Sunday, December 14, 2008

Windy and Wintry

It has been a tough few days for forecasters trying to come to consensus on what will happen Sunday into Monday and beyond. This forecast is about very fine details which will likely be only available using current observations and nowcasting. The forecast is still somewhat going to be generalized.

The tricky part of seeing where the moisture goes and how the Arctic air follows the cold front. Even the weather models runs last evening had several hundred miles in differences in where the freezing line at both the surface and 850mb (5000'). Those two items are critical in deciding what type of precipitation will fall.

As you already know, the day is windy and this is expected to continue through Monday around dawn as the front passes. You can see the forecast position of the front by the GFS Model for 1 AM and 7 AM over Indiana in the two images below.

Monday 1 AM
Monday 7 AM
There is rain ahead of the front. The rain on the surface can freeze with the rapid drop in temperatures. Behind the front is the bigger question. Post frontal precipitation has been all over the forecast maps. Everything from non existent to significant snow and ice has been shown. However the past couple of days it seems there will not be any heavy snow dump (unless it is a BIG surprise) but around 1 inch of accumulation with little to none equally as likely.

The 10AM sounding shows some the potential for freezing rain. While there is cold surface air, the temperature aloft are warm enough for rain. The good news is for Monday the post frontal precipitation is pretty minimal so while things could get tricky on walkways and roads, it is unlikely there will be massive ice damage and power outages as recently experienced out east. It also could be a tricky call for area school making calls for a snow day or 2 hour delay.

I won't beyond Monday in detail but will mention the HPC has our area under a 10% probability for a 1/4" of ice from Tuesday 7 AM to Wednesday 7 AM.

Finally, I want to give you a quick view of the cold front and temperature gradient as it now stands over Iowa. You can see the extreme rapid drop in temperatures on the observation map and the satellite view is very defined.

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