Late Season Snowfall in Chicago and Rockford

Gino Izzi of the NWS office in Romeoville rounded up these statistics for late season snowfall in Chicago and Rockford…

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CHICAGO IL
1128 AM CDT FRI MAR 25 2011 /1228 PM EDT FRI MAR 25 2011/

...LATE SEASON SNOWFALLS...

NOTHING SAYS SPRING TIME IN ILLINOIS MORE THAN TEMPERATURES IN THE
20S AND GRAY CLOUDY SKIES...BUT HISTORICALLY SPEAKING IT MAY BE A
BIT TOO SOON TO STOW AWAY THOSE SHOVELS. IN FACT...PORTIONS OF
MISSOURI AND DOWNSTATE ILLINOIS ARE SEEING SNOW THIS MORNING. SO
HOW COMMON IS IT TO SEE SHOVEL-ABLE SNOW IN OUR AREA THIS TIME OF
YEAR?

IN CHICAGO...54 OF THE PAST 139 YEARS (OR ABOUT 39%) HAVE HAD AT
LEAST ONE DAY WITH AN INCH OR MORE OF SNOWFALL ON OR AFTER MARCH
25TH. IN FACT...17 OF THOSE YEARS HAD MULTIPLE DAYS WITH OVER AN
INCH OF SNOWFALL INCLUDING 6 DAYS WITH OVER AN INCH OF SNOW BACK IN
1926! HERE ARE SOME OF THE RECENT LATE SEASON (AFTER MARCH 25TH)
CHICAGO SNOWFALLS...

SNOWFALL    DATE
 3.0     4/11/2007
 3.0     4/ 7/2003
 2.1     4/ 5/2009
 1.9     3/27/2008
 1.6     4/ 7/2000
 1.2     3/29/2009

HERE ARE THE TOP 10 BIGGEST LATE SEASON SINGLE DAY SNOWFALL TOTALS
FOR CHICAGO...

RANK   SNOW      DATE
 1    13.6    3/25/1930
 2     9.4    4/ 5/1982
 9.4    4/ 2/1975
 4     9.0    4/ 6/1938
 5     8.9    3/26/1970
 6     8.2    4/ 1/1970
 7     7.8    3/30/1926
 8     7.7    3/29/1954
 9     7.1    3/29/1964
10     6.6    3/26/1934

IN ROCKFORD...47 OF THE PAST 118 YEARS (OR ABOUT 40%) HAVE HAD DAYS
WITH AN INCH OR MORE OF SNOWFALL ON OR AFTER MARCH 25TH. OF THOSE
YEARS...10 HAVE HAD MULTIPLE DAYS WITH OVER AN INCH OF SNOW
INCLUDING 4 DAYS IN 1970 AND 1926. HERE ARE SOME RECENT LATE SEASON
ROCKFORD SNOWFALLS...

SNOWFALL    DATE
 1.9     4/11/2007
 1.8     3/29/2009
 1.2     4/ 7/2000
 1.1     4/ 1/2002
 1.0     4/12/2007

HERE ARE THE TOP 10 BIGGEST LATE SEASON SINGLE DAY SNOWFALL TOTALS
FOR ROCKFORD...

RANK   SNOW      DATE
 1    13.5    3/31/1926
 2    10.4    3/29/1972
 3     7.0    4/18/1912
 4     6.3    4/ 5/1982
 5     6.0    4/ 6/1938,  6.0    3/25/1933
 7     5.0    3/29/1954
 8     4.8    3/29/1964
 9     4.6    4/ 2/1975
10     4.5    4/ 2/1936,  4.5    3/28/1894

WHILE NO BIG SNOWS ARE CURRENTLY IN THE FORECAST FOR THE REGION...IT
IS WORTH NOTING THAT LATE SEASON SNOWS ARE NOTORIOUSLY DIFFICULT TO
PREDICT MUCH IN ADVANCE. CURRENT INDICATIONS ARE THAT THE COLD AIR
OVER THE AREA NOW WILL REMAIN IN PLACE THROUGH THE WEEKEND INTO
EARLY NEXT WEEK.

$$

IZZI

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2 thoughts on “Late Season Snowfall in Chicago and Rockford

  1. How is that you show February 2011 state avergae temperature as 29.5 degrees, 0.7 degrees below normal (your March 2nd post) and the NCDC states at their website, “The average temperature in February 2011 was 30.3 F. This was 0.7 F warmer than the 1901-2000 (20th century) average,…”

    I’d appreciate any insight to the huge 1.4F degrees anomaly disparity between the two. TIA

  2. There are two parts to the story. One part has to do with how the state temperature was calculated. I used a gridded temperature data set from the Midwestern Regional Climate Center to estimate the temperature right after the end of the month. Because it’s a gridded product, stations in nearby states may be used to help determine the temperatures at grid points within Illinois. That may help in data sparse areas. But the real advantage is that I can run the product at 10 am the first day of the next month, without waiting for NCDC to post their numbers. NCDC is a few days slower and uses the available stations within the state to calculate their state temperature. For February I arrived at 29.5 degrees and they arrived at 30.3 degrees. Since they are both provisional numbers and will likely change in coming months as more data arrives, I usually don’t pay too much attention to differences of less than a degree at this preliminary stage. Once all the data are in, I go with the final numbers from NCDC.

    The second part is that we are using two different averaging periods for the “normal”. I was using the 1971-2000 average while NCDC was using a 1901-2000 average for that particular comparison.

    Right now we use the 30-year period of 1971-2000 as “normal”. These are updated every 10 years. We will start using the 1981-2010 normals when they are released by NCDC this summer. In this case, NCDC went with the longer averaging period for monitoring long-term changes in climate, which actually makes sense. You really don’t want your base period jumping around every 10 years.

    So the disparity is the result of both the two methods for computing the February temperature in Illinois and the two different average periods. I will be more careful in the future to specify the averaging period in my calculations. Thanks.

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