Area Forecast Discussion
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000 FXUS61 KPBZ 231935 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 235 PM EST TUE DEC 23 2014 .SYNOPSIS... LOW PRESSURE IN THE MIDWESTERN STATES WILL BRING WARMER TEMPERATURES AND OCCASIONAL RAIN CHANCES TO THE REGION THROUGH WEDNESDAY. GUSTY WINDS WILL ACCOMPANY THE PASSAGE OF A COLD FRONT ON CHRISTMAS DAY. && .NEAR TERM /THROUGH WEDNESDAY/... CWA IS MOSTLY IN THE WARM SECTOR NOW...WITH RISING CEILINGS AND GENERAL LACK OF SHRA...SAVE FOR A SMALL CLUSTER NEAR THE OH/PA BORDER. THIS GENERAL TREND WILL CONTINUE INTO THE EARLY EVENING. MODELS GENERALLY AGREE ON INTENSIFYING SURFACE LOW LIFTING THROUGH THE TENNESSEE AND OHIO VALLEYS...WITH THE DEEP CENTER NEAR YYZ BY 00Z THURSDAY. PWATS RISE TO NEAR/ABOVE AN INCH...WHICH IS AROUND 2 STANDARD DEVIATIONS ABOVE NORMAL FOR DECEMBER. THIS COMBINED WITH STRONG 300K ISENTROPIC UPGLIDE WILL BRING RAIN BACK IN BY MIDNIGHT. CONTINUED WITH CATEGORICAL POPS. INITIAL ONSET WILL BE HINDERED BY RAIN SHADOW EFFECT WITH SE FLOW...BUT EXPECT THIS TO FILL IN WITH TIME. INITIAL SHOT PUSHES THROUGH DURING THE MORNING WITH WARM FRONT...WITH ANOTHER GENERAL LULL IN ACTIVITY DURING THE LATE MORNING/EARLY AFTERNOON. HARD-CHARGING COLD FRONT WILL ARRIVE DURING THE LATE AFTERNOON...AND WILL BE IN THE VICINITY OF THE OH/PA BORDER BY 00Z THURSDAY AS THE SYSTEM STARTS TO OCCLUDE. STRONG WIND FIELD TAKES SHAPE WITH H850 WINDS OF 50-55 KNOTS AND H500 AT A SCREAMING 100 KNOTS. WENT WITH A PERIOD OF CATEGORICAL POPS ALONG FRONT FOR EXPECTED THIN LINE OF ACTIVITY. THE QUESTION IS WILL ENOUGH INSTABILITY BE REALIZED TO GENERATE LOW-TOPPED CONVECTION...AND THEREFORE GET THE STRONG WINDS TO THE SURFACE AHEAD OF THE FRONT. WITH DEWPOINTS AROUND 50 DURING THE AFTERNOON AND POSSIBLY EVEN SOME BREAKS IN THE CLOUDS...THINK WE MAY GET JUST ENOUGH INSTABILITY TO GET A RUMBLE OR TWO...AND THUS ADDED THIS POSSIBILITY. SPC HAS MARGINAL SEVERE RISK FOR WIND GUSTS. AGREE ON POTENTIAL BUT UNSURE ENOUGH TO HOLD OFF MENTION OF GUSTY WINDS IN GRIDS...AND WILL KEEP WIND GUST VALUES A BIT LOWER AHEAD OF THE FRONT AS WELL. THINK BETTER RISK OF ADVISORY-LEVEL WIND WILL COME WITH DOWNWARD MOMENTUM TRANSFER BEHIND THE BOUNDARY. PATTERN MATCHES UP FAIRLY WELL WITH LOCAL STUDY WHICH ILLUSTRATES RISK OF WIDESPREAD WIND POTENTIAL IN SIMILAR SITUATIONS. GIVEN TIME TO ONSET...AND IN COLLABORATION WITH NEIGHBORS...ELECTED TO HOLD OFF ON ISSUANCE OF WIND ADVISORY AT THIS TIME. MAINTAINED HWO MENTION AND ALERTED PARTNERS VIA EMAIL OF A POSSIBLE HEADLINE ISSUANCE WITH TIME. TEMPERATURES CHANGED MODESTLY FROM PREVIOUS NUMBERS...OBVIOUSLY WELL ABOVE NORMAL IN THIS REGIME. CL && .SHORT TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... STRENGTHENING SURFACE LOW PRESSURE SYSTEM WILL BEGIN TO CROSS THE GREAT LAKES AND SHIFT NORTHEAST WHILE IT DRAGS A COLD FRONT ACROSS THE FORECAST AREA