Area Forecast Discussion
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000 FXUS61 KPBZ 310122 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 922 PM EDT SAT AUG 30 2014 .SYNOPSIS... SHOWERS AND THUNDERSTORMS WILL BECOME MORE NUMEROUS TONIGHT INTO SUNDAY WITH THE THREAT OF HEAVY RAIN INCREASING AS WELL. && .NEAR TERM /THROUGH SUNDAY/...
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930PM UPDATE...MAINLY MADE TWEAKS TO GRIDS FOR THE NEXT SEVERAL HOURS. NO REAL CHANGE TO PREVIOUS THINKING. INGREDIENTS FOR LOCALLY HEAVY RAINFALL APPEAR LIKELY TO SET UP STARTING AROUND 09Z IN THE WESTERN CWA...AND THEN SPREADING NORTH AND EAST FROM THERE. THESE INCLUDE 2+ INCH PWAT VALUES...WARM CLOUD DEPTHS GREATER THAN 12K FEET...AND SUPPORT FOR VERTICAL MOTION FROM RRQ OF UPPER JET PLUS SEVERAL SHORTWAVE RIPPLES. CAN SEE THE RAIN TRAIN ALREADY STARTING TO SET UP FROM KCMH THROUGH KCVG AND KSDF...WITH HEAVIER RAIN TO THE SOUTHWEST IN TN. A FLASH FLOOD WATCH MAY BE NEEDED FOR AT LEAST PART OF THE CWA ONCE TRENDS BECOME MORE CLEAR. 530 PM UPDATE...ONLY CHANGE WAS TO SLIGHTLY EXPAND THE AREA OF SLIGHT POPS SOUTHWARD TO ENCOMPASS THE METRO PITTSBURGH AREA. HAVE SEEN A COUPLE OF SHRA POP NEARBY...AND PIT DID RECEIVE A TRACE OF RAINFALL. JUST A BIT TOO DRY IN THE LOW LEVELS FOR MORE WIDESPREAD ACTIVITY. THIS WILL NOT BE A PROBLEM LATER TONIGHT WHEN A 2+ INCH PRECIPITABLE WATER AIRMASS ARRIVES. AGREE WITH PREVIOUS SHIFT THAT THE INGREDIENTS FOR HEAVY RAINFALL MAY COME TOGETHER LATER TONIGHT AND SUNDAY. A FLASH FLOOD WATCH IN THE NEXT 9-12 HOURS CANNOT BE RULED OUT. PREVIOUS DISCUSSION... WATER VAPOR IMAGERY REVEALS NEARLY A HANDFUL OF UPPER WAVES AND POTENTIAL VORTICITY FEATURES OF CONCERN FOR THE NEAR TERM PERIOD. THE DEEPEST OF THESE IS DIVING SOUTHEASTWARD THROUGH MINNESOTA AT THE MOMENT...WITH THE LEADING FEATURE NEARING LAKE MICHIGAN...AND YET ANOTHER UPPER PV ANOMALY SCREAMING NORTHEASTWARD INTO THE MID- MISSISSIPPI VALLEY AT THE MOMENT. ALL OF THESE WILL LIKELY PLAY INTO THE FORECAST FOR THE AREA OVERNIGHT AND ESPECIALLY INTO THE DAYTIME ON SUNDAY. HOWEVER...BEFORE WE GET THERE...SOME VERY WARM AND STICKY CONDITIONS WILL PREVAIL ACROSS THE UPPER OHIO VALLEY. A DECENT LAYER OF WARM AIR IS EVIDENT IN THE 6-8 KFT LAYER ON RUC/NAM SOUNDINGS...AND THIS HAS WORKED TO RETARD THE DEVELOPMENT OF CUMULUS THROUGH THE EARLY AFTERNOON HOURS TODAY. WHILE THIS IS SET TO ERODE...IT SEEMS LIKELY TO BE INCOMING MID AND HIGH CLOUDS AHEAD OF THE ACTIVITY TO OUR WEST THAT WILL TURN SKIES PARTLY CLOUDY ACROSS THE AREA BY LATE AFTERNOON/EARLY EVENING...RATHER THAN WIDESPREAD CUMULUS DEVELOPMENT OUTSIDE THE RIDGES. SURFACE DEWPOINTS HAVE ALREADY RISEN INTO THE UPPER 60S TO NEAR 70 AND AS SOUTHWESTERLY FLOW INCREASES AND DEEPER MOISTURE POISED TO OUR SOUTHWEST ADVANCES NORTHEASTWARD...IT SEEMS THAT DEWPOINT VALUES IN THE 70S MAY BECOME MORE COMMON OVER THE NEXT 12-24 HOURS. THIS WILL LIKELY COINCIDE WITH THE EROSION OF THE WARM LAYER ALOFT...AND LIKELY WITH LARGE SCALE DESTABILIZATION OF THE ENVIRONMENT AHEAD OF THE TRIFECTA OF UPPER FEATURES SET TO AFFECT THE AREA. AS THE FIRST WAVE SHEARS OUT ACROSS THE UPPER GREAT LAKES...THE LOW LEVEL FRONTAL ZONE SAGS SOUTHEASTWARD TOWARD OUR NORTHWESTERN PORTIONS OF OUR FORECAST AREA. THIS LOOKS TO INCREASE LOW LEVEL CONVERGENCE INTO THE LATE EVENING AND OVERNIGHT IN AREAS ADVANCING NORTHEASTWARD FROM EASTERN AND SOUTHEASTERN OHIO INTO WESTERN AND SOUTHWESTERN PENNSYLVANIA. IT IS UP THIS CORRIDOR THAT THE NAM/SREF/GFS/LOCAL WRF ALL FOCUS PWAT VALUES OF 2 INCHES OR