Area Forecast Discussion
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000 FXUS61 KPBZ 301936 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 336 PM EDT MON MAR 30 2015 .SYNOPSIS... LOW PRESSURE WILL TRACK ACROSS THE REGION TONIGHT INTO TUESDAY AND BRING ANOTHER ROUND OF PRECIPITATION. A WARMING TREND WILL COMMENCE LATER THIS WORK WEEK. && .NEAR TERM /THROUGH TUESDAY/... MUCH DRIER BOUNDARY LAYER AIR IS CONTINUING TO FLOW INTO THE REGION THIS AFTERNOON BEHIND THIS MORNING`S COLD FRONTAL PASSAGE. LIKEWISE...MUCH COLDER AIR OFF THE DECK IS ADVANCING SOUTHEASTERN TOWARD OUR AREA THIS AFTERNOON...AND GIVEN THE AMOUNT OF LATE MORNING SUNSHINE...HAS RESULTED IN FAIRLY WIDESPREAD REDEVELOPMENT OF FAIR WEATHER CUMULUS. GIVEN UPSTREAM TRENDS...ENOUGH INSTABILITY SEEMS TO BE PRESENT THAT RAIN SHOWERS HAVE DEVELOPED TO OUR NORTHWEST...AND MAY WELL INTERSECT OUR NORTHEASTERN REACHES OF OUR FORECAST AREA LATE THIS AFTERNOON AS LOW LEVEL LAPSE RATES FURTHER STEEPEN...HOWEVER THE BULK OF THE CWA SHOULD REMAIN DRY THROUGH THE EVENING WITH ONLY GUSTY WINDS TO CONTEND WITH. THE REAL FUN PART OF THE FORECAST ROLLS IN LATER TONIGHT. IT FEELS LIKE I HAVE TYPED THIS TOO MANY TIMES THIS WINTER...BUT OUR CWA IS LIKELY TO AGAIN MANAGE TO BE IN THE TRANSITION ZONE FROM RAIN TO SNOW AS THE NEXT IN THE SEEMINGLY ENDLESS SERIES OF NORTHWESTERLY FLOW QUICK-MOVING SYSTEMS CRASHES OUR SUNSHINE PARTY. LUCKILY ENOUGH...MODEL-TO-MODEL VARIANCES IN THE TRACK OF THE SYSTEM HAVE BEEN MINIMIZED...HOWEVER OTHER DISCREPANCIES ON THE HANDLING OF THE BOUNDARY LAYER AS WELL AS THE DEFORMATION FIELD MAKE PARTICULARLY THE SNOWIER SIDE OF THE SYSTEM QUITE A TRICKY BEAST TO DEAL WITH. ROUGHLY SPEAKING...BY 12Z TUESDAY...THE CONSENSUS ALL FAVORS A SURFACE LOW NEAR SANDUSKY OHIO...WITH A TRACK DURING THE DAY ON TUESDAY ROUGHLY DOWN THE OHIO/PENNSYLVANIA TURNPIKES. AS SUCH...THE THERMAL GRADIENT SEEMS TO PACK UP A BIT NORTH OF THERE...ROUGHLY FAVORING THE I-80 CORRIDOR. NORTH OF I-80...EVEN 925 MB TEMPERATURES FAVOR SUB 0C TEMPERATURES FOR A TIME LATE TONIGHT AND INTO TUESDAY MORNING...WHICH MEANS ACCUMULATING SNOW WILL BE LIKELY FOR A PERIOD WITH THIS SYSTEM ALONG AND NORTH OF I-80. HOWEVER...HOW MUCH ACCUMULATION THERE WILL BE IS A BEAST IN AND OF ITSELF. QPF VARIANCES AMONGST THE MODELS ARE VAST. IN FACT...THE NAM IS ROUGHLY TWICE AS WET AS SOME OTHER GUIDANCE...WITH THE MOST LIKELY TOTAL FAR