An area of high pressure on the 18th was off the Mid Atlantic coast, while a cold front was well off to the west. In between these two systems was a surface trough, which helped to focus showers and thunderstorms within a warm and humid air mass.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS PHI wrote it up.
An area of high pressure on the 18th was off the Mid Atlantic coast, while a cold front was well off to the west. In between these two systems was a surface trough, which helped to focus showers and thunderstorms within a warm and humid air mass. The thunderstorms, some severe, marched across the region during the evening hours. The combination of high winds and lightning caused about 27,500 homes and businesses to lose power in Berks County and southeastern Pennsylvania. Power was not fully restored until the 20th.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Aug 18, 4:30 PM EST, 45 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 530 PM EDT TUE AUG 18 2009 .SYNOPSIS... BERMUDA HIGH PRESSURE WILL CONTINUE TO INFLUENCE OUR REGION THROUGH FRIDAY. A COLD FRONT FROM THE NORTHWEST IS FORECAST TO ARRIVE ON SATURDAY. ANOTHER AREA OF HIGH PRESSURE SHOULD BEGIN TO BUILD IN FROM THE WEST ON SUNDAY. THE HIGH IS FORECAST TO REMAIN OVER OUR REGION DURING THE EARLY PART OF THE NEW WEEK. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... THE FORECAST HAS BEEN UPDATED TO REFLECT CURRENT CONDITIONS, WHICH INCLUDED A BROAD AREA OF SHOWERS IN CENTRAL PA WITH QUITE A FEW EMBEDDED TSTMS. THEREFORE THE POPS WERE RAISED TO LIKELY MOST AREAS. OTHERWISE...A VERY WEAK UPPER AIR DISTURBANCE WAS LOCATED OFF OUR COAST THIS AFTERNOON. IT WILL CONTINUE TO DRIFT OUT TO SEA. MEANWHILE, A SURFACE TROUGH WAS DEVELOPING OVER OUR REGION TO THE LEE OF THE MOUNTAINS. AT 300 PM, THERE WAS A FAIR AMOUNT OF SHOWER AND THUNDERSTORM ACTIVITY TO OUR WEST. SOME CONVECTION HAD MADE ITS WAY INTO THE POCONO REGION, AS WELL AS INTO BERKS COUNTY AND LANCASTER COUNTY. LIFTED INDEX VALUES WERE IN THE -3C TO -5C RANGE. SURFACE BASED CAPE VALUES WERE EXCEEDING 2000 J/KG FROM THE DELMARVA REGION UP INTO THE SUSQUEHANNA RIVER VALLEY. THE PRECIPITATION WAS MOVING EASTWARD. WE HAVE INCLUDED THE MENTION OF SCATTERED SHOWERS AND THUNDERSTORMS FOR THIS EVENING. IT APPEARS AS THOUGH THE CONVECTION WILL BEGIN TO DISSIPATE ONCE WE LOSE THE HEAT OF THE DAY. SOME ISOLATED ACTIVITY COULD LINGER BEYOND MIDNIGHT. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/... DURING THE PERIOD FROM WEDNESDAY THROUGH FRIDAY, AN UPPER LEVEL TROUGH IS FORECAST TO DIG OVER THE CENTRAL PART OF THE COUNTRY AS A RIDGE BUILDS OVER THE WESTERN NORTH ATLANTIC. THE PREVAILING FLOW ALOFT WILL BE FROM THE SOUTHWEST. AS THE UPPER FLOW SHARPENS, IMPULSES WILL LIKELY RIDE UP FROM THE SOUTHWEST AND PASS OVERHEAD. THE ATMOSPHERE WILL REMAIN QUITE MOIST THROUGH THE PERIOD AND WE ARE EXPECTING AFTERNOON LIFTED INDEX VALUES OF -2C TO -4C EACH DAY, ALONG WITH SURFACE BASED CAPE VALUES IN THE 1000 TO 2000 J/KG RANGE. AS A RESULT, WE SHOULD SEE SHOWERS AND THUNDERSTORMS EACH DAY, MAINLY IN THE AFTERNOON AND EVENING HOURS. WITH PRECIPITABLE WATER VALUES RISING ABOVE 2 INCHES AT TIMES, SOME HEALTHY DOWNPOURS ARE LIKELY. IT WILL REMAIN WARM AND HUMID. TEMPERATURES AT 850 HPA ARE EXPECTED TO STAY IN THE +17C TO +18C NEIGHBORHOOD. