570
FXUS61 KPBZ 201235
AFDPBZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
835 AM EDT THU AUG 20 2026
WHAT HAS CHANGED
THE FLOOD WATCH HAS BEEN EXPANDED EASTWARD, AS A SWATH OF HEAVY
RAINFALL TOTALS IS EXPECTED TO CONTINUE ACROSS MORE OF SOUTHWEST
PENNSYLVANIA THIS MORNING.
KEY MESSAGES
1) FLOOD WATCH REMAINS IN EFFECT AS A NARROW BAND OF LOCALIZED
HEAVY RAIN ON FAIRLY SATURATED GROUNDS COULD RESULT IN FLOODING.
2) ADDITIONAL ROUNDS OF RAIN FAVORED OVER THE WEEKEND
WITH LIMITED HAZARDOUS WEATHER RISKS.
DISCUSSION
KEY MESSAGE 1...
830AM UPDATE: SWATH OF HEAVY RAIN IS FOLLOWING ALONG AND JUST TO
THE NORTH OF THE TRACK OF AN MCV THAT IS NOW APPROACHING THE
NORTHERN WV PANHANDLE. KPBZ 88D HAS BEEN UNDERESTIMATING
RAINFALL TOTALS AT TIMES, WITH SOME 2 TO 3 INCH TOTALS IN 2 TO 3
HOURS OCCURRING ALONG THE HEAVIER BAND. A FEW FLASH FLOOD
WARNINGS HAVE ALREADY BEEN ISSUED, AND GIVEN THE EXPECTED ENE
TRACK OF THE MCV, HAVE EXPANDED THE FLOOD WATCH TO THE EAST TO
ENCOMPASS SEVERAL MORE COUNTIES. STILL BELIEVE THAT THE THREAT
LARGELY ENDS BY EARLY AFTERNOON AS THE MCV EXITS.
PREVIOUS DISCUSSION...
DRIVEN BY A 700MB SHORTWAVE/MOISTURE PLUME OVER A STALLED
SURFACE BOUNDARY, SHOWERS WITH LIMITED THUNDERSTORMS ARE
UNDERWAY FROM EASTERN OH TO SOUTH CENTRAL IN AND WILL ADVANCE
EAST NORTHEAST THROUGH THE REGION THROUGH THE EARLY AFTERNOON
TODAY. THE COMBINATION OF MOISTURE INFLUX PUSHING PWATS TOWARD
THE 90TH-95TH PERCENTILE (BASED ON LATEST HREF DATA/RAP
MESOANALYSIS AND A NOTED UPTICK FROM PRIOR HREF MEAN VALUES)
PLUS JET-AIDED ASCENT LENDS TO A NARROW BAND OF HEAVIER RAIN
(2-4" TOTAL RANGE) THAT MAY RESULT IN LOCALIZED FLASH FLOODING,
EVIDENCED BY THE FLOOD WATCH. TIMING REMAINS ON TRACK: EASTERN
OH SEES HEAVY RAIN EXPANSION BETWEEN 3AM-5AM, INTENSITIES
PEAKING BETWEEN 5AM-10AM, AND RAIN ENDING AROUND NOON; NORTHERN
WV AND WESTERN PA IS ROUGHLY +2 HOURS TO THAT TIMELINE.
THOUGH VERY SMALL POCKETS OF HEAVIER RAIN COULD OCCUR OUTSIDE
THE WATCH AREA, SLIGHTLY HIGHER FFG THRESHOLDS DUE TO LESS
SATURATED GROUNDS IS ENOUGH TO NOT OFFER NE EXPANSION OF WATCH
EVEN AS GUIDANCE SUGGEST THESE TOTALS MAY ALSO EXTEND INTO WEST
CENTRAL PA. THE FOCUS, THEN, REMAINS ON THE I-70 CORRIDOR (AND
JUST SLIGHTLY SOUTH) AS THE MOST PROBABLE AXIS FROM 2"+ TOTALS
WITHIN 10KMS IN THE 15-30% RANGE.
CONFIDENCE REMAINS HIGH THAT THE FLASH FLOOD THREAT ENDS FAIRLY
QUICKLY AS THE SHORTWAVE EXITS BY THE EARLY AFTERNOON, THROUGH
RESIDUAL LIFT/MOISTURE MAY STILL CAUSE A STRAY SHOWER INTO THE
EVENING WITH LITTLE HAZARDOUS RISK.
KEY MESSAGE 2...
SUBTLE TRENDS IN A WEAK SURFACE HIGH DEVELOPMENT FRIDAY SUGGESTS
THAT THE PREVIOUS EXPECTATION OF AREAWIDE DRY WEATHER MAY BE TOO
BOLD AND THAT THE COMBINATION OF DIURNAL HEATING, TROUGH-INDUCED
LIFT ALOFT, AND ENOUGH AREA MOISTURE COULD RESULT IN ISOLATED TO
SCATTERED STORMS DURING THE AFTERNOON/EVENING MAINLY EAST OF
I-77.
