An upper level disturbance and accompanying cold front will move through the greater Chicago metro area tonight. The big question is: whether this system will produce the first measurable (.1”) snowfall of the winter season for Chicago? The same question raised yesterday. So far at the official station at O’Hare only a trace of snow has been recorded. In other words, no amount deep enough to measure at least .1”. The system tonight looks very weak, with the best chance of a dusting of snow south of Chicago to the south of I80. It looks for the city only about a 20% chance of seeing any measurable snowfall tonight. If we do receive any snow, the best “chance”s look to be from around 1-4 AM. This is when humidity patterns in the atmospheric column look the best to allow snow to reach the surface.
This lack of snow has definitely been tied to the warm weather we have enjoyed lately. The below is the latest release from the NWS. As you can see, we are definitely running quite a bit warmer than last year. Why you may ask? Last year we had a Greenland blocking pattern (ridge) form in the upper atmosphere. This blocking pattern produces a cold northwest flow over about the east ½ of the nation. This blocking pattern is a result of above normal water temperatures in the north Atlantic. So far this year the blocking pattern has not formed, even though the water temperature are above normal. So the basic flow over the country has been more west to east, or warmer. The La Nina (cool) water pattern in the equatorial eastern Pacific is also not as strong as this time last year. The result has been a flatter upper air flow pattern than last year so far this winter.
From the NWS:
The Slow Start to Winter 2011/2012 Continues…
What a difference a year makes…literally! Winter 2011-2012 is certainly getting off to a lackluster start…temperatures so far this month have averaged nearly 6°F per day above normal in Chicago…and 15.4°F warmer than first week of December last year! In fact…so far this month our average low temperature has been 30°F while in comparison 4 out of the first 7 days of December 2010 didn’t even see high temperatures that warm. In fact…by this time last year we had already seen lows in the single digits.
The coldest temperature we have seen so far this year in Chicago was 24°F back on nov 17th! Going back through the 138 years of weather records in Chicago there have only been 4 winters that have failed to produce colder temperatures up this point in the year. The only 4 winters where we saw later dates for the first time reaching 23°F or colder in Chicago are:
1) Dec 31 1923
2) Dec 21 1998
3) Dec 14 1939
4) Dec 9 2001
The average first date for seeing a temperature at or below 23°F in Chicago is November 17th. By this time last year Chicago had already seen 2 days with high temperatures colder than the coldest low temperature so far this year!
In addition…if Chicago fails to get measurable snowfall tonight then all indications are that the threat of measurable snowfall will be very low through next Wednesday Dec 14th…which would put the record latest measurable snowfall record for Chicago in very real jeopardy. The latest first measurable snowfall in Chicago was Dec 16 1965.
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