Flash Flood Warning 4, NWS Pittsburgh, PA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
647 AM EDT THU JUL 5 2007

.SYNOPSIS...
CONDITIONS WILL BE UNSTABLE TODAY AND EARLY TONIGHT AND RIPE FOR THE
DEVELOPMENT OF POSSIBLY SEVERE THUNDERSTORMS AS MOIST LOWLEVEL
AIR OVER OUR UPPER OHIO VALLEY REGION WILL BE TOPPED BY A PASSING
COLD UPPER LOW PRESSURE SYSTEM MOVING EASTWARD THROUGH THE GREAT
LAKES. THEN MIDWEST HIGH PRESSURE WILL BUILD ACROSS THE UPPER OHIO
VALLEY AND PROVIDE DRY WEATHER FOR THIS WEEKEND.

&&

.NEAR TERM /THROUGH TONIGHT/...
WITH DAYTIME HEATING, THE STRATUS/FOG WILL LIFT INTO
STRATOCUMULUS AND CUMULUS. BY AFTERNOON, THE CUMULUS CAN GROW INTO
CUMULONIMBUS AS THE CURRENT MOIST LOWLEVEL AIR MASS OVER THE UPPER
OHIO VALLEY BECOMES TOPPED BY COLDER AIR ALOFT CAUSING A DEEP
UNSTABLE LAYER. THE COLDER AIR ALOFT IS ASSOCIATED WITH A
DEEPENING UPPER LONG-WAVE TROUGH AS SHOWN BY RECENT NAM AND GFS
MODEL RUNS. SEVERAL LOW PRESSURE SYSTEMS MOVING THROUGH THIS
STRENGTHENING NORTHWEST UPPER FLOW WILL PROVIDE THE LIFTING
MECHANISM TO HELP FORM BANDS OF THUNDERSTORMS THIS AFTERNOON AND
EARLY TONIGHT.

CONCUR WITH SPC THINKING THAT THESE COULD BECOME SEVERE AS SREF
MODEL RUNS CONTINUE TO SHOW CAPES GREATER THAN 1000 J/KG AND
EFFECTIVE SHEAR VALUES OF 30-40 KTS. THE HWO WILL HIGHLIGHT THAT
THE PRIME SEVERE THREAT WILL BE DAMAGING WIND AND THE SECONDARY
THREAT LARGE HAIL.

WITH NIGHTFALL, THE THUNDERSTORM THREAT WILL BEGIN TO ABATE AND
LIKELY HAVE ENDED BY AROUND MIDNIGHT. THERE CAN BE PATCHES OF LATE
NIGHT FOG INDUCED BY THE EVAPORATION OF SURFACE MOISTURE IN AREAS
THAT GOT SUBSTANTIAL RAINS.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY/...
NAM AND GFS MODEL RUNS SHOW HIGH PRESSURE FROM THE MIDWEST BUILDING
INTO THE UPPER OHIO VALLEY THIS WEEKEND. THIS WILL HELP DRIER
LOWLEVEL AIR AND MORE STABLE CONDITIONS TO COME INTO THE UPPER OHIO
VALLEY. CONCUR WITH GFS AND NAM MOS VALUES, THAT TEMPERATURES WILL
BE CLOSE TO NORMAL THIS PERIOD.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
BOTH RECENT GFS AND ECMWF MODEL RUNS SHOW THE UPPER JETSTREAM FLOW
PATTERN BECOMING ZONAL EARLY NEXT WEEK. THIS MEANS THE HIGH PRESSURE
THAT BROUGHT DRY, STABLE CONDITIONS THIS WEEKEND WILL MOVE EAST INTO
THE ATLANTIC OCEAN. HENCE GULF MOISTURE AND ATTENDANT INSTABILITY
WILL BE REINTRODUCED INTO THE UPPER OHIO VALLEY EARLY NEXT
WEEK...REALIZING THE POTENTIAL FOR THUNDERSTORMS TO DEVELOP LATE
MONDAY INTO TUESDAY. CONCUR WITH GFS MOS TEMPERATURE VALUES THAT
SHOW A WARMER THAN NORMAL PERIOD EARLY NEXT WEEK.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
RECENT SATELLITE AND SURFACE DATA SHOW THE BANK OF STRATUS AND
FOG ACROSS OHIO HAS EXPANDED INTO WESTERN PENNSYLVANIA AND
NORTHERN WEST VIRGINIA. HOWEVER, DAYTIME HEATING AND MIXING WILL
HELP THE STRATUS BANK TO LIFT INTO STRATOCUMULUS AND CUMULUS WITH
MOST TAF SITES HAVING VFR FLIGHT CONDITIONS (P6SM BKN035) BY 15Z.

