Looking Back at the Lowest Temperature On Record in Chicago and Some Thoughts About the Current Cold Snap

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PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CHICAGO IL
121 PM CST SUN JAN 20 2013 /221 PM EST SUN JAN 20 2013/

ON THIS DATE IN 1985....THE MORNING TEMPERATURE AT O`HARE BOTTOMED
OUT AN INCREDIBLE -27 DEGREES F. THIS SET THE ALL-TIME RECORD LOW
TEMPERATURE IN CHICAGO. IN ADDITION...ROCKFORD PLUMMETED TO
-26...WHICH IS THE SECOND LOWEST TEMPERATURE EVER RECORDED THERE.

INTERESTINGLY...AIR TEMPERATURES LATE TOMORROW OVER NORTHERN
ILLINOIS AT 850 MILLIBARS...OR JUST OVER 4000 FEET ABOVE THE
SURFACE...WILL BE NEAR -25 CELSIUS. TYPICALLY THE 850 MILLIBAR
LEVEL IS AT APPROXIMATELY 5000 FEET ABOVE THE GROUND...AND
TEMPERATURES HERE ARE A GOOD PROXY FOR HOW WARM OR COLD AN AIR
MASS IS. HOWEVER...IN THE CASE OF THE AIR MASS OVERHEAD
TOMORROW...IT WILL BE SO COLD THAT THE 850 MILLIBAR LEVEL WILL
ACTUALLY BE MUCH LOWER AND CLOSER TO 4000 FEET. THE TEMPERATURE AT
THIS LEVEL BACK ON 1/20/85 WAS ONE OF THE LOWEST OBSERVED IN THE
PAST 30 YEARS FOR OUR AREA...AT -31 C.

THEREFORE...THE AIR MASS TOMORROW WILL BE EXTREMELY COLD BY OUR
STANDARDS...ABOVE THE SURFACE...BUT THE LACK OF SNOW COVER IS A
BIG LIMITING FACTOR ON HOW COLD IT CAN GET AT THE SURFACE. ON
1/20/85...THERE WAS 7 INCHES OF SNOW ON THE GROUND AT O`HARE. IF
WE ACTUALLY HAD DEEP SNOW COVER TOMORROW NIGHT...IT IS IMPOSSIBLE
TO ESTIMATE JUST HOW COLD IT WOULD GET...BUT A GOOD INDICATOR IS
THE LAST TIME CHICAGO AND ROCKFORD WERE BELOW ZERO...FEBRUARY 10
2011. ON 2/10/11...THERE WAS NEARLY 20 INCHES OF SNOW ON THE
GROUND ACROSS THE AREA. ROCKFORD SET A NEW RECORD LOW OF -20
DEGREES F AND CHICAGO DIPPED TO -9 DEGREES. 850 MILLIBAR
TEMPERATURES THAT DAY WERE ACTUALLY COMPARATIVELY MILD TO TOMORROW
NIGHT AT AROUND -15 CELSIUS. CLOUD COVER IS ALSO A FACTOR THAT
LIMITS HOW COLD IT CAN GET...BUT BASED ON AIR TEMPERATURES ABOVE
THE SURFACE ALONE...IT IS POSSIBLE IF NOT LIKELY THAT TEMPERATURES
SEVERAL DEGREES COLDER THAN 2/10/11 WOULD HAVE OCCURRED EARLY THIS
TUESDAY IF WE HAD A DEEP SNOWPACK IN PLACE.

IN CLOSING...BECAUSE OF THE LACK OF SNOW COVER...THE ACTUAL AIR
TEMPERATURES DURING THE COLD SNAP WE ARE NOW IN WILL NOT BE NEARLY
AS COLD AS BACK ON 1/20/1985...POSSIBLY GETTING JUST BELOW ZERO IN
CHICAGO ON TUESDAY MORNING. NONETHELESS...WIND CHILL VALUES EARLY
TUESDAY WILL STILL BE AT DANGEROUSLY LOW LEVELS...APPROACHING 20
BELOW ZERO IN THE CITY AND 25 BELOW FOR AREAS NORTH AND WEST OF
CHICAGO.

$$

RC



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