Area Forecast Discussion
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000 FXUS61 KPBZ 172221 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 621 PM EDT FRI OCT 17 2014 .SYNOPSIS... A STRONG COLD FRONT ADVANCES ACROSS THE REGION LATE TONIGHT AND SATURDAY WITH SHOWER CHANCES THROUGH SUNDAY MORNING. && .NEAR TERM /THROUGH SATURDAY/...
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WITH 615PM UPDATE...PRIMARY CHANGE WAS TO INCREASE CLOUD COVER DURING THE EVENING HOURS. MID LEVEL CLOUD HAVE MOVED IN FASTER THAN ANTICIPATED...AND HAVE BUMPED UP CLOUD COVER ACCORDINGLY. THE ADDITIONAL CLOUD COVER SHOULD ALSO SLOW DOWN DROP IN TEMPERATURES DURING THE EVENING HOURS...ALTHOUGH WITH WIDESPREAD CLOUD COVER ALREADY IN THE FORECAST OVERNIGHT...DO NOT THINK THIS WILL IMPACT OVERNIGHT LOWS. PREVIOUS DISCUSSION FOLLOWS. CURRENT VISIBLE SATELLITE IMAGE SHOWS LOW STRATUS ACROSS OUR NERN ZONES ERODING FROM THE WEST WITH CLEAR SKIES TAKING ITS PLACE. STRIATIONS IN THE LLVL CLOUD DECK INDICATE STRONG FLOW AND THE REMNANTS OF AN INVERSION NEAR 2KFT. FURTHER MIXING AND LLVL WAA WILL CONTINUE TO ERODE THE INVERSION AND SUBSEQUENT CLOUD COVER. MOSTLY CLEAR CONDITIONS SHOULD PERSIST ACROSS THE SOUTH THROUGH THE EVENING. VERY BRIEF ZONAL UPPER LEVEL FLOW MODIFIES TONIGHT AS A STRONG SHORTWAVE TROF DROPS SOUTH OUT OF THE WRN GREAT LAKES. COLD FRONT ASSOCIATED WITH THE UPPER LOW IS CURRENTLY SPLAYED THROUGH ERN MI SOUTH THROUGH THE LOWER OHIO VALLEY. LLVLS AHEAD OF THE COLD FRONT HAVE WARMED TODAY THROUGH STRONG WAA. ACROSS THE FRONT...SFC TEMP GRADIENT IS QUITE SMALL...BUT A DEWPOINT GRADIENT AND WIND SHIFT IS NOTED. MOISTURE BEGINS TO POOL TO OUR NW TONIGHT AS THE SHORT WAVE TROF PUSHES SOUTH APPROACHING LAKE ERIE. BEING THAT THE AIR MASS TRANSITION BEHIND THE FRONT IS ONE OF CONTINENTAL ORIGINATION...AND MOISTURE FEED INTO THE LOW IS THE LEFTOVERS FROM THE REMNANT BROAD LOW OVER ERN ONTARIO...MOISTURE AVAILABILITY IS NOT OVERLY IMPRESSIVE. ALSO...WEAK ASCENT IN THE LOW LEVELS WILL INITIALLY QUELL THE ONSET OF PRECIPITATION...BUT AS THE UPPER TROF AXIS APPROACHES WIND FIELDS STRENGTHEN THROUGH THE COLUMN ADVECTING IN MUCH COLDER AIR FROM H9 ON UP. LAPSE RATES WILL ACTUALLY STEEPEN OVERNIGHT INTO EARLY SAT...GENERATING SHOWERS FOR MOST LOCATIONS...BUT WILL BE MORE LIKELY ACROSS THE NORTH WITH THE RELATIVELY DEEPER MOISTURE. MAIN TROF AXIS SHIFTS EAST BY MIDDAY SAT AS LLVL FLOW KEEPS POOLING MOISTURE UP THE HIGHER TERRAIN...THUS EXPECTING SHOWERS TO HOLD IN THE RIDGES A LITTLE LONGER. ANOTHER RE-ENFORCING SW TROF SWINGS THROUGH SAT EVENING GENERATING RAIN SHOWERS OUT OF THE REMAINING MOISTURE. 12Z NAM IS A LITTLE BIT COLDER THAN THE PREVIOUS VERSIONS...BUT ONLY BRINGS IN -1C TO -2C AT H85 BEFORE THE MOISTURE DRIES UP SAT...SO KEPT ALL HYDROMETEORS IN THE LIQUID STATE. TAX
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&& .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... CHANGES IN THE SHORT TERM FORECAST WERE REALLY QUITE MINIMAL AS MODEL TRENDS CONTINUE TO FAVOR A CONTINUATION OF THE GOING FORECAST. THAT SAID...THE SECONDARY SHORT WAVE TROUGH WILL SHIFT SOUTHEAST OF THE AREA THROUGH SATURDAY NIGHT WITH DEEP NORTHWESTERLY FLOW BEHIND. LAYER TEMPERATURES ARE SET TO FALL OFF A CLIFF WITH 850 MB TEMPERATURES DIVING TOWARD THE -2 TO -5C RANGE BY SUNDAY MORNING. WHILE LOW LEVEL LAPSE RATES REMAIN IMPRESSIVE...INVERSION HEIGHTS WILL BE DROPPING LIKE A ROCK THROUGH THE NIGHT SATURDAY NIGHT INTO SUNDAY MORNING. WHILE THIS WILL CONTINUE SHOWER CHANCES EVEN WITH THE SHALLOW MOIST/VERY UNSTABLE