Area Forecast Discussion
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727 FXUS61 KPBZ 212346 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 746 PM EDT THU MAY 21 2015 .SYNOPSIS... COOL TEMPERATURES WILL REMAIN THROUGH LATE WEEK. A WEAK DISTURBANCE WILL BRING SHOWERS MAINLY FROM PITTSBURGH SOUTH THIS MORNING. && .NEAR TERM /THROUGH FRIDAY/... EVENING UPDATE... A RELATIVELY STOUT VORT MAX AND ACCOMPANYING MID- LEVEL WAVE IS DRIVING THROUGH NORTHERN OHIO CURRENTLY. THIS VORT MAX...EVIDENT ON WATER VAPOR IMAGERY...IS RESPONSIBLE FOR RE- GENERATING RAIN SHOWERS THIS EVENING...DESPITE THE RELATIVE LACK OF DEEP MOISTURE. CURRENT MESOSCALE ANALYSIS PROGS SHOW THE PRECIPITABLE WATER VALUES AT 0.75" OR LESS...WITH THE BULK OF THAT MOISTURE CONFINED TO BELOW 700MB. NEVERTHELESS...THE VIGOROUS NATURE OF THE WAVE IS SQUEEZING OUT THE REMNANT MOISTURE...PRODUCING MORE THAN SPRINKLES AND DRIZZLE AT THIS POINT. MODIFIED THE FORECAST TO INCLUDE SHOWERS THROUGH LATE EVENING. THE WAVE WILL PUSH EAST OF THE FORECAST AREA BY MIDNIGHT...SO THE FORECAST THEREAFTER REMAINS LARGELY UNTOUCHED. TAX PREVIOUS DISCUSSION... A WEAKER PIECE OF MID-LEVEL MOISTURE ALONG A 925-700 MB THERMAL GRADIENT SEEMS TO WORK ITS WAY SOUTHWARD ACROSS THE AREA IN THE NORTHWESTERLY FLOW THROUGH FRIDAY MORNING AND EARLY AFTERNOON. MODEL SATURATION OF THE UNSTABLE LOWER FRONTOGENETICAL LAYER IS NOT VERY CONVINCING...NOR IS LIFT PARTICULARLY DEEP WITH THIS FEATURE. IT WOULD SEEM ISOLATED SHOWERS WILL BE THE BEST THAT WILL LIKELY BE MUSTERED...SO POPS WERE MAINTAINED IN THIS CATEGORY. LIKEWISE...WITH LESS THAN CONVINCING SATURATION ALONG THE BOUNDARY...AND A GIGANTIC SLUG OF DRY AIR WITH LAYER MEAN RH VALUES AROUND 15 PERCENT IN ITS WAKE...SKIES SHOULD RAPIDLY CLEAR BY EVENING. FRIES && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... WITH SKIES CLEARING OUT QUICKLY FRIDAY EVENING AND HIGH PRESSURE RESIDING OVER WESTERN PENNSYLVANIA BY SUNRISE SATURDAY...CONDITIONS WILL BE PRIME FOR A NIGHT OF RADIATIONAL COOLING. RAW MODEL GUIDANCE FROM THE 12Z GFS IS NEARLY 10 DEGREES COLDER THAN THE NAM/SREF...ALTHOUGH MOS LOWS BETWEEN THE NAM AND GFS ARE RELATIVELY SIMILAR. WITH THE IDEAL RADIATIONAL COOLING CONDITIONS...HAVE NUDGED THE FORECAST IN THE DIRECTION OF THE RAW GFS FORECAST. OVERALL...THIS GENERALLY BROUGHT FRIDAY NIGHT LOWS DOWN 2-3 DEGREES...BUT DID NOT NEED TO EXPAND MENTION OF FROST TOO MUCH FROM PREVIOUS FORECAST. CONSIDERING LOW TEMPERATURE FORECAST ALONG THE INTERSTATE 80 CORRIDOR IS RIGHT AROUND FREEZING...DECIDED TO ISSUE A FREEZE WATCH FOR SEVERAL COUNTIES. HIGH PRESSURE WILL MOVE OFFSHORE SATURDAY NIGHT...AND LOW PRESSURE DEVELOPING ACROSS THE CENTRAL PLAINS WILL SPREAD A WIDE SWATH OF SHOWERS AND THUNDERSTORMS ACROSS THE CENTRAL UNITED STATES. HIGH CLOUDS WILL MOVE INTO THE AREA SATURDAY AND SATURDAY NIGHT...WITH LOWER CLOUDS MOVING IN FOR SUNDAY. HOWEVER...APPEARS THAT ANY LIFTING MECHANISM WILL REMAIN TOO FAR WEST TO PRODUCE ANY SHOWERS LOCALLY. SUNSHINE ON SATURDAY WILL ALLOW HIGHS TO RISE A FEW DEGREES WARMER THAN FRIDAY...WITH SOUTHERLY WINDS ON SUNDAY BRINGING ANOTHER INCREASE TO HIGHS. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... UPPER RIDGING OVER THE SOUTHEAST CONUS IS EXPECTED TO DEVELOP BY EARLY NEXT WEEK...AS SHORTWAVES ADVANCE ALONG THE NORTHERN PERIPHERY OF THE RIDGE. MAINTAINED CHANCE POPS FOR THE HOLIDAY FORECAST AS THE STRENGTH OF RIDGING AND PLACEMENT OF A SURFACE BOUNDARY ACROSS THE AREA REMAIN IN QUESTION. SOUTHWEST FLOW IS PROGGED TO DEVELOP THEREAFTER WITH OCCASIONAL WEAK EMBEDDED CROSSING SHORTWAVES. THIS WILL CONTINUE PERIODIC CHANCES FOR SHOWERS/STORMS. USED WPC BASED ENSEMBLE MEAN GUIDANCE WITH DOWNWARD ADJUSTMENTS TO POPS TO HELP SMOOTH OUT INDIVIDUAL MODEL DIFFERENCES...ESPECIALLY FOR MONDAY. TEMPERATURES SHOULD AVERAGE ABOVE SEASONAL LEVELS. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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HIGH PRESSURE AND DRIER AIR ARE QUICKLY MOVING IN FROM THE WEST. VFR CIGS WILL BREAK UP TO SCT CLOUDS DURING THE EVENING HOURS...AND VFR WITH LIGHT WINDS WILL CONTINUE THROUGH THE NIGHT. A FRONT SINKS INTO THE AREA ON FRIDAY. MOISTURE IS LIMITED...AND GUIDANCE INDICATES THAT SHRA SHOULD REMAIN WIDELY SCATTERED AND CIGS SHOULD REMAIN VFR. WINDS WILL KICK UP OUT OF THE WEST AS THE FRONT APPROACHES...AND NORTHWEST WINDS GUSTING TO 15-25 KNOTS ARE EXPECTED BEHIND THE FRONT. SKIES WILL THEN CLEAR TOWARDS SUNSET. CL .OUTLOOK.../FRIDAY THROUGH TUESDAY/... RESTRICTIONS ARE POSSIBLE WITH A LATE SUNDAY AND MONDAY WARM FRONT.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...FREEZE WATCH FROM LATE FRIDAY NIGHT THROUGH SATURDAY MORNING FOR PAZ007>009-015-016. WV...NONE. && $$

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