Flash Flooding event of August 12th, 2010

Overview

High moisture values were tapped by a weak Mesoscale Convective Vortex (MCV) that rolled NW to SE through the state during the middle of the day on August 12th. The vortex touched off numerous showers and thunderstorms with torrential rain falling over the lower Susquehanna valley during the afternoon. Capital City Airport (KCXY) recorded over 3 inches of rain in just one hour, while Harrisburg International Airport (KMDT) received less than three-quarters of an inch of rain during the same time. This is a perfect example of how your neighbor can have much different weather than you -- CXY and MDT are only 3 miles apart.

The torrential rain caused flash flooding in many spots -- especially over Harrisburg City, northern York County and southern Adams County. Much of the area received rainfall during the early morning hours which set the stage for the rainfall in the afternoon to quickly run off into the local low spots and small creeks and streams. The urbanized areas of the Harrisburg metro area bore the brunt of the flooding reports, mainly due to the intensity of the rain over the area, but also because the concrete and other impervious surfaces do not allow water to soak into the ground. That rain rapidly runs downhill and collects at low spots and bottle necks/constrictions like underpasses and culverts.

A few water rescues occurred during the late afternoon and evening and a few people were evacuated from their residences in the southern part of the City of Harrisburg. See the preliminary storm reports and some associated images below:


Flooding in Harrisburg, photo courtesy Dauphin County Emergency Management	Dulles (KLWX) sounding at 12z on the 12th	Visible sat. image from the morning of the 12th, note circulation from MCV over Central PA and clearing to the south	IR Sat. image during the morning

Radar and PWATs
Storm Report Summary

Radar and PWATs


Early morning radar reflectivity from KCCX	Afternoon radar reflectivity image from KCCX- heavy rain in Harrisburg and vicinity	KCCX Storm Total Precip from the 11th. Note maxima in the lower Susqehanna Valley.	Precipitable Water values- note over 2 inches of water available overhead in SE PA

Storm Reports

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE STATE COLLEGE PA
600 PM EDT THU AUG 12 2010

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0300 PM     HEAVY RAIN       2 NE HARRISBURG         40.30N 76.86W
08/12/2010  M1.00 INCH       DAUPHIN            PA   TRAINED SPOTTER

            ONE INCH IN 45 MINUTES AT PAXTONIA. MINOR FLOODING NEAR
            FISHING CREEK.

0305 PM     HEAVY RAIN       1 E HARRISBURG          40.28N 76.87W
08/12/2010  E2.00 INCH       DAUPHIN            PA   TRAINED SPOTTER

            TWO INCHES PER HOUR AT PROGRESS.

0335 PM     HEAVY RAIN       SHIREMANSTOWN           40.22N 76.96W
08/12/2010  M0.50 INCH       CUMBERLAND         PA   TRAINED SPOTTER

            0.50 IN 15 MINUTES. STREET FLOODING IN LOWER ALLEN
            TOWNSHIP. TWO INCHES TOTAL SO FAR TODAY.

0351 PM     FLASH FLOOD      MOUNT HOLLY SPRINGS     40.11N 77.18W
08/12/2010                   CUMBERLAND         PA   COUNTY OFFICIAL

            911 CENTER REPORTED UNDERPASS FLOODED. ROADS PONDED.

0400 PM     HEAVY RAIN       YORK SPRINGS            40.01N 77.12W
08/12/2010  M0.87 INCH       ADAMS              PA   NWS EMPLOYEE

            0.87 IN ONE HOUR WITH THUNDERSTORM BETWEEN 3 PM AND 4 PM.


0400 PM     HEAVY RAIN       4 S NEW CUMBERLAND      40.17N 76.88W
08/12/2010  U0.00 INCH       YORK               PA   COUNTY OFFICIAL

            911 CENTER REPORTED WATER RESCUE AND SEVERAL HOUSES
            STRUCK BY LIGHTNING.

0400 PM     HEAVY RAIN       2 SE NEW CUMBERLAND     40.21N 76.85W
08/12/2010  M3.01 INCH       YORK               PA   ASOS

            3.01 INCHES IN ONE HOUR AT CAPITAL CITY AIRPORT.

0400 PM     FLASH FLOOD      GETTYSBURG              39.83N 77.23W
08/12/2010                   ADAMS              PA   COUNTY OFFICIAL

            911 CENTER REPORTED SEVERAL VEHICLES STALLED IN FLOODED
            ROADWAYS.

0400 PM     FLASH FLOOD      7 NW GOLDSBORO          40.23N 76.84W
08/12/2010                   YORK               PA   COUNTY OFFICIAL

            911 CENTER REPORTED 2 PERSONS RESCUED FROM VEHICLE
            STRANDED IN FLOODED ROADWAY AT LEWISBERRY AND FAIRVIEW
            ROADS IN FAIRVIEW TOWNSHIP.

0409 PM     HEAVY RAIN       MECHANICSBURG           40.21N 77.01W
08/12/2010  M1.10 INCH       CUMBERLAND         PA   TRAINED SPOTTER

            1.10 IN 35 MINUTES. URBAN FLOODING. SOME ROADS IMPASSIBLE
            WITH 2 FEET OF WATER.

0430 PM     FLASH FLOOD      HARRISBURG              40.28N 76.88W
08/12/2010                   DAUPHIN            PA   COUNTY OFFICIAL

            911 CENTER REPORTED MULTIPLE STREETS FLOODED THROUGHOUT
            HARRISBURG AND SURROUNDING TOWNSHIPS. INTERCHANGE 283 AND
            83 FLOODED. HOUSES AND VEHICLES FLOODED ESPECIALLY IN
            SWATARA TOWNSHIP. FLOODING BEGAN ABOUT 3 PM AND WAS
            ONGOING AT 430 PM.

0435 PM     HEAVY RAIN       6 WNW GETTYSBURG        39.86N 77.34W
08/12/2010  M1.41 INCH       ADAMS              PA   TRAINED SPOTTER

            1.41 INCHES IN 35 MINUTES AT ORRTANNA.

0500 PM     FLASH FLOOD      HARRISBURG              40.28N 76.88W
08/12/2010                   DAUPHIN            PA   COUNTY OFFICIAL

            911 CENTER REPORTED 8 HOMES WERE EVACUATED IN THE 1200
            BLOCK OF SOUTH 19TH STREET. A SHELTER WAS SET UP FOR THE
            40 PERSONS THAT WERE AFFECTED.

0535 PM     FLASH FLOOD      3 NE HANOVER            39.84N 76.94W
08/12/2010                   YORK               PA   COUNTY OFFICIAL

            911 CENTER REPORTED A WATER RESCUE FROM A FLOODED VEHICLE
            AT THOMAN AND STENNEN ROADS IN HEIDELBERG TOWNSHIP.


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