...Heavy Rainfall from the Remnants
Tropical Storm Lee...
In early September,
ground across Virginia, Maryland, and the District of Columbia was
saturated, following a very wet August even prior to the drenching
from Hurricane Irene. Tropical Storm Lee developed in the northern
Gulf of Mexico on September 1 and made landfall in Louisiana on September
4. Lee moved northeastward into the Tennessee Valley as a Tropical
Depression, eventually becoming extratropical. Southerly flow on the
east side of Lee allowed rich tropical moisture to surge up the Eastern
Seaboard and into the Mid-Atlantic. High pressure centered over eastern
Canada blocked this moisture from moving further north and caused it
to pool across our region. The high pressure also halted Lee’s
northward progress, forcing it to drift northward over the Central
Appalachians for several days. Bands of heavy rain developed and moved
north across the area beginning on September 6th and continued through
September 9th. During this period, rainfall totals were excessive,
exceeding 12 inches in many locations. The most intense rain bands
formed on Wednesday, September 7th and Thursday, September 8th, when
the worst flash flooding occurred.
On Wednesday morning,
September 7th, a torrential rainband set up in a north/south orientation
across Maryland from Charles County into Baltimore County. Rainfall
rates of 2 to 3 inches per hour occurred for several hours within this
band, causing numerous high water rescues, road closures, and flooded
homes. Of note was a rain gauge in Bowie, MD, that observed 4.57 inches
of rain in 3 hours, which is an amount that only has a 0.5 percent
chance of occurring in a given year. Central Charles County (above)
and Ellicott City (left) in Howard County were particularly hard hit.
On Thursday, September
8th, the heaviest rainband set up slightly west of the previous day.
This band brought torrential downpours through much of the daytime
hours along a swath from King George County, VA to Montgomery County,
VA, including the western suburbs of Washington DC. Extremely anomalous
3-hour rainfall amounts were recorded at several automated rain gauges
during the event. One near Franconia observed 5.47 inches, a 3 hour
amount that has approximately a 0.2 percent chance of occurring in
any given year. A similarly rare six hour rainfall amount of 6.57 was
observed at Reston (see right). Also, at Fort Belvoir, an automated
weather station tallied 7.03 inches in only 3 hours. Rainfall of this
magnitude and duration has a 1 in 1000 chance of occurring each year.
This amount of rain caused nearly catastrophic flash flooding in portions
of northern Virginia. Tragically, at least 2 people lost their lives
in floodwaters in Fairfax County. Hundreds of citizens of Fairfax and
Prince William County were evacuated. Numerous major thorough-fares
were flooded and closed, including portions of the Capital Beltway,
Interstate 66, and George Washington Parkway. The Virginia Railway
Express’s Manassas Line and numerous school districts remained
closed into Friday as floodwaters slowly subsided.
Over the course of
the event, 34 Flash Flood Warnings were issued by the Baltimore-Washington
Forecast Office. There were over 100 reports of flash floods received
by our office. This rainfall and the overall wet period of August and
September 2011 will go down in the record books in a number of ways.
At Baltimore-Washington International Airport (BWI), several daily
rainfall records were broken. September 2011 ranked the wettest September
on record, with 13.32” of rain being tallied. The two-month period
set the record wettest 2 months ever recorded at BWI, with 23.70”.
The remnants of Tropical Storm Lee proved to be a reminder for residents
of the Mid-Atlantic of the dangers of flash flooding. Always remember
when high water is observed, turn around, don’t drown.
Allens Fresh (Charles County, MD)
Photo By: NBC Washington
Main Street in Ellicott City
Photo By: JustinWeather
Reston Park and Ride (Fairfax County, VA)
Photo By: NBC Washington
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
932 AM EDT WED SEP 14 2011
...RECENT HEAVY RAINFALL WAS A RARE EVENT...
THE MID-ATLANTIC REGION...ESPECIALLY FROM WASHINGTON AND BALTIMORE
EASTWARD...HAVE RECENTLY BEEN AFFECTED BY BOTH HURRICANE IRENE AND
A WET PERIOD CAUSED LARGELY BY THE REMNANTS OF TROPICAL STORM LEE.
SOME OF THE HIGHEST RAINFALL TOTALS OVER THE LAST 30 DAYS INCLUDE...
TRUMAN DRIVE IN LARGO 24.13
CABIN BRANCH - FORESTVILLE 23.98
LEELAND ROAD IFLOWS 23.62
HERRING ROUTE 40 BALTIMORE 23.11
PRINCE GEORGES JUSTICE CTR 22.87
LAKE ARBOR - LARGO 22.52
DISTRICT HEIGHTS 21.49
KINGSTOWNE - FAIRFAX 20.34
FORT MCHENRY 19.83
FORT BELVOIR 18.61
BWI AIRPORT 17.43
ADDITIONALLY...THE REMNANTS OF LEE BROUGHT TWO INTENSE BANDS OF
RAINFALL TO THE REGION...ONE STRETCHING FROM CHARLES COUNTY
NORTHWARD INTO BALTIMORE COUNTY...AND ANOTHER FROM KING GEORGE
COUNTY NORTHWARD INTO MONTGOMERY COUNTY. THESE RAINBANDS LED TO
RAINFALL THAT IS EXTREMELY UNUSUAL.
ON WEDNESDAY...SEPTEMBER 7TH...THE IFLOWS GAUGE IN BOWIE RECORDED
4.57 INCHES IN THREE HOURS...WHICH HAS APPROXIMATELY A 0.5 PERCENT
CHANCE OF OCCURRING IN ANY GIVEN YEAR /SOMETIMES CALLED A 200-YEAR
FOR ELLICOTT CITY...WHERE SEVERE FLASH FLOODING OCCURRED...THE
RAINS WERE ACTUALLY LESS RARE...ROUGHLY A 1 TO 2 PERCENT CHANCE OF
OCCURRING IN ANY GIVEN YEAR.
THE HEAVY RAINS ON THURSDAY...SEPTEMBER 8TH...WERE EVEN MORE RARE.
THE KINGSTOWNE IFLOWS GAUGE NEAR FRANCONIA IN FAIRFAX COUNTY
RECORDED 5.47 INCHES IN JUST THREE HOURS...WHICH HAS APPROXIMATELY
A 0.2 PERCENT CHANCE OF OCCURRING IN ANY GIVEN YEAR /SOMETIMES
CALLED A 500-YEAR RAINFALL/. THE SAME IS TRUE FOR RAINS IN
RESTON...WHERE 6.57 INCHES WAS RECORDED IN SIX HOURS.
AT FORT BELVOIR...THE AUTOMATED WEATHER STATION THERE RECORDED AN
INCREDIBLE 7.03 INCHES IN JUST THREE HOURS. THIS HAS LESS THAN A
0.1 PERCENT CHANCE OF OCCURRING IN ANY GIVEN YEAR /SOMETIMES
CALLED A 1000-YEAR RAINFALL/.
THESE VALUES ARE PRELIMINARY AND BASED ON PRECIPITATION FREQUENCY
CALCULATIONS FROM THE NEAREST AVAILABLE STATION. THE VALUES ARE
SUBJECT TO CORRECTION.