Area Forecast Discussion
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000 FXUS61 KPBZ 260856 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 356 AM EST WED NOV 26 2014 .SYNOPSIS...
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LOW PRESSURE SYSTEM IS EXPECTED TO DEVELOP AND RIDE NORTHWARD ALONG THE COAST TODAY...BRINGING ACCUMULATING SNOW PRIMARILY TO AREAS SOUTH AND EAST OF PITTSBURGH. A SECOND WEAKER SYSTEM WILL USHER IN COLDER TEMPERATURES AND LINGERING SNOW SHOWER CHANCES THROUGH THE END OF THE WEEK.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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THE 00Z KPBZ RAOB INDICATED A VAST DEPTH WITH DEWPOINT DEPRESSIONS THAT REVEALED A VERITABLE CHASM BETWEEN THE TEMPERATURE AND DEWPOINT CURVE IN THE VERTICAL. AS SUCH...A DEEP LAYER OF DRY AIR HAS ESTABLISHED RESIDENCE OVER THE AREA...WHICH SHOULD ALLOW FOR THE VAST MAJORITY OF THE CWA TO REMAIN DRY THROUGH SUNRISE IN ADVANCE OF THE SYSTEM ADVANCING NORTHEASTWARD UP THE EASTERN SEABOARD. THE EXCEPTION TO THIS IS LIKELY TO BE THE SOUTHEASTERN RIDGES OF THE CWA...WHERE KPBZ WSR-88D IS ALREADY SHOWING RETURNS. GIVEN THE COLUMN AT THE ONSET...THIS IS LIKELY MOSTLY STILL VIRGA FOR THE MOMENT. THIS VIRGA ON RADAR IS WELL OUT IN ADVANCE OF THE SYSTEM THAT IS SET TO SWING UP THE EAST COAST TODAY. THE UPPER FEATURE RESPONSIBLE FOR THIS SYSTEM IS STILL LARGELY A STRETCHING UPPER PV FEATURE THAT HAS NOT EVEN YET MANAGED TO CONGEAL INTO A BALL IN THE BASE OF THE UPPER TROUGH...AND IS STILL WELL CONNECTED WITH THE UPPER POLAR FRONT...WHICH SEEMS LIKELY TO PRECLUDE THIS PV FEATURE FROM ACQUIRING SIGNIFICANT ENOUGH DEPTH TO LOWER THE DYNAMICAL TROPOPAUSE TO ALLOW FOR SIGNIFICANT OCCLUSION OF THE SYSTEM AS IT EJECTS UP THE EAST COAST. THIS HAS IMPORTANT RAMIFICATIONS FOR OUR AREA AS WILL WILL BE DEPENDENT UPON MOISTURE BEING WRAPPED BACK WESTWARD OVER OUR AREA TO ESTABLISH A PRECIPITATION SHIELD OF ANY STRENGTH OR DURATION. GIVEN THAT AN OCCLUSION AND SIGNIFICANT BENDING BACK OF THE WARM CONVEYOR BELT INTO THE COLD AIR SEEMS RATHER UNLIKELY...THIS MEANS OUR WINDOW FOR SIGNIFICANT PRECIPITATION WILL BE FLEETING TODAY...AND THE MODEL SOLUTIONS HAVE LARGELY CONVERGED ON THE WINDOW BEING ALMOST EXCLUSIVELY THE 12Z-18Z TIME FRAME. GIVEN THE EASTWARD ADVANCE OF THE UPPER FRONT...AND THE SIGNIFICANT AMOUNT OF MIDDLE AND UPPER DRYING ASSOCIATED WITH IT...IT SEEMS THE WESTERN FLANK OF THE PRECIPITATION SHIELD IS LIKELY TO BE VERY STARK. THE GFS AND ECMWF BOTH ALLOW FOR THE BACK END OF THE PRECIPITATION TO PUSH WESTWARD ACROSS THE ENTIRE FORECAST AREA BY LATE THIS MORNING...WHILE THE CANADIAN AND NAM ARE NOT QUITE SO AGGRESSIVE...AND THE HIGHER RESOLUTION MODELS FAIL TO ALLOW IT TO EVEN REACH THE PA/OH BORDER. THAT SAID...MODEL MASS FIELDS FROM THE NAM/CANADIAN/GFS/ECMWF ARE ALL RATHER SIMILAR IN SUGGESTING A VERY STRONG UPWARD BRANCH OF A FRONTOGENETICAL CIRCULATION SPREADING RATHER BROAD LIFT INTO THE AREA CENTERED AROUND 15Z. MODEL CROSS SECTIONS ACROSS THIS FEATURE REVEAL A GOOD BIT OF INSIGHT INTO WHAT IT COULD MEAN. FIRST OF ALL...WHILE LIFT IS RATHER PERVASIVE AND SO IS SATURATION...ALL MODELS FAVOR LIFT WELL ABOVE THE DENDRITIC GROWTH ZONE ACROSS