GREATER...THEN MANAGE TO PERSIST THEM OVER THIS AXIS FOR 12-18 HOURS AS THE FIRST WAVE SHEARS OFF TO THE EAST AND EASTWARD PROGRESS OF THE MOISTURE AXIS SEEMS LIKELY TO STALL. WITH THE MOISTURE AXIS STILL IN PLACE...AND FRONTAL ZONE LIKELY STALLED SOMEWHERE IN THE CENTRAL PORTION OF THE FORECAST AREA...THE THREAT FOR HEAVY RAIN IS VERY REAL. NOT ONLY DO A SUCCESSION OF WAVES SEEM LIKELY TO TRACK DOWN THIS FRONT AFTER IT BECOMES STALLED AS A COUPLE OF WAVES MOVE UP FROM THE MISSISSIPPI VALLEY BEFORE THE WAVE CURRENTLY OVER MINNESOTA KICKS THE FRONT EASTWARD...BUT WE REMAIN LOCATED IN THE RIGHT ENTRANCE REGION OF AN UNSEASONABLY STRONG UPPER JET STREAK...WITH MODEL TIME HEIGHT SECTIONS SUGGESTING STRONG LIFT IN THE 0-10C LAYER. THIS IS INDICATIVE OF COLLISION AND COALESCENCE PROCESSES...AND COMBINED WITH LOW LEVEL CONVERGENCE...UPPER LEVEL DIVERGENCE...AND A STRONG MOISTURE STREAM WITH AT LEAST MODERATE INSTABILITY IN PLACE...A FLASH FLOOD WATCH IS BEING SERIOUSLY CONSIDERED. WITH DEEP MOISTURE IN PLACE THROUGH SUNDAY...TEMPERATURES WILL BE RATHER SLOW TO FALL OVERNIGHT...AND ALSO SLOW TO RISE TOMORROW. EXPECTED THE DIURNAL SPREAD TO BE LITTLE MORE THAN 10 DEGREES DURING THAT TIME FRAME. FRIES
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&& .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... A WEAK BOUNDARY STALLS OUT OVER THE AREA SUN NIGHT AS THE BERMUDA HIGH SLOWS ITS PROGRESSION. MULTIPLE WAVES ARE PROGGED TO TRAVERSE ALONG THE BOUNDARY...AND THE LAST SUCH WAVE WILL ENTER FROM THE SW NEAR 00Z MON. NEAR TERM MODELS HAVE PULLED THE WRN EXTENT OF THIS WAVE BACK...DRAWING MORE MOISTURE IN THAN PREVIOUSLY FORECAST. WHILE THE DEEPEST MOISTURE IS DRAWN EASTWARD OVER COASTAL NEW ENGLAND...PWAT VALUES FROM THE 12Z NAM/SREF HOVER NEAR 1.75" THROUGH EARLY MON MORNING. WILL CONTINUE WITH LIKELY POPS IN OUR SE ZONES SUN NIGHT/MON MORNING AND HAVE INCLUDED HEAVY RAIN MENTION. THIS HAPPENS TO BE OUR WETTEST AREA WITH 3HR FFG UNDER 2"...BUT ANY FLASH FLOODING ISSUES THROUGH EARLY MON MORNING COULD BE HIGHLY DEPENDENT ON ANTECEDENT RAIN EXPECTED THROUGH THE AREA TONIGHT/SUN. WILL HIGHLIGHT HEAVY RAIN/LOCALIZED FLASH FLOOD THREAT IN HWO FOR NOW. MONDAY A WARM AND HUMID AIR MASS WILL REMAIN WITH AFTERNOON SHOWERS AND THUNDERSTORMS...BUT WITH LIMITED COVERAGE DUE TO LACK OF ANY REAL UPPER LEVEL SUPPORT...WILL KEEP SLGHT AND CHC POPS. ANOTHER UPPER LEVEL TROUGH AND COLD FRONT WILL APPROACH TUES. MODELS HAVE REMAINED LARGELY CONSISTENT IN TIMING AND STRUCTURE OF THE FRONT...SO HAVE NO REASON TO DEVIATE FROM LIKELIES AT THIS TIME. TAX && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... SHOWERS AND THUNDERSTORMS WILL LINGER TUESDAY NIGHT AS COLD FRONT PUSHES EAST. HIGH PRESSURE MOVES OVER THE EAST COAST WEDNESDAY...AND SHOULD MANAGE TO DEFLECT PRECIPITATION ASSOCIATED WITH A COLD FRONT TO THE NORTH UNTIL AT LEAST FRIDAY. COLD FRONT COULD GET CLOSE ENOUGH BY FRIDAY AFTERNOON TO BRING AT LEAST AN ISOLATED SHOWER TO WESTERN AREAS. TEMPERATURES WILL BE A FEW DEGREES ABOVE NORMAL. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/... VFR CONDS WL DCR AS A SHRTWV ADVNS ACRS THE RGN OVRNGT INTO SUN...BRINGING SHWRS AND WDSPRD MVFR/PTCHY IFR CONDS. A FEW TSTMS ARE PSBL BUT WITH INSTBY/COVG LMTD ONLY INCLUDED A VCTS MENTION SUN AFTN ATTM. OUTLOOK.../SUNDAY NIGHT THROUGH THURSDAY/... RSTRNS WL CONT SUN NGT INTO MON MRNG AS A DISTURBANCE EXITS. RSTRNS ARE PSBL AGAIN WITH A TUE CDFNT. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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