STILL LOOKS AS THOUGH THUNDERSTORMS WITH SEVERE POTENTIAL CAN
DEVELOP THIS AFTERNOON AND EARLY TONIGHT AS THE CURRENT MOIST
LOWLEVEL AIR MASS BECOMES TOPPED BY COLDER TEMPERATURES
ALOFT...RESULTING IN UNSTABLE CONDITIONS. THIS COLDER AIR ALOFT
WILL BE ACCOMPANYING THE EASTWARD PASSAGE OF AN UPPER LOW PRESSURE
SYSTEM THROUGH THE GREAT LAKES. RECENT RUC MODEL RUNS SHOW
DEEP LAYER OF LIFT MOVING SOUTHEASTWARD THROUGH THE UPPER OHIO
VALLEY THIS AFTERNOON, 17Z-20Z THROUGH THE KFKL-KDUJ CORRIDOR AND
20Z-23Z KZZV-KPIT-KLBE CORRIDOR AND SOUTH TO KMGW. THIS CONCURS
WITH MOST SREF MEMBERS THAT SHOWED THE BEST CHANCE OF THUNDERSTORMS
18Z-03Z WITH SUFFICIENT EFFECTIVE SHEAR FOR SEVERE DEVELOPMENT AS
INDICATED BY RECENT SPC THINKING. BRIEF IFR FLIGHT CONDITIONS CAN
DEVELOP IN THE THUNDERSTORMS WITH HEAVIER RAIN.

WINDS WILL GO FROM SOUTHSOUTHWEST 4-7KTS TO SOUTHWEST AND
WESTSOUTHWEST 9-15KTS MIDDAY INTO EARLY EVENING. WIND GUSTS MAY
BRIEFLY EXCEED 50KTS IN SEVERE THUNDERSTORMS. HOWEVER, IN TAFS,
MENTIONED GUSTS NEAR 30KTS.

LATE TONIGHT, THERE CAN BE POST-RAIN EVAPORATION-INDUCED PATCHY FOG
AND STRATUS WITH MAINLY MVFR FLIGHT CONDITIONS. WITH RENEWAL OF
DAYTIME HEATING AND MIXING FRIDAY, EXPECT VFR FLIGHT CONDITIONS,
THAT WILL PERSIST THROUGH SUNDAY.

AS LOWLEVEL MOISTURE BEGINS TO INCREASE LATE SUNDAY, THERE WILL BE A
POSSIBILITY OF PATCHY MVFR FLIGHT CONDITIONS IN LATE NIGHT FOG EARLY
MONDAY AND TUESDAY...AND IN MAINLY AFTERNOON AND NIGHTTIME
THUNDERSTORMS.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...NONE.
OH...NONE.
PA...NONE.
WV...NONE.

&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE PITTSBURGH PA
427 AM EDT THU JUL 5 2007

MDZ001-OHZ039>041-048>050-057>059-068-069-PAZ007>009-013>016-020>023-
029>032-WVZ001>004-012-021>023-041-060830-
GARRETT-TUSCARAWAS-CARROLL-COLUMBIANA-COSHOCTON-HARRISON-
JEFFERSON OH-MUSKINGUM-GUERNSEY-BELMONT-NOBLE-MONROE-MERCER-VENANGO-
FOREST-LAWRENCE-BUTLER-CLARION-JEFFERSON PA-BEAVER-ALLEGHENY-
ARMSTRONG-INDIANA-WASHINGTON-WESTMORELAND-GREENE-FAYETTE-HANCOCK-
BROOKE-OHIO-MARSHALL-WETZEL-MARION-MONONGALIA-PRESTON-TUCKER-
427 AM EDT THU JUL 5 2007

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN MARYLAND...EAST
CENTRAL OHIO...NORTHWEST PENNSYLVANIA...SOUTHWEST
PENNSYLVANIA...WEST CENTRAL PENNSYLVANIA...NORTHERN WEST VIRGINIA
AND NORTHERN PANHANDLE OF WEST VIRGINIA.

.DAY ONE...TODAY AND TONIGHT

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR THIS AFTERNOON
INTO THIS EVENING. THE PRIMARY SEVERE WEATHER THREAT WILL BE
DAMAGING WINDS AND LARGE HAIL.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

THERE IS NO SEVERE WEATHER EXPECTED AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NEEDED TODAY.