THE VAST MAJORITY OF THE FORECAST AREA THROUGHOUT THE EVENT...EXCEPT IN THE SOUTHEASTERNMOST PORTIONS OF THE CWA. THIS MEANS THAT WHILE THE MODELS ARE FAVORING GENERAL AMOUNTS OF BETWEEN 0.1 AND 0.25 INCHES OF QPF FOR MUCH OF THE LOWER ELEVATIONS OF WESTERN PA AND NORTHERN WV...POOR MICROPHYSICAL DENDRITIC GROWTH AND WARM GROUND TEMPERATURES WILL LIKELY WORK AGAINST SNOW RATIOS BEING PARTICULAR LILY EFFICIENT AT TURNING THAT QPF INTO SNOW. ADDITIONALLY...BOUNDARY LAYER TEMPERATURES MAY WELL TURN OUT TO BE PROBLEMATIC. WHILE WE ARE STARTING OFF THE DAY A BIT COLDER THAN THE MODELS WOULD SUGGEST AND WE MAY WELL WET BULB COOLER...TEMPERATURES ARE LIKELY TO STRADDLE FREEZING FOR MUCH OF THE EVENT IN THE LOWER ELEVATIONS. THIS SET OF CIRCUMSTANCES IS NOT EXACTLY A RECIPE FOR EFFICIENT ACCUMULATIONS. THAT SAID...DAYTIME RATIOS ON THE ORDER OF 7-9 TO 1 WERE FAVORED FOR MOST OF THE AREA. THE STORY QUICKLY CHANGES THE FARTHER SOUTHEAST YOU GO. NOT ONLY IS FRONTOGENETICAL FORCING DEEPER AND STRONGER...IT MUCH MORE CLOSELY OVERLAYS THE DENDRITIC GROWTH ZONE. FURTHERMORE...A DEEP LAYER OF WEAK STATIC STABILITY IS MANIFEST THROUGH THE STRONGEST PORTION OF THE UPWARD BRANCH OF SAID CIRCULATION. UNFORTUNATELY FOR THE SNOW LOVERS...THE RESIDENCE TIME OF THIS COMBINATION OF FEATURES OVER THE SOUTHEASTERN PORTIONS OF THE AREA IS ONLY ON THE ORDER OF 6 HOURS OR SO. THAT SAID...SNOW RATES OF AN INCH PER HOUR STILL SEEM REASONABLE ON AVERAGE OVER ABOUT 6 HOURS GIVEN QPF EXPECTATIONS AND RATIOS MORE ON THE ORDER OF 12-13 TO 1. THIS SEEMS LIKELY TO YIELD UP TO AROUND 8 INCHES IN PORTIONS OF GARRETT AND TUCKER COUNTIES...WITH THE NUMBERS QUICKLY FALLING OFF NORTHWEST OF THERE. GIVEN ALL THE AFOREMENTIONED LOGIC FOR ACCUMULATIONS...HEADLINES WERE ALLOWED TO REMAIN AS IS FOR THIS SHIFT. WHILE ADVISORY AMOUNTS GVEN THE QPF WITH MICROPHYSICAL PROBLEMS MAY NOT BE REACHED IN PORTIONS OF THE ADVISORY AREA...THE QPF SPREAD WAS STILL WIDE ENOUGH TO ALLOW FOR THE POSSIBILITY OF ADVISORY AMOUNTS EVERYWHERE THEY WERE PREVIOUSLY ISSUED...AND GIVEN THE BASIC 100 PERCENT CHANCE OF SNOW...IT MAY WELL BE A DISSERVICE TO CANCEL SAID ADVISORIES WHEN IT IS STILL CLEARLY GOING TO SNOW...AND LIKELY ACCUMULATE AT LEAST LIGHTLY. ADDITIONALLY...WHILE THE WARNINGS MAY NOT BE ACHIEVED IN PORTIONS OF THE RIDGES...HERE TOO THE SAME LOGIC HOLDS. SO...HEADLINES REMAIN IN PLACE WITH ACCUMULATION FORECAST JUST HEAVILY SKEWED TOWARD THE SOUTHEASTERNMOST PORTIONS OF THE AREA...QUICKLY TAPERING BACK TO AN INCH OR LESS BY THE TIME YOU GET AS FAR NORTHWEST AS JUST WASHINGTON PA. TEMPERATURES WERE CONSTRUCTED USING NAM CURVES GENERALLY. HOWEVER...A LOWER STARTING POINT AND EXPECTATIONS FOR WET BULB EFFECTS EARLY THIS MORNING COMBINED WITH DYNAMICAL COOLING FOR A GOOD PORTION OF THE LATE MORNING AND EARLY AFTERNOON SHOULD PRECLUDE ANYONE FROM WANDERING MUCH ABOVE FREEZING WHERE IT IS SNOWING. AS A RESULT...ALL SITES ARE EXPECTED TO REMAIN ALL SNOW WHERE THE PRECIPITATION REMAINS COHERENT. THINGS SHOULD QUICKLY WIND DOWN FROM WEST TO EAST AS THE UPPER FRONT MOVES SWIFTLY FROM WEST TO EAST THIS AFTERNOON. FRIES