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... AN UPPER LEVEL TROUGH FROM THE WEST IS FORECAST TO MOVE OVER THE NORTHEASTERN STATES FOR THE WEEKEND AND FOR THE EARLY PART OF THE NEW WEEK. THE ASSOCIATED SURFACE COLD FRONT IS EXPECTED TO PASS THROUGH OUR REGION ON SATURDAY. THE BOUNDARY MAY TAKE ITS TIME PASSING OFF THE COAST ON SATURDAY NIGHT INTO SUNDAY. WE WILL CONTINUE TO MENTION CHANCE POPS FOR OUR EASTERN COUNTIES INTO SUNDAY. && .AVIATION /22Z TUESDAY THROUGH SUNDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. HIGH PRESSURE, SITTING OFF TO OUR SOUTHEAST, WILL CONTINUE TO KEEP OUR AREA UNDER A PREDOMINANTLY SOUTHWEST FLOW THROUGH TONIGHT. A BOUNDARY TO OUR NORTH WILL TRY TO MAKES ITS WAY INTO THE AREA BY TOMORROW BUT IS LIKELY TO STALL, LEAVING A TROUGH OVER US. THERE HAS BEEN SOME ACTIVITY FORMING IN EAST-CENTRAL PENNSYLVANIA AND AS DAYTIME HEATING PROGRESSES THESE STORMS WILL START TO EXPAND IN COVERAGE, BUT SHOULD REMAIN SCATTERED ACROSS THE REGION. WE HAVE COVERED THIS WITH A CB IN THE TAFS FOR ALL SITES WITH THE EXCEPTION OF KABE AND KRDG, AS SOME CONVECTION HAS ALREADY STARTED FORMING JUST OFF TO THEIR WEST. A SEA BREEZE HAS DEVELOPED THIS AFTERNOON, MAKING ITS WAY INTO KACY, BUT IT CONTINUES TO STRUGGLE MOVING FARTHER INLAND. THE CUMULUS, ALONG WITH ANY SHOWERS OR THUNDERSTORMS, SHOULD START TO DISSIPATE AS WE HEAD TOWARD EVENING. OUTLOOK... A TROUGH WILL REMAIN OVER THE AREA FOR THE NEXT DAY OR TWO. A STRONG COLD FRONT IS EXPECTED TO ARRIVE LATE FRIDAY OR SATURDAY. SCATTERED SHOWERS AND THUNDERSTORMS CAN BE EXPECTED WITH THE FRONTAL PASSAGE. && .MARINE... HIGH PRESSURE WILL REMAIN OFF TO OUR SOUTHEAST MAINTAINING A SOUTHWESTERLY FLOW. THE LEE SIDE TROUGH, WHICH DEVELOPED TODAY, IS EXPECTED TO HANG AROUND FOR THE NEXT DAY OR TWO AS A FRONTAL BOUNDARY STALLS OFF TO THE NORTH OF THE REGION. WINDS SHOULD INCREASE ON THE WATER AS THE GRADIENT TIGHTENS BETWEEN THESE TWO FEATURES. HOWEVER, WINDS SHOULD REMAIN BELOW 25 KTS AND NO SMALL CRAFT ADVISORIES ARE ANTICIPATED. A RELATIVELY STRONG COLD FRONT IS EXPECTED TO CROSS THE WATERS LATE FRIDAY OR SATURDAY. WINDS AND SEAS ARE EXPECTED TO INCREASE AS THE SYSTEM APPROACHES THE REGION. WAVEWATCH GUIDANCE CONTINUES TO BE CONSISTENT IN ADVERTISING SEAS BUILDING TO BETWEEN 5 AND 7 FEET LATER THIS WEEK. HURRICANE BILL REMAINS WAY OUT IN THE ATLANTIC, HOWEVER IT CONTINUES TO BE FORECAST TO STRENGTHEN AND EVENTUALLY TURN MORE TOWARD THE NORTHWEST AND NORTH LATER THIS WEEK. THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO HOW CLOSE IT WILL GET TO THE EAST COAST. HOWEVER, THIS SYSTEM WILL PUSH SIGNIFICANT OCEAN SWELLS UP THE MID-ATLANTIC SEABOARD LATE THIS WEEK AND ESPECIALLY OVER THE WEEKEND. WE CONTINUE TO MAKE MINOR ADJUSTMENTS TO THE WIND AND WAVE HEIGHT GRIDS. DEPENDING ON WHETHER THE STORM TRACKS FARTHER WEST WILL HAVE AN IMPACT ON OUR WAVE HEIGHTS. ALSO, SWELLS WILL INCREASE AS THE SYSTEM MOVES NORTHWARDS IN ITS TRACK THIS WEEKEND. THIS WILL INCREASE THE RIP CURRENT RISK AND WILL NEED TO BE MONITORED CLOSELY. