Area Forecast Discussion
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000 FXUS61 KPBZ 190343 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 1043 PM EST THU DEC 18 2014 .SYNOPSIS... A WEAK DISTURBANCE WILL CROSS THE REGION TONIGHT. BUILDING HIGH PRESSURE IS EXPECTED THROUGH THE WEEKEND...THOUGH CLOUDS WILL REMAIN. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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AN UTTERLY POOR SNOW CRYSTAL GROWTH HABIT EXISTS OVER THE CWA THIS EVENING...AS EVIDENCED BY THE PINHEAD SIZED SNOWFLAKES/GRAINS THAT HAVE BEEN SCATTERED ACROSS THE AREA IN THE FORM OF "FLURRIES." IN ADDITION...AREAS JUST TO THE NORTH AND NORTHWEST OF OUR CWA HAVE STARTED TO REPORT FREEZING DRIZZLE. THIS SEEMS LIKELY TO CONTINUE TO FILTER INTO AT LEAST THE NORTHERN REACHES OF THE CWA AS MOISTURE INCREASES...BUT STILL LITTLE MOISTURE EXISTS ABOVE ABOUT THE -8C ISOTHERM. AS SUCH...WHILE DEEPER MOISTURE MAY ALLOW SOME TREND TOWARD SNOW SHOWER ACTIVITY...IT SEEMS INSUFFICIENT TO PRECLUDE FREEZING DRIZZLE FROM CONTINUING TO DEVELOP IN THE NORTHERN PORTIONS OF THE CWA. ADDITIONALLY...WITH AN INCOMING SHORT WAVE TROUGH AND A BIT OF ELEVATED MOISTURE...SOME SEEDER-FEEDER TYPE OF SNOW FLURRY ACTION DOES SEEM POSSIBLE FOR A SHORT WINDOW AS THE WAVE TRACKS THROUGH...HOWEVER MODEL CROSS SECTIONS...TIME HEIGHT SECTIONS...AND PLAN DEPICTIONS ALL GENERALLY REVEAL A QUICK TREND TOWARD ZERO MICROBARS/SEC OF LAYER NEGATIVE OMEGA VALUES BETWEEN 00Z AND 06Z THIS EVENING. AS A RESULT...WITH DRY AIR BETWEEN THE TWO MOIST LAYERS...WEAKENING LIFT ALOFT...AND POOR CRYSTAL GROWTH BELOW...IT WOULD SEEM JUST A GENERAL EXPANSION IN THE FLURRIES FOR A FEW HOURS AS THE WAVE TRACKS THROUGH SHOULD BE THE MAIN PROBLEM FOR THE NIGHT OVER THE REST OF THE CWA. UNFORTUNATELY...ROAD TEMPERATURES HAVE TRENDED NEAR FREEZING THROUGH THE EVENING...SO REPORTS OF NUMEROUS ACCIDENTS HAVE COME IN ACROSS THE I-70 AND SOUTH CORRIDOR AS LIGHT SNOW INITIALLY MELTED THEN REFROZE ON ROADWAYS. LUCKILY ENOUGH...THIS WAVE IS TRACKING THROUGH RATHER QUICKLY...AND AFTER ITS PASSAGE...EVEN BOUNDARY LAYER RH VALUES START TO FALL OFF A BIT...SO IT WOULD SEEM EVEN THE POOR GROWTH HABIT MAY DETERIORATE BY MORNING. FRIES