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/... ANY LINGERING SNOW IN THE FAR EAST THIS EVENING WILL END QUICKLY AS THE LIFT AND MOISTURE HEAD TOWARD THE COAST WITH THE EXITING LOW. HUGE DRY SLOT WILL OVERTAKE THE ENTIRE AREA THIS EVENING AND MIXING HEIGHTS WILL BE WELL BELOW THE DENDRITIC GROWTH ZONE... ENDING THE ATMOSPHERES ABILITY TO CREATE SNOW. CAA WILL BE GOING ALL NIGHT...BUT WITH THE LACK OF DEEP MOISTURE AND A RATHER WEAK AND UNORGANIZED FLOW ALOFT...DO NOT EXPECT ANY SNOW SHOWERS TO REDEVELOP. FOR THE MOST PART...THURSDAY MORNING LOOKS DRY AS THE FLOW ALOFT REMAINS UNORGANIZED AND MIXING HEIGHTS ARE JUST TOO LOW TO REACH INTO THE SNOW GROWTH ZONE. SHORTWAVE TROUGH WILL SWING THROUGH LATE THURSDAY...INCREASING LARGE SCALE ASCENSION AND BRINGING ANOTHER SHOT OF COLDER AIR THURSDAY NIGHT. WINDS ALOFT WILL INCREASE IN SPEED AND BECOME WEST-NORTHWESTERLY. THE SNOW GROWTH ZONE BEGINS TO LOWER AND LAKE ERIE WILL COME INTO PLAY. WILL BRING POPS BACK IN ACROSS THE NORTH AND RIDGES THURSDAY AFTERNOON. CAA CONTINUES THURSDAY NIGHT BEHIND EXITING SHORTWAVE TROUGH. FOR A TIME...WINDS ALOFT LOOK GOOD FOR LAKE INFLUENCES AND THE SNOW GROWTH ZONE WILL DROP INTO AN AREA OF HIGHER SATURATION. EXPECT AN INCREASE IN SNOW SHOWER COVERAGE THURSDAY NIGHT...WITH THE NORTH AND RIDGES THE MOST LIKELY LOCATIONS FOR ACCUMULATING SNOW. SURFACE HIGH WILL BUILD IN ON FRIDAY AND THE ENTIRE ATMOSPHERE WILL DRY OUT DURING THE MORNING HOURS. COULD SEE SOME LINGERING FLURRIES ACROSS THE NORTH AND RIDGES FRIDAY MORNING...BUT THESE TOO WILL END AS SUBSIDENCE AND THE DRIER AIR TAKES OVER. A COLD DAY FRIDAY WITH TEMPS WELL BELOW SEASONABLE LEVELS. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... MODELS COMING INTO GOOD AGREEMENT WITH ANOTHER WARM UP FOR THE WEEKEND. MODELS ARE STILL TRYING TO FIGURE OUT HOW MUCH PRECIP WILL DEVELOP SATURDAY NIGHT AND SUNDAY WITH PASSING WAVES IN THE ZONAL FLOW ALOFT. EXPECT MOST OF SATURDAY NIGHT TO BE DRY WITH A SHOWER POSSIBLE BY SUNDAY MORNING. LOOKING LIKE A BETTER CHANCE FOR SHOWERS SUNDAY...PARTICULARLY IN THE AFTERNOON...AHEAD OF AN APPROACHING COLD FRONT. AREA WILL DRY OUT MONDAY WITH ANOTHER COOL DOWN ON TUESDAY. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... VFR CONDITIONS SHOULD CONTINUE UNTIL DAWN AT MOST SITES. SNOW WILL ADVANCE NORTH-NORTHWESTWARD THROUGH THE MORNING HOURS AND AFFECT MOST SITES OTHER THAN KZZV IN THE EARLY AFTERNOON. SOME DETERIORATION TO IFR IS LIKELY GENERALLY EAST OF KPIT DURING THE HEAVIEST WINDOW OF SNOW...HOWEVER THE WINDOW FOR ACCUMULATION AT ALL SITES OTHER THAN KMGW SHOULD BE FAIRLY BRIEF. AS SNOW SHUTS OFF BY EVENING...ABUNDANT LOW LEVEL MOISTURE SHOULD ALLOW FOR STRATUS TO REMAIN A FIXTURE AT MOST SITES. FRIES OUTLOOK.../WEDNESDAY NIGHT THROUGH SUNDAY/... PERIODIC RSTRNS ARE PSBL THRU THE WKEND WITH SVRL FAST MOVG DISTURBANCES. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...WINTER STORM WARNING UNTIL 6 PM EST THIS EVENING FOR MDZ001. OH...NONE. PA...WINTER STORM WARNING UNTIL 6 PM EST THIS EVENING FOR PAZ074- 076. WINTER WEATHER ADVISORY UNTIL 6 PM EST THIS EVENING FOR PAZ023- 073-075. WV...WINTER STORM WARNING UNTIL 6 PM EST THIS EVENING FOR WVZ023- 041. WINTER WEATHER ADVISORY UNTIL 6 PM EST THIS EVENING FOR WVZ021- 022. && $$

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