Area Forecast Discussion
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000 FXUS61 KPBZ 211423 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 923 AM EST SUN DEC 21 2014 .SYNOPSIS... HIGH PRESSURE WILL BE IN CONTROL THROUGH MONDAY. EXPECT INCREASING PRECIPITATION CHANCES TUESDAY. STORM SYSTEM BRINGS RAIN AND WIND CHRISTMAS EVE...AND SNOW SHOWERS CHRISTMAS DAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... NO MAJOR CHANGES NEEDED TO THE FORECAST FOR TODAY. A FEW BREAKS IN THE STRATUS DECK ARE EVIDENT IN THE SAT LOOP. CLOUD DECK IS FAIRLY SHALLOW...EVIDENCED BY VERY DRY AIR ABOVE H700 ON SOUNDING. NEW NAM RUN SEEMS TO HAVE A GOOD HANDLE ON H850 MOISTURE CURRENTLY...AND DRIER AIR AT THAT LEVEL IN OHIO IS SET TO ARRIVE THIS AFTERNOON ON LIGHT W FLOW. THUS...STILL EXPECT A DECREASING TREND IN CLOUD COVER WITH TIME. TEMPERATURES ON TRACK FOR HIGHS IN THE LOWER TO MID 30S. && .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY/... SURFACE HIGH WILL SLIDE EASTWARD TONIGHT...AS WILL THE UPPER LEVEL RIDGE. WAA WILL GET STARTED MONDAY MORNING AND CONTINUE THROUGHOUT THE DAY. THE WARMER AIR WILL ALSO PULL MOISTURE NORTHWARD... INCREASING CLOUDS ONCE AGAIN. BIG QUESTION ON MONDAY IS THE OUTER EDGE OF THE PRECIP SHIELD THAT IS ASSOCIATED WITH EASTERN SEABOARD SYSTEM. IT APPEARS THAT SOME OF THIS PRECIP COULD CLIP THE SOUTHEASTERN RIDGES ON MONDAY. THE PROBLEM WILL BE THE COLD AIR TRAPPED IN THE RIDGES...THANKS TO THE LOW-LEVEL EASTERLY FLOW. CONFIDENCE IS LOW HERE...AS MODELS HAVE BOUNCED BACK AND FORTH WITH HOW FAR WEST TO SPREAD THE PRECIP. THERE IS VERY LITTLE UPPER LEVEL SUPPORT THIS FAR WEST...BUT WITH THE EASTERLY FLOW... SHOWERS MAY BE ABLE TO CLIP THE AFOREMENTIONED RIDGES. SURFACE TEMPS WILL BE CRITICAL...BUT IT DOES APPEAR THAT AT LEAST THE SHELTERED VALLEYS WILL BE ABLE TO HOLD ON TO TEMPS AT OR BELOW FREEZING. WILL NEED TO INCLUDE THE MENTION OF FREEZING RAIN IN THESE AREAS MONDAY. LOW-LEVEL JET WILL GET STRONG WAA GOING MONDAY NIGHT AND TUESDAY. STILL TOUGH TO GET A GOOD HANDLE ON HOW MUCH RAIN WILL REACH THE REGION ON TUESDAY. THE UPPER LEVEL SUPPORT IS WEAK AND UNORGANIZED...BUT THE LOW-LEVEL JET WILL PULL MOISTURE NORTHWARD AS WELL. WILL INCREASE RAIN CHANCES FROM WEST TO EAST TUESDAY...BUT KEEP EVERYTHING IN THE CHANCE CATEGORY. