Area Forecast Discussion
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000 FXUS61 KPBZ 231851 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 251 PM EDT THU APR 23 2015 .SYNOPSIS... COLD AND BRISK WEATHER WILL CONTINUE...EVEN AS CLOUD COVER DECREASES ACROSS THE REGION.THE NEXT CHANCE FOR RAIN IS FORECAST SATURDAY AS LOW PRESSURE APPROACHES. && .NEAR TERM /THROUGH FRIDAY/... TEMPERATURES AT 850 HPA NEVER RECOVERED MUCH ABOVE -5 TO -9C TODAY...AND LIKEWISE 700 HPA TEMPERATURES REMAINED MIRED IN THE NEGATIVE MID TEENS CELSIUS...RESULTING IN AN ATMOSPHERE THAT REACHED ITS CONVECTIVE TEMPERATURE VERY EARLY ON. WHILE SOME CLEARING HAD BEEN NOTED ON VISIBLE IMAGERY EARLY DURING THE DAY...THIS WAS FLEETING AS DESTRUCTIVE SUNSHINE QUICKLY TURNED THE ENTIRE UPPER OHIO VALLEY INTO A STRATOCUMULUS WASTELAND. IF YOU THOUGHT IT SEEMED LIKE FEBRUARY TODAY...YOU WOULD BE ENTIRELY CORRECT. IN FACT...HIGHS WHERE CONVECTIVE PRECIPITATION LINGERED THE LONGEST...NORTH OF THE TURNPIKE...WERE GENERALLY ONLY IN THE 30S...WHILE THE 40S WERE RELEGATED TO PITTSBURGH AND SOUTHWARD. INDEED...THIS WAS REMINISCENT OF FEBRUARY...NOT LATE APRIL. WITH ALL THAT IN MIND...STRONG LOW-LEVEL LAPSE RATES LINGER RIGHT THROUGH THE NEAR-TERM PORTION OF THE FORECAST. HOWEVER...A FEW SMALLER DETAILS DO CHANGE. THIS WILL ALLOW FOR VAST IMPROVEMENT AT LEAST FROM A SKY-COVER PERSPECTIVE. WHILE CYCLONIC DEEP-LAYER FLOW WILL REMAIN ENTRENCHED OVER OUR REGION...WE WILL QUICKLY TRANSITION BEYOND THE INFLECTION POINT IN THE 500 HPA FLOW PATTERN. ONCE THIS OCCURS...WE WILL START TO SEE 500 HPA HEIGHTS RISING...AND LIKEWISE A STABILIZATION OF THE ATMOSPHERE ABOVE THE STRATOCUMULUS LAYER. BECAUSE OF HOW THE THERMODYNAMIC PROFILES ARE WORKING OUT WHEN OVERLAID WITH THE MOISTURE PROFILE...THIS MEANS FROZEN HYDROMETEORS WILL BE TOUGHER TO GENERATE AS THE UNSTABLE LAYER BEGINS TO FAIL TO INTERSECT THE UNDERSIDE OF THE DENDRITIC GROWTH LAYER. THIS IS DUE TO DRIER AIR ENCROACHING FROM ABOVE AS WELL AS DIFFERENTIAL NEGATIVE VORTICITY ADVECTION. THIS WILL BRING AN END IN SHOWER ACTIVITY FROM WEST TO EAST PROGRESSIVELY INTO TONIGHT...AND LIKEWISE START TO SCATTER OUT STRATOCUMULUS AFTER A TIME ONCE THE MIXING OF DRY AIR INTO THE MORE UNSTABLE BOUNDARY LAYER STARTS TO TAKE PLACE. BY FRIDAY...RAPID CLEARING SHOULD OCCUR FROM WEST TO EAST AS 700 HPA TEMPERATURES RAPIDLY RISE THROUGH THE DAY AND THERMODYNAMIC PROFILES MAKE SATURATING ANY LAYER OF THE ATMOSPHERE MUCH LESS LIKELY. THIS WILL ALLOW FOR SOME WARMING RELATIVE TO TODAY... ESPECIALLY GIVEN A DRASTIC INCREASE IN SUNSHINE. THAT SAID...HIGHS WILL STILL BE BELOW NORMAL AS NORTHWEST FLOW RULES THE ROOST ACROSS THE REGION. FRIES && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... GIVEN THE NON-PROGRESSIVE PATTERN IN PLACE...STRONG CLOSED LOW PRESSURE AT 500 MB WILL REMAIN RELATIVELY STATIONARY OVER THE NORTHEASTERN CONUS THROUGH THE WEEKEND...WITH SEVERAL SHORTWAVE TROUGHS ROTATING AROUND THE LOW. NWLY FLOW WILL DOMINATE THIS PERIOD AS THE REGION REMAINS BETWEEN THIS LOW AND A RIDGE TO THE WEST...BUT THIS FLOW WILL BE SUPPLANTED BRIEFLY ON SATURDAY NIGHT AS ONE SUCH SHORTWAVE TROUGH APPROACHES FROM THE NORTHERN PLAINS AND PHASES INTO THE STRONG UPPER LOW. PRECIPITATION WILL BEGIN TO SPREAD NORTHEASTWARD SATURDAY MORNING AS WARM AIR ADVECTION INCREASES...BUT WILL HAVE CONSIDERABLE LOW-LEVEL DRY AIR TO OVERCOME BEFORE MEASURABLE RAIN IS REALIZED. AS A RESULT...ONSET OF PRECIPITATION WAS SLOWED FOR SATURDAY MORNING. MODELS ARE IN GENERAL AGREEMENT THAT THE CENTER OF THE APPROACHING SHORTWAVE TROUGH WILL PASS JUST SOUTH OF THE FORECAST AREA SATURDAY