Area Forecast Discussion
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000 FXUS61 KPBZ 242209 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 609 PM EDT FRI APR 24 2015 .SYNOPSIS... DRY AND COOL WEATHER CAN BE EXPECTED TODAY AS HIGH PRESSURE SLIDES ACROSS THE REGION...BUT THE REPRIEVE WILL BE BRIEF AS LOW PRESSURE IS FORECAST TO GENERATE RAIN ON SATURDAY. && .NEAR TERM /THROUGH SATURDAY/...
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MINOR ADJUSTMENTS MADE TO HOURLY TEMPERATURES TO BETTER ALIGN WITH OBSERVATIONS...BUT NO OTHER CHANGES ARE NEEDED AT THIS TIME. PREV DISCUSSION FOLLOWS.. A FEW PUFFS OF DIURNALLY-INDUCED FLAT CUMULUS HAVE BEEN NOTED GENERALLY ALONG THE I-80 CORRIDOR...WITH CLEAR SKIES CONTINUING SOUTH OF THERE. GIVEN THE COLDER MID-LEVEL TEMPERATURES RETREATING A BIT TO THE NORTHEAST...FURTHER DEVELOPMENT OF CUMULUS IS NOT REALLY ANTICIPATED...WITH CLEARING LIKELY RESUMING ACROSS THE NORTHEASTERN REACHES OF THE AREA EVEN BEFORE THE NORMAL DIURNAL CONVECTIVE MAXIMUM. AN UNSTABLE BOUNDARY LAYER THIS AFTERNOON HAS BEEN ALLOWING FOR FREQUENT NORTHWESTERLY WIND GUSTS OF AROUND 20 KTS. THESE SHOULD TAPER INTO THE EVENING AS CLEARING ALLOWS FOR WEAK INVERSIONS TO SET UP. HIGH CLOUDS WILL ALSO BEGIN TO ENCROACH FROM THE WEST OVERNIGHT AS THE NEXT SYSTEM STARTS TO COME INTO VIEW. HOWEVER...BEFORE THEIR ARRIVAL...TEMPERATURES SEEM POISED TO NOSE DIVE FOR SEVERAL HOURS. THE DURATION OF THE CLEAR SKIES WILL BE THE MOST IMPORTANT MODULATOR TO THE DEPTHS TO WHICH THE MERCURY WILL PROBE TONIGHT. AS SUCH...A RATHER LOW CONFIDENCE FORECAST EXISTS FOR TEMPERATURES OVERNIGHT. HOWEVER...THE COLDEST READINGS ARE VERY LIKELY TO BE IN THE 20S IN THE VALLEYS OF THE NORTHEASTERN CWA...WHILE THE WARMEST READINGS ARE LIKELY TO BE MAINLY IN OHIO. WHILE THE SURFACE REFLECTION OF THE NEXT SYSTEM THAT WILL PLAGUE THE AREA WILL LIKELY REMAIN WELL BACK IN THE MID-MISSISSIPPI VALLEY BY SATURDAY AFTERNOON...A WARM FRONT WILL LIKELY EXTEND WELL TO ITS EAST INTO THE CENTRAL APPALACHIANS. DIFFERENTIAL THETA-E ADVECTION AS WELL AS PVA WILL STREAM NORTH OF THIS FRONT OVER GENERALLY THE SOUTHWESTERN HALF OF THE CWA BY SATURDAY AFTERNOON. THIS WILL RESULT IN RAIN INCREASING FROM THE SOUTHWEST TO NORTHEAST THROUGH THE AFTERNOON. HOWEVER...DUE TO ABUNDANT DRY AIR IN PLACE UPON ITS ARRIVAL...SEVERAL HOURS OF VIRGA ARE LIKELY BEFORE THE RAIN ACTUALLY HITS THE GROUND. BECAUSE OF THIS...PRECIPITATION OUTBREAK IN THE FORECAST MORE CLOSELY ALIGNS WITH THE SLOWER-DEVELOPING AND HIGHER LOW LEVEL RESOLVING NAM. ASSUMING THIS VERIFIES...RAIN MAY HOLD OFF IN PITTSBURGH ITSELF FOR THE MAJORITY OF THE DAY ON SATURDAY. EVEN STILL...A CONTINUATION OF BELOW NORMAL TEMPERATURES WILL BE LIKELY. FRIES
