, so the west-to-east movement of the GFS seems less likely.

In any case, it will be quite telling in the next couple of days as to what happens—whether we'll have two feet of snow on the ground by Christmas. Okay, that's an exaggeration. Maybe.

(There might be a couple inches before then, but it appears more likely to go south of us.)

Update: the latest Canadian GEM model seems to split the difference, although it has a pretty dramatic precipitation cutoff near the Twin Cities.

No comments:

Post a Comment