NIGHT AND SUNDAY. SUCH A TRACK WILL KEEP THE FORECAST AREA ALONG OR JUST NORTH OF THE ASSOCIATED WARM FRONT...AND IN PRIME TERRITORY FOR A BAND OF MODERATE RAIN OWING TO DEFORMATION AND MID-LEVEL WARM AIR ADVECTION. THE SYSTEM WILL EMERGE RAPIDLY INTO THE ATLANTIC SUNDAY AFTERNOON...SPELLING A RAPID END TO PRECIPITATION. GIVEN THIS EXPECTED STORM TRACK...PRECIPITATION CHANCES SATURDAY NIGHT AND SUNDAY MORNING SEEM INCREASINGLY LIKELY. ALTHOUGH A SHARP NORTHERN PRECIPITATION GRADIENT IS LIKELY TO EXIST...IT IS UNCLEAR EXACTLY HOW FAR NORTH PRECIPITATION WILL REACH. NEVERTHELESS...POPS WERE INCREASED TO CATEGORICAL LEVELS IN THE SOUTH AND TO LOW-END CHANCE LEVELS IN THE NORTH...WHICH DEPICTS A MILD WEAKENING OF THE GRADIENT FROM THE PREVIOUS FORECAST GIVEN UNCERTAINTY IN NORTHWARD EXTENT OF RAIN. TEMPERATURES PRIOR TO THE WEEKEND RAIN WILL BE TRICKY ON FRIDAY NIGHT. INITIAL CONDITIONS OF DRY AIR...LIGHT WIND AND CLEARISH SKY WILL BE QUITE FAVORABLE FOR RADIATIONAL COOLING...BUT CIRRUS ARE EXPECTED TO BECOME INCREASINGLY ABUNDANT DURING THE NIGHT AND WILL LIMIT COOLING LATER. IT IS POSSIBLE ALL AREAS WILL DROP BELOW FREEZING...BUT THE MOST LIKELY LOCATIONS WILL BE THE TYPICAL NORTHERN ZONES. APART FROM THE DISTURBANCE SATURDAY NIGHT AND SUNDAY...NO MAJOR PRECIPITATION IS EXPECTED. DAYTIME MAXIMA WILL REACH THE UPPER 50S AND MINIMA WILL BE IN THE MID-UPPER 30S AS CLOUD COVER AND MODEST WARM AIR ADVECTION OFFSET DIURNAL COOLING. && .LONG TERM /MONDAY THROUGH THURSDAY/... THE UPPER-LEVEL LOW WILL SIT AND SPIN OVER NEW ENGLAND SUNDAY NIGHT INTO MONDAY NIGHT. MOISTURE WILL SPILL INTO WESTERN PENNSYLVANIA BY MONDAY MORNING...AND A SHORTWAVE TROUGH PIVOTING AROUND THE UPPER LOW WILL BRING AT LEAST A SLIGHT CHANCE OF PRECIPITATION FOR PENNSYLVANIA COUNTIES MONDAY INTO MONDAY NIGHT. CANNOT RULE OUT A FEW SNOWFLAKES MIXING IN RIGHT AROUND SUNRISE FOR FOREST...CLARION...AND JEFFERSON COUNTIES BOTH MONDAY AND TUESDAY MORNINGS. BY THE TIME THE UPPER LOW MOVES OFFSHORE TUESDAY...A SURFACE LOW WILL BEGIN DEVELOPING ALONG THE GULF COAST AND MOVE TO THE NORTHEAST. THE ECMWF KEEPS THE LOW WELL EAST OF THE REGION WITH NO RAIN LOCALLY...BUT THE GFS...UKMET AND THE NAEFS KEEP THE LOW CLOSER TO THE COAST AND SPREAD RAIN INTO THE REGION. HAVE BUMPED UP POPS WEDNESDAY...ALTHOUGH FUTURE MODEL RUNS WILL BE NEEDED TO DETERMINE THE EXACT TRACK OF THE LOW. WITH SOME SORT OF NORTHERLY FLOW EXPECTED THROUGH THE EXTENDED FORECAST AND MUCH CLOUD COVER WITH THE TWO MAIN SYSTEMS... TEMPERATURES WILL CONTINUE TO BE BELOW NORMAL BUT WILL RISE CLOSER TO CLIMATOLOGICAL VALUES BY THE END OF THE PERIOD. THE EASTERN CONUS TROUGH AND RESULTING NORTHWESTERLY WIND WILL CONTINUE TO HOLD TEMPERATURES UNDER THE AVERAGES. HAVE TRENDED TOWARDS WPC NUMBERS...WHICH KEEP AT LEAST SOME CHANCE OF PRECIPITATION IN THE FORECAST. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... SNOW SHOWER ACTIVITY IS WINDING DOWN ACROSS THE NORTHERN SITES AS DRIER AIR ALOFT MOVES OVER THE SITES. THIS IS ALLOWING FOR STRATOCUMULUS TO RAISE OFF THE GROUND...AND ALL SITES WILL BE VFR WITHIN THE NEXT COUPLE OF HOURS. WIND GUSTS WILL CONTINUE INTO THE EVENING DUE TO STRONG LOW-LEVEL INSTABILITY...HOWEVER THOSE TWO SHOULD SLACKEN OVERNIGHT. VFR CONDITIONS WITH MAINLY CLEAR SKIES WILL CONTINUE INTO FRIDAY...WITH AFTERNOON WIND GUSTS AGAIN POSSIBLE. FRIES .OUTLOOK.../FRIDAY THROUGH MONDAY/... RESTRICTIONS ARE POSSIBLE SATURDAY AFTERNOON THROUGH SUNDAY AFTERNOON AS LOW PRESSURE TRACKS ACROSS THE OHIO VALLEY. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$ FRIES/KRAMAR

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