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&& .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... VERY MINOR ADJUSTMENTS MADE THROUGH THE SHORT TERM AS THE MODELS CONTINUE TO HAVE A SURFACE LOW MOVE JUST SOUTH OF THE FORECAST AREA OVER KY/WV SATURDAY NIGHT. WHILE THERE MAY INITIALLY BE CONCERNS FOR DRY AIR AT THE SURFACE...AS MENTIONED ABOVE...THIS SHOULD BE OVERCOME BY NIGHTFALL WITH THE FOCUS OF PRECIPITATION REMAINING ALONG A STALLED WARM FRONT OVER THE REGION. ISENTROPIC LIFT WILL PROVIDE FOR LIGHT TO MODERATE RAINFALL...ALBEIT FOR A BRIEF PERIOD...AS THE SYSTEM SHIFTS TO THE EAST BY SUNDAY MORNING. POPS WERE KEPT CATEGORICAL BUT SHIFTED A BIT FURTHER WEST AT THE START OF THE NIGHT...ESPECIALLY OVER POINTS SOUTH AND WEST OF PITTSBURGH AS THE MODELS SUGGEST A MORE NW-SE LOW TRACK AND PRECIP AXIS. DRY AIR WILL ENCROACH RATHER QUICKLY BY SUNDAY MORNING WITH BUILDING HIGH PRESSURE...CUTTING OFF THE CHANCE FOR PRECIPITATION SAVE FOR THE RIDGES. NORTHERLY FLOW ALOFT SHOULD LIMIT CLOUD COVERAGE BEYOND SOME DIURNAL CU SUNDAY AFTERNOON. ANOTHER SHORTWAVE IS FORECAST TO DROP SOUTHWARD OUT OF THE EASTERN GREAT LAKES LATE SUNDAY NIGHT/MONDAY PROVIDING FOR A REINFORCING SHOT OF COLD AIR. WHILE THERE WILL BE LITTLE MOISTURE PRESENT WITH THIS FEATURE...A RETURN OF NW FLOW WILL PROMOTE THE CHANCE FOR RAIN SHOWERS...AND A BRIEF PERIOD OF SNOW SHOWERS..FOR POINTS NORTH AND IN THE THE RIDGES. TEMPERATURE THROUGHOUT THE ENTIRE PERIOD WERE PRODUCED USING A COMBINATION OF THE NAM AND GFS MOS...WHICH WAS CLOSE TO 5 TO 10 DEGREES BELOW SEASONAL AVERAGES. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... UPPER LEVEL LOW OVER NEW ENGLAND SHOULD BEGIN TO MOVE OFFSHORE TUESDAY...WHICH SHOULD ALLOW FOR AT LEAST SOME PARTIAL CLEARING TO OCCUR OVER THE REGION. THE CLEARING WOULD LIKELY BE SHORT LIVED...AS MODELS PROJECT A SURFACE LOW WILL DEVELOP ALONG THE GULF COAST AND MOVE TO THE NORTHEAST. STILL LOTS OF UNCERTAINTY WITH THE TRACK OF THIS LOW...AND RECENT MODEL TRENDS HAVE SUGGESTED THE LOW MAY STAY FARTHER TO THE SOUTHEAST. WILL TREND THE FORECAST IN THIS DIRECTION...REDUCING BUT NOT ELIMINATING SHOWERS WEDNESDAY AND THURSDAY. ALTHOUGH TEMPERATURES ARE STILL EXPECTED TO BE A FEW DEGREES BELOW NORMAL THROUGH THE EXTENDED FORECAST...THE MORE SOUTHERLY TREND OF THE MID WEEK SURFACE LOW SHOULD ALLOW FOR MORE SUNSHINE LOCALLY...AND HAVE BROUGHT TEMPS UP A FEW DEGREES TO COMPENSATE. PRECIPITATION IN THE FORECAST. && .AVIATION /22Z FRIDAY THROUGH WEDNESDAY/...
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VFR CONDITIONS ARE EXPECTED THROUGH EARLY SATURDAY AFTERNOON AS HIGH PRESSURE SLOWLY EXITS. MID LEVEL CLOUDS SHOULD INCREASE LATE TONIGHT IN WARM ADVECTION AHEAD OF APPROACHING LOW PRESSURE. THE LOW IS EXPECTED TO BRING SHOWERS MAINLY SOUTH OF I 80 BY MID TO LATE AFTERNOON AS IT TRACKS ACROSS THE OHIO VALLEY...WITH EVENTUAL MVFR CONDITIONS SOUTH OF PIT. .OUTLOOK.../SATURDAY NIGHT THROUGH WEDNESDAY/... RESTRICTIONS ARE LIKELY TO CONINUE SOUTH OF PIT SATURDAY NIGHT WITH THE LOW PRESSURE. RESTRICITIONS ARE POSSIBLE MONDAY WITH AN UPPER TROUGH.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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