Area Forecast Discussion
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000 FXUS61 KPBZ 041533 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 1133 AM EDT MON MAY 4 2015 .SYNOPSIS... WARM TEMPERATURES WILL CONTINUE INTO THE AFTERNOON. A SLOW MOVING FRONTAL BOUNDARY WILL BRING RAIN CHANCES TONIGHT THROUGH WEDNESDAY. && .NEAR TERM /THROUGH TONIGHT/... TEMPERATURES HAVE JUMPED THIS MORNING WITH MIXING...ESPECIALLY FAR NORTH WHERE READINGS ALREADY IN THE MID 70S. ADJUSTED MAX TEMPS UP A DEGREE OR 2 MAINLY NORTH AND WEST OF KPIT. NO OTHER CHANGES. PREVIOUS BELOW. WESTERLIES TRY TO SETTLE SOUTH LATER THIS AFTERNOON AND TONIGHT WITH A WEAK FRONT ATTEMPTING TO PUSH SOUTH. THERE IS UNCERTAINTY IN HOW FAR AND HOW QUICKLY THE FRONT MAKES SOUTHERLY PROGRESS. FOR NOW...NAM/GFS AND SREF POINT TO A 12Z TUESDAY POSITION IN THE CWA BUT NORTH OF PIT...AND WILL TAKE THIS SUGGESTION. PLACED LIKELY POPS NORTH OF A PHD/BTP/DUJ LINE DUE TO THIS THINKING...WITH CHANCE POPS TO THE SOUTH. ALTHOUGH THE FIRST RAINDROPS COULD ARRIVE PRIOR TO 00Z...MOST CONFIDENT IN SEEING ACTIVITY AFTER THAT TIME AS WE SHOULD ONCE AGAIN SEE DEEP MIXING AND A DRY BOUNDARY LAYER DURING THE AFTERNOON. MODELS HAVE REALLY STRUGGLED WITH DEWPOINTS THE LAST COUPLE OF DAYS...UNDERESTIMATING THE DOWNWARD MIXING OF DRY AIR. WENT LIGHT ON THE THUNDER CHANCES THIS AFTERNOON/EVENING WITH UNIMPRESSIVE INSTABILITY NOTED ON MODEL SOUNDINGS. STILL LOOKS TOO WARM ALOFT TO SUPPORT DEEP CONVECTION. ONLY TWEAKED PREVIOUS HIGH TEMPERATURE FORECAST...AS IT WAS ALREADY WELL-PLACED ON THE WARMER SIDE OF GUIDANCE. H850 TEMPS LOOK TO BE 1-2C WARMER THAN YESTERDAY...AND WITH GOOD MIXING...THAT WOULD CORRESPOND TO HIGH TEMPERATURES 3-5 DEGREES WARMER THAN YESTERDAY IN MANY CASES. STUCK CLOSER TO MODEL CONSENSUS WITH LOW TEMPS TONIGHT WITH CONTINUED MIXING AND THICKENING CLOUDS. && .SHORT TERM /TUESDAY THROUGH THURSDAY NIGHT/... THE REGION WILL REMAIN ON THE NORTHERN PERIPHERY OF A FLAT H5 RIDGE OVER THE CAROLINAS THROUGH THE MID-WEEK PERIOD...WHICH WILL LEAD TO MODEST MID-LEVEL FLOW AND LITTLE SOUTHWARD PROGRESS OF THE FRONTAL BOUNDARY. ANY MOVEMENT OF THE BOUNDARY IS MORE LIKELY TO BE DRIVEN BY CONVECTION THAN JET DYNAMICS. UPSWINGS IN CONVECTIVE COVERAGE ON TUESDAY WILL HINGE UPON THE TIMING OF LOW-AMPLITUDE SHORTWAVE TROUGHS THAT WILL MOVE THROUGH THE WNWLY FLOW. CLOUD COVER WILL BE CONSIDERABLE AND WILL SERVE TO LIMIT DESTABILIZATION...BUT MOISTURE AND AN UNUSUALLY WARM BOUNDARY LAYER ARE LIKELY TO PROVIDE SUFFICIENT INSTABILITY TO GENERATE SHOWERS AND STORMS ON TUESDAY AS THE FRONTAL BOUNDARY SAGS SOUTHWARD. POPS WERE INCREASED SLIGHTLY FROM THE PREVIOUS FORECAST. THE FRONTAL BOUNDARY WILL MOVE LITTLE DURING THE PERIOD...BUT WILL BECOME MORE DIFFUSE WITH TIME. AS