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  • Tim McGill

Storm Nudges North & Takes Most Of The Snow With It

Temperatures Tumble Today

This month has been relatively "mild" so far. December is now 6.5° above average. We start off in the 40s today but quickly fall into the 30s with wind chills dropping into the 20s today and teens by tomorrow night.

Here are the forecast temperatures for 3 this afternoon as gusty northerly and northeasterly winds begin to bring in the coldest air of the season so far.

After our slide into the 30s we don't really recover all the way through next week. Highs will be in the 30s Sunday through at least Friday. This will be the coldest stretch of weather we have seen since February.

Models More In Agreement

We are now 3.6 inches below average on snowfall going back to the start of November. It doesn't appear we will make much of that up today. The GFS model favors mostly cold rain with today's storm. We might even here some rumbles of thunder south and east of the city early on Saturday. More on the snowfall numbers below. After today the model keeps us dry through next weekend.

The accumulated snowfall meteogram for O'Hare has once again lowered the range of solutions for today's snowfall. On the low end some models have as little as a a couple of tenths of an inch and on the high end at least one model is suggesting just under two inches.

The NAM model is spitting out the most snowfall for our storm today followed by the European model and then finally the GFS model.

The latest GFS model run has no measurable snow forecast in and around Chicago. It suggests the most snow will fall well northwest of the city near the Wisconsin state line with up to three or four inches from near Rockford and westward to the Iowa border.

The European model has the city getting a couple tenths of an inch and far northern reaches of Lake and McHenry county getting up to a couple of inches of snow. Most of us get less than a half inch of snow with this model.

The NAM model is the most robust with its snowfall projections. The city and most suburbs still only get a dusting but much of Lake, McHenry counties and a portion of northern Kane and DeKalb counties get two to three inches.

Since most models have nudged the storm track further north keeping us on the milder side of this system I think we are looking at a lot of cold rain but little snow. My best guess is a dusting in the city and adjacent suburbs. Norther Lake county and much of McHenry and DeKalb county may see an inch or two but heavies amounts out toward Boone and Winnebago counties with two to five inches of slushy wet snow there. Boone and McHenry county have a winter weather advisory in effect until 3 pm today.

White Christmas Dreaming

I included this in yesterday's post and want to share it again. After getting updated long range temperature forecasts (see below) it looks like having a white Christmas will be more of a dream than a reality this year.

The National Weather Service has an interactive map of the U.S. that will show the probability of a white Christmas for a place just by clicking on it. It is based on the latest (1981-2010) U.S. Climate Normals from NOAA's National Centers for Environmental Information (NCEI).

This probability is based on past climatological records for O'Hare. This suggests the probability of O'Hare having a white Christmas is 37%. As stockbrokers will tell you "past performance doesn't guarantee future results". We could end up having a white Christmas for the next ten years in a row or go without any snow on Christmas day for the next ten. Probability is the chance that something will happen or put another way, the probability of an event is a measure of the likelihood that the event will occur.

While the map shows the historical probability that a snow depth of at least one inch will be observed on December 25, the actual conditions in any year may vary widely from these because the weather patterns present will determine the snow on the ground or snowfall on Christmas day. These probabilities are useful as a guide only to show where snow on the ground is more likely.

Here is the wide range of possibilities for a white Christmas in our area from least to greatest:

Kankakee 34%

Elgin 35%

Crystal Lake 39%

Midway Airport 41%

Antioch 42%

Harvard 46%

DeKalb 49%

Rockford 55%

Winds Still Set To Whip Up Big Waves

One thing that hasn't changed is the forecast for strong winds whipping up big waves along Lake Michigan's shoreline. Waves could build to 10-14 feet along the shores of northern Cook county up through Lake county in Illinois and Kenosha county in Wisconsin. The lake level might sure up to a foot as well.

A lakeshore flood warning is in effect from midnight tonight until 3 am Sunday for Kenosha County, Lake County and the northern portion of Cook County. A lakeshore flood advisory is in effect until Sunday morning for the rest of Cook County and also the shoreline of Indiana.

Strong Signal For "Mild" End To Month

Our cold snap that is coming would normally not even make us think twice. We have been relatively mild for so long now that a drop to about average temperatures will seem like a big deal. After next week the cold eases an we could begin a turn towards a relatively mild pattern that may last through the end of December. The 6-10 day temperature probability forecast has us outlooked for about average temperatures overall from December 17th through December 21st. The longer range 8-14 day forecast from December 19th through Christmas day keeps us (and the vast majority of the country) above average overall. The probabilities are even stronger in this forecast for relatively mild temperatures in this forecast leading up to the holiday.

More Active Pattern?

The longer range precipitation probabilities forecasts trend us towards higher than average precipitation towards the end of the month. The 6-10 day forecast from December 17th through December 21st has us the Chicago area split between being outlooked for below average precipitation overall and being outlooked for above normal precipitation.. The 8-14 day forecast from December 19th through December 25th has outlooked for above average precipitation overall. The corresponding temperature probabilities forecasts both have us outlooked for above average temperatures so much of this possible precipitation could be in the form of rain.

Here is my 7 day forecast:

Today: Rain mixing with/changing to snow, blustery High: 42 (falling into the 30s)

Sunday: Partly to mostly sunny Low: 27 High: 37

Monday: Mostly sunny Low: 25 High: 32

Tuesday: Mostly cloudy Low: 24 High: 33

Wednesday: Partly to mostly sunny Low: 28 High: 35

Thursday: Partly to mostly cloudy Low: 28 High: 36

Friday: Cloudy, breezy, colder, rain mainly pm and northwest High: 39



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