Area Forecast Discussion
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FXUS61 KPBZ 302137
AFDPBZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
536 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 /UNTIL 6 AM SUNDAY MORNING/...
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

&&

.SHORT TERM /6 AM SUNDAY MORNING 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 /22Z SATURDAY 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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