Friday, January 8, 2010

Why I am not too, too concerned about the thaw next week

That's a map of snow cover this morning. The gradients are 0-2 (brown), 2-4, 4-8, 8-16, 16-24 and 24+ (light purple). Look at the snow cover to our southwest. There's at least 16 inches from northwest Missouri to northwest South Dakota. There's at least 8 inches from northwest Arkansas to northwest Nebraska. So any warm air, coming from the southwest, will have to cross hundreds of miles of cold, deep snow pack, moderating it dramatically (melting is endothermic, too, which can cool the air just above the snow). In addition, moisture released form melting snow may build a deck of stratus clouds, muting the ability of the sun to heat the air. This is why, despite 1000-500 mb thicknesses of 552, the air temperatures are not forecast to rise much past freezing. And although we don't have the snowpack in the Twin Cities that they have over the Buffalo Ridge in Southwest Minnesota, the snow down there should help to protect us, to some degree, from a total meltdown.

I hope.

More locally, check out the latest snow depth maps from the U.

That's more than 30" of snow over some of Southwest Minnesota (and, yes, not even a foot here in the Twin Cities), where the warm will come from. That's way more than normal—in the 99th percentile. In recent years, we've often seen a lot of snow over Northeast Minnesota (and, more interestingly, the north has often fallen in to high percentiles, with the south in lower ones), with little down south—a steep gradient which is often exacerbated by warm air which blows over the snow-free areas and eats in to the first snow it finds. This year, the southwest has more snow than up north, which should be good for snow for everyone.

I hope.

In any case, we are entering a dreaded split-flow situation, which brings warm, dry air to the area. We can only hope it is short-lived and does not do much to our base, and good, cold, moist weather returns soon.

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