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  • Tim McGill

The First Real Snowstorm Of The Season (For Some)

Falling From 40s To 30s This Weekend


Our mild march through December continued yesterday. The high of 57° on Thursday was 21° above average. 9 of first 10 days of the month have been above average. 22 of the past 23 days have been above average going back into November. So far December is running 6° above average.


We slowly start to slide back to reality over the next couple of days. Today's highs will be in the middle 40s with clouds, rain and a wind off the lake cooling things off.


We will be hovering around 40° at midnight tonight and early Saturday morning. Watch for temperatures to fall from near 40° early tomorrow into the 30s during the afternoon. Then it's 30s for highs starting on Sunday and we don't recover from those 30s all the way through Thursday. The last time we had five days in a row with highs in the 30s was early February. This is the coldest air of the season so far but actually right around average for the middle of December.




Models Still Not In Agreement On Snowfall


We are now 3.4 inches below average on snowfall going back to the start of November. Some models suggest we could make up most of that this weekend while others suggest much lighter snowfall here. The GFS model favors mostly rain with Saturday's storm. We might even here some rumbles of thunder south and east of the city early tomorrow. It has the rain changing to snow around midday Saturday well northwest of the city and in the city itself late in the afternoon. After this storm it keeps things quiet here until it brings back plain old rain a week from Monday.



The accumulated snowfall meteogram for O'Hare has overall lowered the range of solutions for Saturday'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 nearly two inches.


The European model is spitting out the most snowfall for Saturday's storm followed by the NAM model and then finally the GFS model.


The latest GFS model run has dropped its snowfall numbers dramatically. It now shows a dusting near the north side of the city to nearly two inches along the Wisconsin border in Lake County. It has the heaviest swath of snow staying well north in southern and central Wisconsin.

The European model brings the heaviest swath of snow further south with higher totals. It has about an inch of snow in the city then ramping up quickly to around 7 to 8 inches near the Wisconsin state line.


The NAM model is somewhere in-between the other two models. It has about an inch of snow in the city and then about 4 to 6 inches north and west of there.


Once again I need to caution that since there isn't better agreement from the models the snowfall forecast will continue to be revised right up to the arrival of the first flakes. As of today, I am favoring a forecast that is somewhere between the GFS and European models. That would mean about a half inch to an inch in the city with most of that coming late Saturday into Saturday night. Higher amounts, about 2 to 5 inches for areas north and west of the city. That would include an area from DeKalb to Lake Zurich to Lake Forest and north and west of there. The transition to snow from rain will occur early Saturday in those areas.



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


Still looking at big waves whipped up by strong northeast winds especially later Saturday. North and northeasterly winds with our storm will strengthen later today into tomorrow. These winds could whip up waves up to 10 to 12 feet high on the southern end of Lake Michigan and cause some lakeshore flooding. The strongest winds and biggest waves will hit late Saturday afternoon into Saturday night.

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 watch is in effect for the same period for the rest of Cook County and also Lake County, Indiana.




Trending Mild Towards End Of The Month


After a return to about average temperatures starting Sunday and through Thursday the long range outlook has us bouncing back. The 6-10 day temperature probability forecast has us outlooked for about average temperatures overall from December 16th through December 20th. The longer range 8-14 day forecast from December 18th through the 24th outlooked to return to above average temperatures overall.



There are signs this relatively mild pattern could continue into the new year. The experimental multi-model ensemble has us outlooked for above average temperatures overall through both the week ending on Christmas day and the week ending on New Year's Day. This forecast will be revised this weekend but if it holds, it suggests a pattern that would not support much snowfall during the period. At the very least it suggests that overall temperatures will be above average. That doesn't rule out a couple of colder than average days but it favors more days above than below average. The area of orange and red that represents temperatures at least 1° to 5° or more above average grows bigger and moves more over the Midwest during this two week period.






Trending Towards More Precip?


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 16th through December 20th has us outlooked for below average precipitation overall. The 8-14 day forecast from December 18th through December 24th has outlooked for above average precipitation overall but it still isn't a very strong signal.




Here is my 7 day forecast:


Today: Cloudy, breezy, colder, rain mainly pm and northwest High: 47


Saturday: Rain mixing with/changing to snow, blustery Low: 40 High: 43 (falling into the 30s)

Sunday: Partly to mostly cloudy Low: 27 High: 36

Monday: Mostly sunny Low: 25 High: 35


Tuesday: Partly to mostly sunny Low: 24 High: 36


Wednesday: Mostly cloudy Low: 28 High: 36


Thursday: Partly to mostly cloudy Low: 28 High: 37


#ilwx

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