LOWER THAN THIS OWING TO UNREASONABLY HIGH OMEGA VALUES IN THE DEFORMATION ZONE ON THE NORTH SIDE OF THIS SYSTEM IN THE NAM. THIS IS A SYSTEMATIC ISSUE THAT HAS BEEN NOTICED THIS WINTER...AND GIVEN THAT THE IRRATIONAL OMEGA VALUES WOULD BE WHAT WOULD BE REQUIRED TO GENERATE LIFT OF THE DEPTH REQUIRED TO FULLY BISECT THE DENDRITIC GROWTH ZONE...HIGH SNOW-TO-LIQUID-RATIO ACCUMULATIONS SEEM FAIRLY UNLIKELY. THIS IS FURTHER SUPPORTED BY WARM ANTECEDENT CONDITIONS...AND THE FACT THAT THE BULK OF THE PRECIPITATION FALLS DURING THE DAY ON TUESDAY. THAT SAID...IT IS WELL KNOWN THAT IF YOU GET A SLUSHY INCH OF SNOW...ACCUMULATION CAN OCCUR VERY READILY ON TOP OF THAT...SO WE ARE RUNNING A VERY FINE LINE ACROSS THE NORTHERN TIER. ALL OF THIS CONSIDERED...SNOW AMOUNTS WERE GENERATED AND BASED UPON A FAIRLY ELEVATION DEPENDENT SCHEME DUE TO THE FACT THAT SURFACE TEMPERATURES WILL BE SO HIGHLY MARGINAL. THIS HAS ALLOWED FOR SOME AREAS OF 2-3 INCH ACCUMULATIONS IN FOREST/NORTHERN JEFFERSON/NORTHERN VENANAGO...WITH LOWER AMOUNTS IMMEDIATELY SOUTH OF THERE. IT MUST BE NOTED...SHOULD HIGH PRECIPITATION RATES DEVELOP PRIOR TO SUN UP...THERE IS POTENTIAL FOR QUICK ADVISORY CRITERIA AMOUNTS...HOWEVER THIS DOES NOT SEEM LIKE THE MOST LIKELY SCENARIO AT THIS TIME. SOUTH OF THE TRACK OF THE SYSTEM...LOWER CHANCES OF MOSTLY RAIN ARE LIKELY THROUGH THE MORNING AS DEFORMATION AND ISENTROPIC ASCENT FAVOR THE NORTHERN REACHES OF THE CWA. BY AFTERNOON HOWEVER...THE MAIN SHORT WAVE TROUGH STARTS TO SWING THROUGH AND LAPSE RATES INCREASE RAPIDLY ALONG THE COLD FRONT...ALLOWING FOR THE EXPANSION OF RAIN SHOWERS OVER MOST OF THE CWA FOR A TIME. GUSTY WINDS WILL AGAIN BE COMMON ALONG THE FRONT AS THE MIXED LAYER DEPTH INCREASES. HOWEVER...BEFORE THE COLD FRONT ARRIVES...TEMPERATURES AT LEAST IN THE WARM SECTOR SHOULD APPROACH BALMY NORMAL LEVELS BEFORE GETTING KNOCKED BACK BY THE FRONT. FRIES && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... PRECIPITATION SHOULD END RATHER ABRUPTLY TUESDAY NIGHT...SAVE FOR THE HIGHER TERRAIN...AS THE LOW PRESSURE MOVES WELL OFF TO THE EAST AND NW FLOW ENSUES. THE BACK EDGE OF THE UPPER TROUGH AND SOME MID LEVEL MOISTURE SHOULD MAINTAIN SOME CLOUD COVER THROUGH MUCH OF THE NIGHT BEFORE HIGH PRESSURE RIDGE BUILDS EASTWARD AND DRIES OUT THE LOW-MID LEVELS. DESPITE MOSTLY SUNNY SKIES BY MIDDAY WEDNESDAY...NW FLOW SHOULD KEEP TEMPERATURES CLOSE TO OR JUST