Area Forecast Discussion
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000 FXUS61 KPBZ 011834 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 234 PM EDT MON SEP 1 2014 .SYNOPSIS... SCATTERED SHOWERS AND STORMS WILL DIMINISH IN COVERAGE GOING INTO THE EVENING...WITH HUMID AND SOMEWHAT UNSTABLE CONDITIONS CONTINUING THROUGH EARLY TUESDAY. A COLD FRONT WILL ADVANCE ACROSS THE AREA TUESDAY...WIDESPREAD SHOWERS AND STORMS. && .NEAR TERM /THROUGH TUESDAY/... THE 12Z KPBZ RAOB INDICATED A NEARLY SATURATED ATMOSPHERE BELOW 700 MB...AND WHILE SOME OF THE MOISTURE HAS LIKELY MIXED OUT A BIT AS STRATUS HAS BROKEN UP ACROSS THE AREA...THE SOURCE REGION AIR MASS FOR THE LARGE SCALE FLOW CONTINUES TO HAVE DEWPOINTS LARGELY IN THE LOWER 70S THIS AFTERNOON. AS SUCH...RATHER DEEP INSTABILITY WAS NOTED ON THE 12Z SOUNDING...AND IT LIKELY STILL EXISTS OVER THE AREA THIS AFTERNOON. WHILE FORCING FOR ASCENT IS RATHER NEBULOUS TO FIND AT THE MOMENT...A COUPLE OF WEAK EMBEDDED VORTEXES SEEM TO BE MORE THAN ENOUGH TO HARNESS THIS INSTABILITY AND TRANSLATE IT INTO RATHER EFFICIENT HEAVY RAIN PRODUCING SHOWERS AND THUNDERSTORMS GIVEN PWAT VALUES OF AROUND OR ABOVE 2 INCHES. THE FIRST OF THESE HAS ALREADY BASICALLY EXITED THE AREA TO THE NORTHEAST. THE SECOND IS CURRENTLY MOVING INTO EASTERN OHIO AND SHOULD TRANSLATE ACROSS THE BULK OF THE REST OF THE AREA SOUTH OF A NEW PHILADELPHIA TO INDIANA LINE THIS EVENING. BRIEF HEAVY RAINFALL WILL BE LIKELY WITH THESE CELLS. HOWEVER...THEY ARE RATHER TRANSIENT...SO FLOODING CONCERNS ARE MINIMIZED AS A RESULT. DEEP INSTABILITY AND A VERY HUMID AIR MASS REMAIN BEHIND THE ACTIVITY THIS EVENING...SO WHILE SHOWERS AND A STORM OR TWO CANNOT BE RULED OUT...THE CHANCES DO LOOK TO DWINDLE OWING TO A LACK OF FORCING OTHER THAN ANOTHER TRANSIENT WAVE THAT MAY BRING SOME SHOWERS THROUGH AROUND MIDNIGHT. THE REAL FOCUS BY TONIGHT LOOKS TO BE TURNING TO THE DAYTIME ON TUESDAY...WHEN THE MID-LEVEL SYSTEM APPROACHES FROM THE GREAT LAKES ALONG WITH THE ASSOCIATED COLD FRONT. MODEL SOLUTIONS SEEM TO BE KEYING IN ON A LATE MORNING/EARLY AFTERNOON ARRIVAL INTO NORTHWEST PORTIONS OF THE AREA...WITH A MID-AFTERNOON CROSSING OF THE PITTSBURGH AREA...AND A LATE AFTERNOON/EARLY EVENING EXIT OF THE CWA FOR THE FRONTAL BOUNDARY. INSTABILITY ALONG THE FRONT WHILE LIKELY CONTINUING TO BE RATHER DEEP...SHOULD ALSO BE RATHER UNIMPRESSIVE OWING IN LARGE PART TO A LACK OF INSOLATION AHEAD OF THE FRONT THROUGH THE EARLY PORTIONS OF THE DAY. THIS SHOULD LIMIT ADDITIONAL DESTABILIZATION. ADDITIONALLY...OWING TO THE FACT THAT THE MID-LEVEL WAVE LARGELY SHEARS OFF TO OUR NORTH...0-6 KM BULK SHEAR VALUES REALLY ONLY RUN UP TOWARD 35 KTS OF WEAKLY VEERED FLOW AS THE BOUNDARY IMPINGES