THAT THEN SERVES AS A PRECURSOR TO ADDITIONAL SHORTWAVE TROUGH
MOVEMENT ALOFT COMBINING WITH A MORE DEFINED SURFACE COLD FRONT
THAT PASSES THROUGH SOMETIME SATURDAY AFTERNOON INTO SUNDAY.
VARIANCES IN UPPER TROUGH DEPTH/POSITIONING AND TIMING OF THE
WAVE MAKES IT DIFFICULT TO ASSESS POTENTIAL STORM ENVIRONMENTS
AND POTENTIAL HAZARDOUS WEATHER THREATS. ENSEMBLE GUIDANCE,
DRIVEN PRIMARILY BY CANADIAN MODELING, DOES SHOW SCENARIOS WITH
ENOUGH CAPE/DCAPE FOR A DAMAGING WIND THREAT (THOUGH WEAK SHEAR
WON'T BE A BOON) THAT SEEMS TO BE CAPTURED BY A FEW OTHER
MACHINE-LEARNING CONVECTIVE OUTLOOK MODELS. THIS SCENARIO IS
MAINLY TIED TO CLEAN HEATING PRIOR TO THE FRONT/SHORTWAVE AND
SATURDAY AFTERNOON/EARLY EVENING STORM OCCURRENCE; EARLIER/LATER
ARRIVALS OR MORE CLOUD COVER IS JUST AS LIKELY THAT WOULD
DAMPEN OR COMPLETELY SQUASH THAT RISK. TRENDS WILL BE MONITORED
TO UNDERSTAND HOW THINGS MAY TRACK.
CONFIDENCE REMAINS FAIRLY HIGH THAT HIGH PRESSURE UNDERNEATH
WEAKENING UPPER TROUGHING FAVORS DRIER WEATHER FOR THE FIRST
HALF OF NEXT WEEK.
AVIATION /12Z THURSDAY THROUGH MONDAY/
SHOWERS WILL EXPAND EASTWARD THROUGH 13Z IN ASSOCIATION WITH AN
MID-LEVEL SHORTWAVE OVERRUNNING A STALLED BOUNDARY NEAR THE I-70
CORRIDOR. THE COMBINATION OF RAIN AND MOISTURE CONVERGENCE WILL
CAUSE CEILINGS TO FALL INTO THE MVFR/IFR REALM WITH POTENTIAL
FOR HEAVY RAIN (AT RATES IN THE 1-2"/HR VICINITY) TO CAUSE
QUICKER/BRIEFER VISIBILITY RESTRICTIONS OF 2 MILES OR LESS.
THERE MAY BE SLIGHT DIFFERENCES IN RAIN ONSET AND PEAK INTENSITY
FROM LATEST TAF BUT TRENDED AN HOUR OR SO LATER FROM PREVIOUS
TAFS TO ACCOUNT FOR LATEST GUIDANCE.
MODEL SOUNDINGS SUGGEST VERY LIMITED VERTICAL BUOYANCY THAT MAY
MAKE LIGHTNING PRODUCTION DIFFICULT AND MORE LOCALIZED. LOW
PROBABILITY IN OCCURRENCE RESULTED IN THE LACK OF TAF MENTION
BUT BEARS MONITORING AS THE WAVE CROSSES MAINLY BETWEEN 08Z-14Z.
SHORTWAVE EXITING EAST DURING THE LATE MORNING TO EARLY
AFTERNOON HOURS RESULTS IN A END IN MOST PRECIPITATION CHANCES
AS RESTRICTIONS SLOWLY IMPROVE. THERE MAY BE ENOUGH LIFT ON THE
BACKSIDE OF THE SYSTEM TO CREATE ISOLATED AFTERNOON SHOWERS, SO
LEFT VARIOUS PROB30S OR VICINITY SHOWERS TO ACCOUNT FOR THIS
POTENTIAL.
OUTLOOK...
A STAGNANT MOIST AIRMASS FAVORS THE DEVELOPMENT AND LIKELY
WESTWARD EXPANSION OF SOME SORT OF MVFR TO LIFR STRATUS LAYER
THURSDAY NIGHT INTO FRIDAY, WITH A LOWER PROBABILITY FOR
POCKETS OF FOG.
GENERALLY DRY CONDITIONS WITH VFR RETURNING BY THE AFTERNOON
FAVORED, THOUGH A FEW AFTERNOON SHOWERS ALONG THE HIGHER TERRAIN
CAN'T BE RULED OUT. ADDITIONAL ROUNDS OF RAIN AND RESTRICTIONS
LIKELY THIS WEEKEND.
PBZ WATCHES/WARNINGS/ADVISORIES
PA...FLOOD WATCH UNTIL 2 PM EDT THIS AFTERNOON FOR PAZ021-029-031-
073>076.
OH...FLOOD WATCH UNTIL 2 PM EDT THIS AFTERNOON FOR OHZ039-040-
048>050-057>059-068-069.
WV...FLOOD WATCH UNTIL 2 PM EDT THIS AFTERNOON FOR WVZ001>004-012-
021-509>512.
DISCUSSION...FRAZIER/CL
AVIATION...FRAZIER
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