BELOW SEASONAL AVERAGES. A RETURN OF SOUTHWEST FLOW IS EXPECTED AHEAD OF THE NEXT COLD FRONT FORECAST TO MAKE IT`S APPROACH THURSDAY NIGHT. WHILE WE MAY DECOUPLE WEDNESDAY NIGHT...ALLOWING FOR TEMPERATURES TO FALL BACK INTO THE 30`S...HIGH TEMPERATURES THURSDAY ARE EXPECTED TO BE WELL INTO THE 60`S AND APPROACHING 70 FOR POINTS FROM PITTSBURGH ON SOUTH. THIS IS IN PART DUE TO STRONG WARM ADVECTION ALOFT WITH 850HPA TEMPERATURES APPROACHING +10C. IN ADDITION TO THE WARMING TEMPERATURES...A 50KT LOW LEVEL JET MAY PROVIDE FOR SOME GUSTY WINDS OUT AHEAD OF AND ALONG THE SLOW MOVING COLD FRONT. HOWEVER...THE INTRICATE DETAILS OF THE FORECAST ON THURSDAY WILL BE DETERMINED LATER AS THERE IS STILL A BIT OF UNCERTAINTY IN THE FRONTAL TIMING BETWEEN THE THREE DETERMINISTIC MODELS AND THE ENSEMBLES. THIS TIMING WOULD IMPACT THE DEVELOPMENT OF ANY SHOWERS AND STORMS AHEAD OF THE FRONT...AND SUBSEQUENT ISSUES WITH WIND. THUS...HAVE OPTED TO STAY RELATIVELY CLOSE TO THE PREVIOUS FORECAST BUT WANTED TO KEEP VERY LOW POPS UNTIL LATE THURSDAY AFTERNOON. && .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... DECENT PRECIPITATION CHANCES WILL CONTINUE INTO THE WEEKEND AS THE FRONT SAGS ACROSS THE REGION AND PROVIDES FOCUS FOR SUBSEQUENT LOW PRESSURE PASSAGE LATER ON FRIDAY...WITH AN EASTWARD EXIT ON SATURDAY. THIS FORECAST WAS BASED ON A BLEND OF GFS TRENDS AND WPC GUIDANCE WITH SHORTWAVE TIMING AND BOUNDARY PLACEMENT LIKELY TO REMAIN QUESTIONABLE FOR A FEW DAYS. THE THURSDAY WARMTH WILL THUS BE SHORT LIVED...BUT THE COOLDOWN WILL NOT BE AS DRAMATIC AS THIS PAST WEEKEND GIVEN THE ZONAL FLOW THAT IS PROGGED ALOFT. TWEAKED WPC GUIDANCE WAS USED FOR THE SEASONABLY COOL TEMP FORECAST. && .AVIATION /19Z MONDAY THROUGH SATURDAY/... VFR CONDITIONS HAVE MOVED OVER THE AREA AS LAST NIGHT`S SYSTEM HAS EXITED QUICKLY. VFR CONDITIONS ARE LIKELY TO CONTINUE UNTIL THE NEXY SYSTEM SPREADS RAIN AND SNOW ACROSS THE AREA BY EARLY TUESDAY. LIFR CONDITIONS ARE QUITE LIKELY IN SNOW AT KFKL AND KDUJ AS SNOW RATES INCREASE ON TUESDAY MORNING...WITH VERY GUSTY WINDS LIKELY EVERYWHERE SOUTH OF KBVI AS THE WARM SECTOR AND RAIN SHOWERS ADVANCE NORTHWARD. FRIES .OUTLOOK.../TUESDAY THROUGH FRIDAY/... RESTRICTIONS ARE LIKELY WITH CROSSING LOW PRESSURE ON TUESDAY AND AGAIN LATE THURSDAY AND FRIDAY. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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