A SHORTWAVE TROUGH EJECTS FROM THE SOUTHERN PLAINS TO THE NORTHERN PLAINS...A H5 RIDGE WILL AMPLIFY OVER THE GREAT LAKES. THE NET RESULT WILL BE INCREASING INFLUENCE OF HIGH PRESSURE AND A SUPPRESSION TO THE SOUTH AND WEST OF CONVECTION. THUS...POPS BY LATE WEDNESDAY AFTERNOON WERE SHIFTED TO THE RIDGES AND TO THE SOUTHWEST OF THE FORECAST AREA. UNTIL THE RIDGE AXIS TRANSLATES EAST OF THE FORECAST AREA... DIURNAL INCREASE IN CU CAN BE EXPECTED BUT CHANCES FOR RAIN WILL BE MINIMAL DESPITE LINGERING MOISTURE AND QUITE-AMPLE HEATING. IN FACT...OWING TO INCREASING H9-H8 TEMPERATURES AND QUALITY MIXING...MAXIMA ARE EXPECTED TO ECLIPSE CLIMATOLOGICALLY-NORMAL VALUES BY SEVERAL DEGREES F. LINGERING MOISTURE AND CLOUD COVER ALSO WILL SUPPORT MINIMA SEVERAL DEGREES ABOVE NORMAL VALUES. && .LONG TERM /FRIDAY THROUGH SUNDAY/... A BROAD H5 RIDGE WILL DOMINATE THE NERN CONUS INTO THE WEEKEND. AGGRESSIVE WARM AIR ADVECTION /WITH H9 TEMPS REACHING 20-22C/ IS INDICATED IN BOTH ECMWF AND GFS DESPITE DIFFERENCES IN THE DETAILS OF THE OVERALL PATTERN EVOLUTION. THESE PATTERN DIFFERENCES NOTWITHSTANDING...THERE IS HIGH CONFIDENCE IN PERSISTENT WARMTH... WITH MAXIMA IN THE 80S AND MINIMA IN THE UPPER 50S-LOWER 60S. THIS WARM AIR AND INCREASING LOW-LEVEL MOISTURE WILL COMBINE TO FUEL OCCASIONAL SHOWERS AND STORMS THROUGH THE EXTENDED PERIOD. ANY SUCH STORMS WILL BE TIMED WITH SUBTLE WAVES THAT ARE NOT RESOLVED PARTICULARLY WELL BY THE MODELS AT THIS TIME. THUS...POPS IN THIS FORECAST FOLLOW A GENERAL DIURNAL TREND AND ARE FOCUSED HIGHEST IN PROXIMITY TO LARGER-SCALE UPPER-LEVEL TROUGH POSITION. && .AVIATION /16Z MONDAY THROUGH FRIDAY/... VFR TO START...WITH ONLY PATCHY HIGH CLOUDS EARLY ON. DEEP MIXING WILL LEAD TO ANOTHER CU FIELD TODAY...WITH BETTER VERTICAL DEVELOPMENT THAN LAST COUPLE OF DAYS. WINDS WILL ALSO PICK UP OUT OF THE SOUTHWEST AND GUST TO AROUND 20 KNOTS DURING THE AFTERNOON. MID AND UPPER CLOUDS THICKEN THIS EVENING. A FRONTAL BOUNDARY WILL APPROACH FROM THE NORTH...AND TAKE UP RESIDENCE NORTH OF PIT BY 12Z TUESDAY. CANNOT RULE OUT TSRA AT FKL/DUJ PRIOR TO 02Z THIS EVENING BUT CONFIDENCE IS NOT HIGH ENOUGH YET TO INCLUDE. MVFR CIG/VIS IN SHRA POSSIBLE BY THAT TIME FROM PIT ON NORTH. THINK SOUTHERN TERMINALS LIKE ZZV/HLG/MGW HOLD ON TO VFR THROUGH THE TAF PERIOD. .OUTLOOK.../TUESDAY THROUGH THURSDAY/... OCCASIONAL CIG/VSBY RESTRICTIONS IN SHOWERS AND STORMS CAN BE EXPECTED THROUGH TUESDAY NIGHT AS A FRONTAL BOUNDARY SAGS SOUTHWARD INTO THE REGION. WITH LINGERING MOISTURE AND LIGHT OVERNIGHT WIND...RADIATIONAL FOG WILL BE POSSIBLE WEDNESDAY NIGHT AND THURSDAY NIGHT AND MAY LEAD TO OCCASIONAL VSBY RESTRICTIONS. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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