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... HIGHLIGHTS: - A WET CHRISTMAS EVE - TEMPERATURES RETURN BACK TO NORMAL VALUES CHRISTMAS DAY - SLIGHTLY ABOVE NORMAL TEMPERATURES RETURN BY FRIDAY STRONG LOW PRESSURE SYSTEM THAT HAS BEEN MUCH ADVERTISED...GIVEN THE TIME PERIOD WHICH IT AFFECTS OUR AREA...IS STILL ON TRACK. LONG RANGE ENSEMBLE PREDICTION SYSTEMS AND OPERATIONAL MODELS ARE CONVERGING ON A TRACK THROUGH IND / ILL THEN NORTH INTO CANADA. THIS WILL MAKE FOR ALL LIQUID PRECIPITATION ON THE FRONT END. TEMPERATURES WILL SOAR TO NEARLY 20 DEGREES ABOVE NORMAL WEDNESDAY WITH 50S COMMON. COUPLED WITH INFLUX OF MOISTURE FROM THE GULF OF MEXICO...SOUNDINGS ILLUSTRATE ELEVATED INSTABILITY WHICH PUSHES TEMP DIFFERENTIAL BETWEEN H8 AND H5 TO NEARLY 0C. THIS COULD LEAD TO ISOLATED THUNDERSTORMS AHEAD OF THE BAROCLINIC ZONE. FOR THE TIME BEING...OPTED TO LEAVE THUNDER OUT OF THE FORECAST AND WAIT FOR BETTER CONTINUITY GIVEN THIS STILL IS FIVE DAYS OUT. DESPITE PWATS NEARLY TWO STANDARD DEVIATIONS ABOVE NORMAL...COMBINATION OF KATABATIC WINDS AND DRY CONVEYOR BELT WILL LIMIT QPF POTENTIAL TO NEGATE ANY FLOOD THREAT. LONG RANGE HYDROLOGIC MODELS SHOW VERY LITTLE RISE IN AREA RIVERS...WHICH CORRELATES WELL TO THE PROJECTED QPF OF 0.50 TO 1.00". WITH THE TRACK OF THE CYCLONE DUE N...COLD AIR INJECTED INTO THE AREA WILL BE OF MIDWEST ORIGIN...SO TEMPS WILL RETREAT BACK TO SEASONAL BENCHMARKS CHRISTMAS DAY. FOR THOSE WHO ARE LOOKING FOR SNOW ON CHRISTMAS...A CHANCE OF SNOW SHOWERS EXIST...BUT GIVEN TRAJECTORY FROM THE SOUTHWEST ANY ACCUMULATION ASSOCIATED WITH THE UPPER LEVEL SYSTEM WILL BE LIGHT AT BEST. HISTORICALLY...THERE IS ONLY A 17 PERCENT CHANCE OF AT LEAST ONE INCH OF SNOW ON THE GROUND AT PITTSBURGH CHRISTMAS MORNING. H8 WINDS PUSH 50KTS WEDNESDAY NIGHT INTO THURSDAY AND HIGHEST ANOMALIES RESIDE ALONG THE LAKE SHORE...SO EXPECT HIGHEST GUSTS WOULD DO SO AS WELL. NONETHELESS...PERIOD FROM WEDNESDAY NIGHT INTO THURSDAY STRONG ISALLOBARIC FLOW COULD YIELD GUSTS IN EXCESS OF 40 MPH ESPECIALLY IN THE MOUNTAINS OF WRN MD AND NRN WV. BALANCE OF THE EXTENDED IS CHARACTERIZED BY UNCERTAINTY AS ONE WOULD EXPECT IN THE DAY 6 -7 TIME FRAME. ECMWF HAS A STRONGER SYSTEM CROSSING FRIDAY NIGHT VERSUS GFS WHICH IS MUCH WEAKER. EITHER CASE PTYPE SHOULD BE RAIN WITH SNOW SHOWERS ON THE BACKSIDE. NO PLUNGES OF ARCTIC AIR ARE FORECAST IN THE NEXT 10 DAYS. MAX/MIN TEMPERATURES WERE A BLEND OF THE FORECAST AND ECMWF BIAS CORRECTED DATA SET. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... SPOTTY MVFR CIG RESTRICTIONS WILL IMPROVE THIS MORNING AS WINDS ALOFT TRANSITION TO THE SOUTHWEST AHEAD OF A LOW PRESSURE SYSTEM DEVELOPING OVER THE CENTRAL CONUS. OUTLOOK.../SUNDAY NIGHT THROUGH THURSDAY/... NEXT STORM SYSTEM WILL BEGIN TO IMPACT THE REGION ON TUESDAY AND RESULT IN WIDESPREAD RAIN...CIG/VIS RESTRICTIONS...AND PERIODS OF LLWS INTO THURSDAY. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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