A Look Back at Below Zero Temperatures and Snow Cover in Chicago

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PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CHICAGO IL
1207 PM CST THU JAN 17 2013 /107 PM EST THU JAN 17 2013
LONGER RANGE FORECAST MODELS CONTINUE TO SUGGEST A VERY STRONG
BLAST OF ARCTIC AIR WILL COME BARRELING INTO THE REGION LATER
THIS WEEKEND AND ESPECIALLY INTO EARLY NEXT WEEK. THIS WILL LIKELY
BE THE COLDEST AIRMASS TO HIT CHICAGO SINCE THE BIG COLD SNAP BACK
AROUND FEBRUARY 10TH 2011 WHEN THE LOW TEMPERATURE DROPPED TO -9
OFFICIALLY IN CHICAGO. WHILE CURRENT FORECASTS ARE NOT THAT
COLD...THEY DO SUGGEST LOWS COULD APPROACH ZERO IN CHICAGO. THE
PROSPECTS OF ANY MEANINGFUL SNOWFALL BETWEEN NOW AND EARLY NEXT
WEEK APPEAR VERY LOW...SO JUST HOW UNUSUAL WOULD IT BE FOR
TEMPERATURES TO GET BELOW ZERO IN CHICAGO WITHOUT ANY SNOW COVER?

BETWEEN 1960 AND 2010 THERE HAVE BEEN 469 DAYS THAT HAVE RECORDED
LOW TEMPERATURES BELOW ZERO AT CHICAGO O`HARE...OF WHICH ONLY 16
HAVE OCCURRED WITH AN OFFICIAL SNOW DEPTH OF ZERO. THERE WERE 45
DAYS WITH JUST A TRACE OF SNOW ON THE GROUND AT O`HARE...WHICH
MEANS ONLY ABOUT 3 PERCENT OF ALL SUBZERO DAYS IN THAT 50 YEAR
PERIOD OCCURRED WITHOUT ANY SNOW ON THE GROUND AT O`HARE AND ONLY
ABOUT 13 PERCENT WITH ONLY A TRACE OF SNOW ON THE GROUND.

WHILE SNOW ON THE GROUND AT A GIVEN REPORTING STATION OBVIOUSLY
HAS A LARGE IMPACT ON TEMPERATURES...EVEN SNOW COVER UPSTREAM
WILL PLAY A ROLE IN THE LIKELIHOOD OF SUBZERO TEMPERATURES. OF THE
17 DAYS THAT O`HARE SAW SUBZERO TEMPERATURES WITH NO SNOW ON THE
GROUND...THE MAJORITY OF THEM (10) ROCKFORD HAD AT LEAST AN INCH
OF SNOW ON THE GROUND AND ONLY 3 OF THOSE 17 DAYS DID MADISON
WISCONSIN NOT HAVE AT LEAST 5 INCHES OF SNOW ON THE GROUND. IN
FACT...5 OF THE DAYS MADISON ACTUALLY HAD 10 INCHES OR MORE OF
SNOW ON THE GROUND...SO THE UPSTREAM SNOW COVER LIKELY HELPED
ALLOW TEMPERATURES TO FALL BELOW ZERO. CURRENTLY THERE IS MINIMAL
AMOUNT OF SNOW COVER UPSTREAM WITH NO SNOW COVER ON THE GROUND
OFFICIALLY IN ROCKFORD AND ONLY AN INCH ON THE GROUND IN MADISON
WISCONSIN.

ONE OF THE TOOLS FORECASTERS COMMONLY USE TO FORECAST HIGH AND LOW
TEMPERATURES ARE THE TEMPERATURES AT 850MB (AROUND 5000FT ABOVE
THE GROUND). NOT UNEXPECTEDLY...LOOKING BACK AT THE APPROXIMATE
850MB TEMPERATURES DURING DAYS WITH SUBZERO TEMPERATURES AND NO
SNOW COVER AT O`HARE...IT TOOK COLDER 850MB TEMPERATURES ON
AVERAGE TO PRODUCE SUBZERO LOWS THAN IT DOES WHEN THERE IS SNOW ON
THE GROUND. FOR NON-SNOW COVER SUBZERO DAYS THE AVERAGE 850MB
TEMPERATURE WAS AROUND -21C AND AS COLD AS -29C...WHILE THE
AVERAGE 850MB TEMPERATURE FOR ALL SUBZERO DAYS IN LATE JANUARY IS
-17C.

WHILE IT IS EXTREMELY RARE TO SEE SUBZERO TEMPERATURES WITHOUT ANY
SNOW ON THE GROUND IN CHICAGO...HISTORY SHOWS IT HAS HAPPENED AND
IT CAN HAPPEN AGAIN. CERTAINLY THE LACK OF SNOW COVER HERE AND
UPSTREAM WILL MAKE THE TASK OF GETTING BELOW ZERO OFFICIALLY IN
CHICAGO MUCH HARDER THAN IT WOULD BE IF THERE WAS A BIG SNOW PACK
UPSTREAM.

$$

IZZI


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