Posted at 8:32 AM on November 23, 2009
by Paul Huttner
Twin Cities NWS graphical forecast indicates snow Wednesday.
Welcome to my weather nightmare.
First of all, it's Thanksgiving. One of the busiest travel weeks of the year. The magnifying glass is on weather this week.
Secondly, we have a very non-traditional storm system to decipher. Twin low pressure systems are spinning through the Upper Midwest this week. The forecast models, and the forecasters, don't like "non-traditional" weather systems. Give us a nice clean Alberta Clipper or panhandle hook any day.
Here are some of the interesting (and challenging) aspects to this week's evolving weather systems.
-Twin lows developing and deepening right over the Upper Midwest.
-Warm air in place through Tuesday, making for a later changeover to snow.
-Models performance can be questionable in these situations. To quote a recently famous military leader; "We don't know what we don't know."
-There are still significant model differences in timing, precip type and location of precipitation.
NAM forecast model shows "twin lows" rotating into Upper Midwest Wednesday.
So here's the early crack at deciphering the weather code this week for holiday travel. Buyer beware, this forecast may change tomorrow.
Monday & Tuesday: Scattered rain, warm enough for all rain except in far northwest Minnesota, where some icing may occur.
Tuesday Night & Wednesday: Rain changes to snow. The GFS model has been leaning toward a period of 24 hours from late Tuesday night to late Wednesday with periods of snow. The NAM pushes everything east into Wisconsin. The European model would seem to favor a quick 12 hour burst moving off to the east.
The trump card in all of this may be unusually warm ground for late November and temperatures just above freezing for most of the day Wednesday. That could mean mostly wet roads, instead of mostly icy roads.
Thanksgiving Day: Colder with snow east of the Twin Cities. Travel could be slippery between the Twin Cities and Chicago. Highs in the mid-30s with a biting wind. This should be a pretty typical Thanksgiving Day weatherwise. We have a white Thanksgiving about one in every three years in the Twin Cities.
Friday & Saturday: More sun, milder with highs into the 40s. The best days for travel!
Sunday: A chance of snow.
Keep an eye on the forecasts this week. Things may change quickly.
At the Northern Plains Winter Storm Conference earlier this month, there was a presentation about ground temperature at snowfall accumulation. It tried to correlate ground temperatures (as measured 4 inches below the ground) and if snowfall would accumulate. There was not as much of a correlation as the author thought there would've been, and he concluded that a temperature closer to the top of the ground might be a better determiner if snow would accumulate. He did note that it seemed like just two inches of snow on the ground would insulate the ground sufficiently, at any temperature, such that the new snow on top of that wouldn't see the ground's temperature. However, above-freezing ground temperatures would cause melting at the bottom of the snow pack.
Good comment. Makes perfect sense to me. Decent snowfall rates will always overcome above freezing ground with below freezing air temps.
So far with this system the models are cranking out air temps above freezing during the entire "event." Also, snowfall rates do not appear to be that impressive, as of yet. So any snow will be fighting above freezing temperatures from top and bottom. That should put a dent in what eventually may accumulate.
I remember the Halloween Mega-Storm in '91, result with warm ground was rapidly falling snow overcoming warm surfaces, meting then freezing from above. That left a nasty layer of ice underneath the heavy snow.