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...NONE. && SYNOPSIS...IOVINO NEAR TERM...IOVINO/STAUBER SHORT TERM...IOVINO LONG TERM...IOVINO AVIATION...MEOLA MARINE...MEOLA
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 413 AM EDT TUE AUG 18 2009 DEZ001>003-MDZ008-012-015-019-020-NJZ001-007>010-012-013-015>023-027- PAZ054-055-060>062-067>071-190815- NEW CASTLE-KENT-INLAND SUSSEX-CECIL-KENT MD-QUEEN ANNES-TALBOT- CAROLINE-SUSSEX-WARREN-MORRIS-HUNTERDON-SOMERSET-MIDDLESEX- WESTERN MONMOUTH-MERCER-SALEM-GLOUCESTER-CAMDEN- NORTHWESTERN BURLINGTON-OCEAN-CUMBERLAND-ATLANTIC-CAPE MAY- SOUTHEASTERN BURLINGTON-CARBON-MONROE-BERKS-LEHIGH-NORTHAMPTON- CHESTER-MONTGOMERY-BUCKS-DELAWARE-PHILADELPHIA- 413 AM EDT TUE AUG 18 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR CENTRAL DELAWARE...NORTHERN DELAWARE...SOUTHERN DELAWARE...NORTHEAST MARYLAND...CENTRAL NEW JERSEY...NORTHERN NEW JERSEY...NORTHWEST NEW JERSEY...SOUTHERN NEW JERSEY...EAST CENTRAL PENNSYLVANIA...NORTHEAST PENNSYLVANIA AND SOUTHEAST PENNSYLVANIA. .DAY ONE...TODAY AND TONIGHT. NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY. NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS NOT EXPECTED AT THIS TIME. DEZ004-NJZ014-024>026-190815- DELAWARE BEACHES-EASTERN MONMOUTH-ATLANTIC COASTAL CAPE MAY- COASTAL ATLANTIC-COASTAL OCEAN- 413 AM EDT TUE AUG 18 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTHERN DELAWARE...CENTRAL NEW JERSEY AND SOUTHERN NEW JERSEY. .DAY ONE...TODAY AND TONIGHT. NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY. AN INCREASE IN WAVE ACTION AND RIP CURRENTS ARE POSSIBLE LATE FRIDAY AND INTO THE WEEKEND AS HURRICANE BILL PASSES WELL OFF THE MID-ATLANTIC COAST. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS NOT EXPECTED AT THIS TIME. ANZ430-431-450>455-190815- DELAWARE BAY WATERS NORTH OF EAST POINT NJ TO SLAUGHTER BEACH DE- DELAWARE BAY WATERS SOUTH OF EAST POINT NJ TO SLAUGHTER BEACH DE- COASTAL WATERS FROM SANDY HOOK TO MANASQUAN INLET NJ OUT 20 NM- COASTAL WATERS FROM MANASQUAN INLET TO LITTLE EGG INLET NJ OUT 20 NM- COASTAL WATERS FROM LITTLE EGG INLET TO GREAT EGG INLET NJ OUT 20 NM- COASTAL WATERS FROM GREAT EGG INLET TO CAPE MAY NJ OUT 20 NM- COASTAL WATERS FROM CAPE MAY NJ TO CAPE HENLOPEN DE OUT 20 NM- COASTAL WATERS FROM CAPE HENLOPEN TO FENWICK ISLAND DE OUT 20 NM- 413 AM EDT TUE AUG 18 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR ATLANTIC COASTAL WATERS AND DELAWARE BAY WATERS. .DAY ONE...TODAY AND TONIGHT. NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY. LONG PERIOD SWELLS ARE POSSIBLE LATE FRIDAY AND INTO THE WEEKEND AS HURRICANE BILL PASSES WELL OFF THE MID-ATLANTIC COAST. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS NOT EXPECTED AT THIS TIME.