LAYER...IT HAS THE INTERESTING CHARACTERISTIC OF LIMITING THE MOISTURE JUST A FEW DEGREES CELSIUS WARMER THAN THE DENDRITIC GROWTH LAYER. NORTHWESTERLY AND MODESTLY CONVERGENT BOUNDARY LAYER FLOW IS FAVORED FROM LAKE HURON...ACROSS LAKE ERIE...AND INTO WESTERN PENNSYLVANIA LATE SATURDAY NIGHT WHICH COULD FAVOR SOME AMOUNT OF COHERENT LAKE BANDING...HOWEVER THERMAL PROFILES SUGGEST THIS SHOULD REMAIN RAIN SHOWER ACTIVITY FOR THE MOST PART. IT REALLY ONLY GETS PROBLEMATIC TOWARD THE 09-15Z TIME FRAME OVER THE RIDGES AS BOUNDARY LAYER TEMPERATURES FALL CLOSE TO FREEZING WHILE LOW LEVEL MOISTURE IN THE UPSLOPE FLOW SLOWLY ERODES. IT WOULD SEEM GIVEN THE SMALL MELTING LAYER THAN SOME AMOUNT OF POORLY FORMED ICE CRYSTAL GROWTH WILL BE POSSIBLE EVEN WITHOUT INSTABILITY/MOISTURE EXTENDING UP TO THE BOTTOM OF THE DENDRITIC LAYER...HOWEVER WARM GROUND TEMPERATURES AND POOR ICE MICROPHYSICS WITH LIMITED AVAILABLE MOISTURE WILL ALL WORK AGAINST ANYTHING MORE THAN A VERY LOCAL/VERY SHORT-LIVED COATING. HIGH PRESSURE THEN FOLLOWS SATURDAY NIGHT`S SYSTEM THROUGH SUNDAY AND SUNDAY NIGHT WITH CLEARING AND A FAIRLY STRONG RADIATION NIGHT LIKELY AT LEAST IN THE SOUTHERN TWO-THIRDS OF THE CWA ON SUNDAY NIGHT. WHILE HIGH CLOUDS IN ADVANCE OF THE NEXT SYSTEM WILL LIKELY FILTER INTO THE NORTHERN ZONES...SUBSIDENCE AND CLEARING SHOULD REMAIN OVER THE SOUTH. THIS SHOULD SET THE STAGE FOR ONE LAST FREEZE CHANCE AT THE END OF THE GROWING SEASON FOR THE LOWER ELEVATIONS OF SOUTHWEST PA/EASTERN OH/NORTHERN WV. FROST WAS ADDED TO THE FORECAST...BUT IT IS MUCH TOO FAR OUT TO BE CERTAIN ENOUGH ABOUT FOR ANY HEADLINES. MOSTLY DRY CONDITIONS WITH INCREASING MID AND HIGH CLOUDS SEEM LIKELY TO CONTINUE INTO MONDAY. THE 12Z MODEL SUITE FAVORED AN INFLECTION POINT IN THE MID-LEVEL DEVELOPMENT OF THE NEXT WAVE JUST FAR ENOUGH WEST WHEREBY IT TRANSITIONS FROM STRONGLY POSITIVELY-TILTED TO A MUCH MORE NEUTRAL TILT BY THE TIME IT EVENTUALLY KICKS TOWARD OUR AREA ON MONDAY NIGHT. FRIES && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... THE AFOREMENTIONED SYSTEM IS SET TO PUSH ACROSS THE AREA MONDAY NIGHT INTO TUESDAY AND LIKELY CLOSE OFF OVER THE ALLEGHENIES BY TUESDAY NIGHT. AS IT DOES SO...IT LOOKS TO GRADUALLY BECOME CUT OFF FROM THE POLAR JET FLOW. THE GLOBAL CONSENSUS FAVORS IT STALLING ALONG THE MID-ATLANTIC COAST FOR MUCH OF THE NEXT WEEK...WHICH MEANS A FAIRLY ABUNDANT AMOUNT OF CLOUDS SEEM LIKELY...WITH AT LEAST A SHOT AT SHOWERS IN THE EASTERN HALF OF THE CWA UNTIL LATE IN THE WEEK. FRIES && .AVIATION /22Z FRIDAY THROUGH WEDNESDAY/... LAST BIT OF MVFR STRATUS LINGERING ACROSS NRN PORTS. GENERALLY VFR IS EXPECTED FROM THROUGH THE EVENING WITH CLEARING SKIES. GUSTY SW WINDS CURRENTLY REPORTED THROUGH THE AREA ARE EXPECTED TO CONTINUE THROUGH TONIGHT...ALTHOUGH THEY SHOULD BEGIN TO VEER TO THE WEST AS THE NEXT SYSTEM APPROACHES TONIGHT. LLVL MOISTURE APPROACHES LATE TONIGHT AS AN UPPER LEVEL TROF DROPS OUT OF THE GREAT LAKES. EXPECTING CIGS TO LOWER IN RESPONSE TO THE ADDED MOISTURE IN THE COLUMN. IN GENERAL...EXPECTING MVFR CIGS ACROSS THE SOUTH INCLUDING KPIT...BUT NRN PORTS SHOULD DROP TO IFR. RAIN SHOWERS ALSO POSSIBLE FROM 06Z THROUGH THE REMAINDER OF THE TAF PERIOD. TAX .AVIATION /FRIDAY NIGHT THROUGH TUESDAY/... PERIODIC RESTRICTIONS POSSIBLE THROUGH SUNDAY MORNING WITH ANOTHER WEAK SYSTEM SAT NIGHT. RESTRICTIONS ALSO POSSIBLE AGAIN MON NIGHT/TUES AS ANOTHER LOW PRESSURE SYSTEM APPROACHES FROM THE NW. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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