Historic Rainfall Frequency amounts set in September

Link to USGS Press Release

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
200 PM EDT WED NOV 4 2009

...HISTORICAL RAINFALL FREQUENCY AMOUNTS OCCUR OVER PORTIONS OF
NORTH GEORGIA IN SEPTEMBER 2009...

THE NATIONAL WEATHER SERVICE HAS BEEN REVIEWING THE EXTREME HEAVY
RAINFALL THAT OCCURRED IN SEPTEMBER 2009 WHICH CAUSED EPIC FLOODING
IN PORTIONS OF NORTH GEORGIA. NUMEROUS LOCATIONS IN DOUGLAS...EAST
PAULDING...WEST COBB...EAST CARROLL...CENTRAL GWINNETT AND SOUTHWEST
WALKER COUNTIES EXCEEDED 10 INCHES OF RAIN DURING A 24 HOUR PERIOD.
THE GREATEST AMOUNT WAS 16.7 INCHES JUST WEST OF DOUGLASVILLE.

THE FOLLOWING AMOUNTS ARE FOR 24 HOUR RAINFALL FREQUENCY PERIODS IN
NORTH GEORGIA AND THE CHANCES OF RECURRENCE IN ANY GIVEN YEAR...

7.2 INCHES IS 1.0 PERCENT...100 YEAR RAIN EVENT
7.7 INCHES IS 0.5 PERCENT...200 YEAR RAIN EVENT
8.2 INCHES IS 0.2 PERCENT...500 YEAR RAIN EVENT
8.7 INCHES IS 0.1 PERCENT...1000 YEAR RAIN EVENT
9.7 INCHES IS 0.05 PERCENT...5000 YEAR RAIN EVENT

USING RAINFALL FREQUENCY CALCULATIONS...IT HAS BEEN DETERMINED THE
CHANCES OF 10 INCHES OR MORE OCCURRING AT ANY GIVEN POINT ARE LESS
THAN ONE HUNDRETH OF ONE PERCENT. THIS MEANS THE ODDS ARE 1 IN 10000
OR MORE OF RECURRENCE IN ANY GIVEN YEAR...OR A 10000 YEAR RAIN EVENT.

WEATHER RADAR ESTIMATES COMBINED WITH AUTOMATED HOURLY RAINGAGE
REPORTS INDICATED THE 24 HOUR PERIOD WITH THE MOST INTENSE RAINFALL
FOR THE WHOLE STORM EVENT WAS FROM 800 PM EDT ON SEPTEMBER 20
THROUGH 800 PM EDT ON SEPTEMBER 21.

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