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 507 PM EDT TUE AUG 18 2009 MDZ008-012-015-182230- CECIL MD-KENT MD-QUEEN ANNES MD- 507 PM EDT TUE AUG 18 2009 ...A STRONG THUNDERSTORMS WILL AFFECT CECIL...KENT AND QUEEN ANNES COUNTIES... AT 503 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED SEVERAL STRONG THUNDERSTORMS NEAR BALTIMORE...MOVING EAST AT 10 MPH. VERY HEAVY RAIN WILL FALL FROM THIS STORM...AND WILL CAUSE WATER TO POND ON ROADS AND QUICKLY FILL ROADSIDE DITCHES AND SMALL STREAMS. WIND GUSTS TO 50 MPH ARE POSSIBLE WITH THIS STORM. EXPECT INTENSE LIGHTNING FROM THIS STORM. DO NOT SEEK SHELTER UNDER TREES WHEN LIGHTNING THREATENS...MOVE INDOORS WHEN A THUNDERSTORM APPROACHES.
The State College radar (KCCX), 158 km away, was scanning. The volume nearest the first event began at 22:16 UTC.
6 thunderstorm wind, 2 lightning, 1 hail. Local standard time, as filed.
A severe thunderstorm knocked down a couple of trees in Tulpehocken Township.
A severe thunderstorm tore down several trees and numerous tree limbs along the southern tier of Chester County in East Nottingham. Elk and Franklin Townships. In Elk Township, downed trees tore down a half a mile power lines on Strickersville Road. Most of the down tree limbs in Franklin Township were on the south side.
A severe thunderstorm toppled more than one hundred trees and damaged buildings in Boyertown. A few homes were damaged by downed trees. Two buildings on the 100 block of East Philadelphia Avenue were damaged. There were about twenty weather related calls in the borough. Residents were evacuated from both buildings after bricks were scattered across the alley between them. Both roofs were damaged as well as the exterior wall and chimney of one of the buildings. One building was closed until repairs were made. On another block of East Philadelphia Avenue, a downed tree damaged a roof and a vehicle. Numerous trees were knocked down on Third Street (making it impassable), Fourth Street and Fifth Street as well within the Boyertown Community Park. At the park, a dugout pavilion rolled over onto its roof and damaged a fence. About 1,300 homes and businesses lost power. Power was not fully restored until the 20th. Property damage in the borough was estimated at $500,000. No injuries were reported.
A severe thunderstorm tore down several trees in northwest Montgomery County. In Gilbertsville, one downed tree fell through a home. A tree was down on High Street in Pottstown. Trees were down along Pennsylvania State Routes 73 and 100. About 5,000 homes and businesses lost power in Pottstown, Lower Pottsgrove, West Pottsgrove and Lower Frederick Townships.
A severe thunderstorm tore down large tree limbs in Boothwyn Borough.
A 59 mph wind gust was measured and penny size hail fell in Skippack Township.
A lightning strike and the ensuing fire damaged a home in Lansdowne Borough.
Lightning struck a home in Silverdale Borough and caused a fire. No injuries were reported.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Pennsylvania in 2009Every state in 2009This day in weather historyWarnings in Pennsylvania, August 2009
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 193929, 193931, 192390, 193939, 193932, 193933, 193935, 193936, 193938.