UPON THE AREA. AGAIN WITH THIS VARIABLE...THOSE VALUES DO NOT EXACTLY MAKE THIS FORECASTER PARTICULARLY JUMPY. FURTHERMORE...WITH SHEAR VALUES THAT HIGH AND A FAIRLY QUICK MOVING WAVE PASSAGE...WHILE THE CELLS THAT DO MOVE THROUGH ARE LIKELY TO BE HEAVY RAIN PRODUCERS...PROLONGED HEAVY RAINFALL IN ANY PARTICULAR LOCATION AS WELL AS TRAINING OF ECHOES SEEMS UNLIKELY. THIS SHOULD MINIMIZE THE RISK OF FLASH FLOODING AS WELL. IT IS WORTH NOTING THAT SPC HAS PLACED THE AREA IN A SLIGHT RISK FOR TOMORROW...HOWEVER OWING TO LESS THAN STELLAR INSTABILITY VALUES...MODERATE SHEAR...A LACK OF INSOLATION...FORCING THAT FOCUSES HEIGHT FALLS LARGELY TO THE NORTH OF THE AREA ALONG WITH THE HIGHEST SHEAR VALUES...IT SEEMS LIKE THIS SLIGHT RISK IS A LESS CONFIDENT SLIGHT RISK THAN NORMAL. FRIES && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... THE NEXT SHORTWAVE TROUGH WILL MOVE THROUGH TUESDAY WITH A WEAK FRONTAL BOUNDARY. MODELS CONTINUE TO SHOW AN UNSTABLE ATMOSPHERE AS THE HIGH SURFACE DEWPOINTS REMAIN ENTRENCHED. TIMING ON THE BOUNDARY APPEARS TO BE IN THE AFTERNOON VIEWABLE WITH A LINE OF LOW LEVEL FORCING. TUESDAY LOOKS LIKE A GOOD BET TO SEE WIDESPREAD SHOWERS WITH THE RISK FOR STORMS. SHOULD BE PLENTY OF CLOUD COVER TUESDAY WHICH WILL LIMIT THE STRENGTH OF CONVECTION. WOULD THINK THE GREATER THREAT WOULD BE HEAVY DOWNPOURS AS PWATS WILL RISE TO NEAR 2 INCHES RIGHT AHEAD OF THE CROSSING BOUNDARY. UPPER LEVEL SUPPORT WILL QUICKLY MOVE TO THE EAST IN THE EVENING...CARRYING THE BOUNDARY WITH IT. THIS WILL BRING AN END TO THE THREAT FOR PRECIPITATION. DRIER AIR WILL EVENTUALLY SETTLE IN OVER THE REGION ON WEDNESDAY. TEMPS WILL STILL BE WARM BUT CONDITIONS WILL BE NOTICEABLY MORE COMFORTABLE. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... HIGH PRESSURE WILL REMAIN ALONG THE EAST COAST THROUGH FRIDAY... ALLOWING CONDITIONS TO REMAIN DRY. COLD FRONT WILL BRING SHOWERS AND STORMS FOR THE BEGINNING OF THE WEEKEND...WITH COOLER DRIER AIR FOR THE END OF THE LONG TERM. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... VFR CONDITIONS WILL LIKELY BE LOCALLY MARRED BY CONVECTION THROUGH THE AFTERNOON HOURS...THOUGH TIMINIG IS DUBIOUS AT BEST AT THE MOMENT FOR ANY PARTICULAR TERMINAL. MVFR VIS ARE LIKELY TO RETURN OVERNIGHT AS CONVECTION DWINDLES AND ABUNDANT LOW LEVEL MOISTURE AGAIN RETURNS. HOWEVER...A COLD FRONT WILL ADVANCE ON THE AREA ON TUESDAY...BRINGING A BETTER RISK OF SHOWERS/STORMS AT ALL SITES NEAR/AFTER THE END OF THE CURRENT FORECAST VALID PERIOD. FRIES OUTLOOK.../TONIGHT THROUGH FRIDAY/... SHOWERS AND STORMS WILL BE LIKELY AT ALL SITES TUESDAY AFTERNOON. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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