WEDNESDAY EVENING. DEEP MOISTURE AND LIFT WILL ALLOW FOR RAIN TO MOVE ACROSS THE REGION WEST TO EAST BEFORE CHANGING OVER TO A MIX OF RAIN/SNOW AND SNOW SHOWERS EARLY THURSDAY AS COLD ADVECTION PERMEATES THE ATMOSPHERE. LOW INVERSION LEVELS...LIMITED MOISTURE...AND SURFACE TEMPERATURES WILL LIMIT ANY ACCUMULATION TO A TRACE OR NULL. MAIN CONCERN IS GUSTY SURFACE WINDS AS PRESSURE GRADIENT TIGHTENS AND LOW LEVEL WINDS BEGIN TO MIX DOWN BEHIND THE FRONT. THE MENTION OF GUSTY WINDS REMAINS IN THE HWO WILL CONTINUE TO MONITOR FOR POSSIBLE ADVISORIES. LOCAL RESEARCH INDICATES THAT THE SYNOPTIC SETUP CALLS FOR A WIDESPREAD WIND EVENT. SURFACE HIGH PRESSURE WILL BUILD RAPIDLY OVER THE SOUTH BY THURSDAY LATE AFTERNOON/EVENING...THUS ELIMINATING PRECIPITATION CHANCES THURSDAY NIGHT THROUGH FRIDAY. A SHORTWAVE QUICKLY CROSSING THE GREAT LAKES MAY CAUSE RAIN/SNOW SHOWERS ON SATURDAY. TEMPERATURES FOLLOWING THE FRONTAL PASSAGE WILL LOWER TOWARDS SEASONAL AVERAGES BUT REMAIN SLIGHTLY WARMER THAN NORMAL. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... DRY CONDITIONS WILL CONTINUE FRIDAY NIGHT BEFORE ANOTHER FRONTAL SYSTEM IMPACTS THE REGION OVER THE WEEKEND. MODELS CONTINUE TO SHOW RELATIVELY POOR CONSENSUS WITH THIS SYSTEM SO HAVE OPTED TO WEIGH HEAVILY ON ENSEMBLE GUIDANCE IN MAKING ANY ADJUSTMENTS TO THE ONGOING FORECAST. IN ANY CASE TEMPERATURES LOOK TO REBOUND ON SATURDAY UNDER SOUTHWESTERLY FLOW...WITH FORECAST TEMPERATURES BACK UP TO AROUND 15 DEGREES ABOVE NORMAL EVEN IF THE FASTER OPERATIONAL GFS SOLUTION WERE TO VERIFY. IF THE OVERALL CONSENSUS TRENDS TOWARDS THE SLOWER EC...TEMPERATURES WOULD NEED TO BE INCREASED SEVERAL DEGREES FROM CURRENT FORECAST VALUES. IN ADDITION TO TIMING...THERE IS CONSIDERABLE DISAGREEMENT IN PRECIPITATION FIELDS. THE EC FAVORS A MORE NORTHERLY TRACK WHICH WOULD RESULT IN MUCH MORE WIDESPREAD PRECIPITATION OVER THE WEEKEND...WHILE THE GFS MAINTAINS A SOUTHERLY TRACK ...WHICH WOULD RESULT IN A MUCH SMALLER WINDOW OF ACTIVE WEATHER/PRECIPITATION. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... CIGS LIFTING AS WARM SECTOR OVERSPREADS AREA. MOST TERMINALS WILL BE VFR THIS AFTERNOON/EARLY EVENING FOR A TIME AT LEAST...SAVE FOR PERHAPS FKL. RAIN RETURNS LATER TONIGHT AS THE NEXT SURFACE LOW GETS ORGANIZED OVER THE TENNESSEE VALLEY...DROPPING CIG/VIS INTO MVFR AND THEN IFR CATEGORY AFTER 07Z OR SO. HAVE SOME SLIGHT IMPROVEMENT BACK TO MVFR AROUND 14/15Z AHEAD OF COLD FRONT...WHICH SWEEPS THROUGH AFTER THE TAF PERIOD. FOR THE PIT 30 HOUR TAF...INSERTED VCTS AFTER 24/21Z WITH SOME MEAGER INSTABILITY PERHAPS INDUCING LOW-TOPPED CONVECTION. OUTLOOK.../WEDNESDAY THROUGH SUNDAY/... CIG/VIS RESTRICTIONS THROUGH THURSDAY WILL IMPROVE TO VFR FRIDAY AND CONTINUE UNTIL ANOTHER FRONTAL SYSTEM CROSSES THE REGION SATURDAY AFTERNOON. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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