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/... SFC HIGH PRES IS PROGGED TO BLD IN UNDER UPR TROFG OVR THE GT LKS FRI...ENDING ANY LINGERING MRNG SNW SHWRS. THE SFC HIGH IS EXPD TO BRING DRY WEA THRU THE WKEND...THOUGH PERSISTENT MSTR BLO AN INVERSION MAY KEEP SKIES MCLDY. A SHRTWV MOVG ACRS THE SERN CONUS LT SAT/SAT NGT IS NOW PROGGED TO STAY FAR ENOUGH S TO REMOVE ALL PCPN FM THE FCST FOR AREAS S OF PIT. A WK SHRTWV IS PROGGED FOR ERLY SUN ACRS THE GT LKS...BUT WITH A LACK OF DEEP MSTR AND THE SFC HIGH OVERHEAD KEPT THE FCST DRY. SLGTLY BLO AVG TEMPS SHOULD RTN TO NR SEASONAL LVLS BY SUN. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... WHILE THE MODELS AGREE ON A DEEPENING UPPER LEVEL TROUGH TO CLOSE OFF OVER THE GREAT LAKES THROUGH THE PERIOD THE DETAILS OF HOW THIS EXACTLY UNFOLDS WILL BE CRITICAL. BY MONDAY SOUTHERN STREAM SHORTWAVE AHEAD OF MAIN NORTHERN STREAM UPPER TROUGH DIGGING INTO THE NORTHERN PLAINS WILL PRODUCE LOW PRESSURE ACROSS THE CAROLINAS MOVING OFF SHORE OF THE EAST COAST. THE SOUTHEAST RIDGES MAY BE BRUSHED BY LIGHT RAIN FROM THIS SYSTEM. NEXT UP IS DEEPENING LOW PRESSURE OVER THE OHIO VALLEY AHEAD OF SHARPLY DIGGING UPPER TROUGH AND CLOSED LOW MOVING INTO THE WESTERN GREAT LAKES. UPPER OHIO VALLEY LOOKS TO BE ON THE WARM SIDE OF THE SYSTEM TUESDAY INTO WEDNESDAY WITH RAIN...FOLLOWED BY STRONG COLD FRONT AND COLD ADVECTION LATER WEDNESDAY NIGHT AND THURSDAY WITH SNOW SHOWERS. WITH SUCH A STRONG SYSTEM ALOFT AND FAST DEEPENING LOW PRESSURE AT THE SURFACE TIMING OF COLD AIR AND EXACT TRACK WILL LIKELY CHANGE AS WE GET CLOSER TO CHRISTMAS. ALL IN ALL PREVIOUS FORECAST IS LITTLE CHANGED WITH ONLY MINOR ADJUSTMENTS BASED ON NEWEST WPC EXTENDED PROGS. && .AVIATION /04Z FRIDAY THROUGH TUESDAY/... MVFR CIGS WILL BE EXPECTED AT ALL TERMINALS OVERNIGHT ALONG WITH LIGHT SNOW. THINK THAT IFR CIGS WILL OCCUR ACROSS NORTHERN SITES OF FKL/DUJ...AND AN ISOLATED IFR VIS CANNOT BE RULED OUT IN A HEAVIER SNOW SHOWER ELSEWHERE. SCATTERED SNOW SHOWERS COULD PERSIST NORTH AND EAST OF PITTSBURGH AFTER DAYBREAK...BUT COVERAGE WILL BE MINIMAL. ALTHOUGH MODEL GUIDANCE INDICATES THAT CEILINGS SHOULD BE SCATTERING OUT FRIDAY AFTERNOON...DO NOT HAVE HIGH CONFIDENCE IN THIS CONSIDERING WINDS WILL REMAIN OUT OF THE WEST/NORTHWEST...AND HAVE KEPT TAFS MORE PESSIMISTIC. OUTLOOK.../FRIDAY NIGHT THROUGH TUESDAY/... STRATUS AND RESTRICTIONS MAY PERSIST INTO THE WEEKEND. NEXT STORM SYSTEM WILL BRING RESTRICTIONS TUESDAY. && .PBZ WATCHES/WARNINGS/ADVISORIES...
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MD...NONE. OH...NONE. PA...WINTER WEATHER ADVISORY UNTIL 6 AM EST FRIDAY FOR PAZ007>009- 013>016-022. WV...NONE.
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