-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPBZ 211938
AFDPBZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
338 PM EDT TUE MAY 21 2013

.SYNOPSIS...
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SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE INTO THE EVENING...
WITH INCREASING CHANCES WEDNESDAY AND THURSDAY AS A COLD FRONT
APPROACHES AND MOVES THROUGH. HIGH PRESSURE WILL RETURN FRIDAY.
-- End Changed Discussion --

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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AN AREA OF MORE STABLE AIR IS CURRENTLY SHIFTING EASTWARD FROM
EASTERN OHIO INTO WESTERN PENNSYLVANIA...WHICH REPRESENTS THE
REMNANTS OF THE COOL DOME UNDER THIS MORNING`S CONVECTIVE DEBRIS
THAT COVERED EASTERN OHIO. WITH INSOLATION QUICKLY WORKING TO WARM
THIS COOL DOME...IT IS EXPECTED EVEN THIS AREA WILL DEVELOP
CUMULUS WITHIN THE NEXT FEW HOURS. BEHIND THIS AREA...A RATHER
UNDERSTATED LOW LEVEL MOISTURE CONVERGENCE AREA REPRESENTED BY A
DEWPOINT DISCONTINUITY WITH VALUES IN THE MID TO UPPER 50S BEHIND
IT AND MID 60S AHEAD OF THE AREA OF WIND CONVERGENCE. GIVEN CAPE
VALUES FROM LAPS GUIDANCE PUSHING 3000 J/KG ACROSS PORTIONS OF THE
CWA...THIS MAY JUST BE ENOUGH TO SPARK OFF A CLUSTER OF
THUNDERSTORMS ALONG THIS BOUNDARY AS IT EDGES EASTWARD THIS
EVENING. THE FORECAST WAS ADJUSTED FOR THIS IDEA...AND THE HRRR
CONCURS WITH ITS DEVELOPMENT CONSISTENTLY OVER THE PAST SEVERAL
HOURS OF RUNS.

TEMPERATURES OVERNIGHT WERE ALLOWED TO STAY UP IN THE MID TO UPPER
60S ACROSS MUCH OF THE AREA OWING TO STRONG LOW LEVEL MOISTURE AND
CONTINUOUS SOUTHWESTERLY FEED INTO THE AREA. THIS MEANS ANOTHER
WARM AND SOUPY NIGHT ACROSS THE REGION. FRIES
-- End Changed Discussion --

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
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A VERY PRONOUNCED LOW LEVEL JET IS EVIDENT ON ALL GUIDANCE BY
WEDNESDAY MORNING NOSING INTO SOUTHWESTERN OHIO AHEAD OF AN
INCOMING UPPER LEVEL WAVE. GUIDANCE CONTINUES TO BE A BIT VARIED
ON ITS PLACEMENT AND EVOLUTION GOING INTO THE LATER MORNING
HOURS...HOWEVER ENOUGH GUIDANCE MEMBERS BRING IT INTO CENTRAL OHIO
BY LATE MORNING THAT THUNDERSTORMS MAY WELL DEVELOP ALONG THE NOSE
OF IT. IN CONJUNCTION...SEVERAL CONSECUTIVE NAM RUNS HAVE DEPICTED
A COMPACT SHORTWAVE EMBEDDED INTO THE SOUTHWESTERLY FLOW TO TRACK
OVER WESTERN AND CENTRAL OHIO FROM LATE MORNING THROUGH EARLY
AFTERNOON. AS THIS OCCURS...CONVECTION IS LIKELY TO BREAK OUT. IN
ADDITION...WITH CAPE VALUES AROUND 1500 J/KG AND STRONGER 0-6 KM
SHEAR LIKELY OVER OHIO THAN POINTS FARTHER EAST...BETTER CHANCES
OF THUNDERSTORM DEVELOPMENT EXIST IN OHIO THAN THE REST OF THE
FORECAST AREA THROUGH MOST OF THE DAYTIME ON WEDNESDAY. WITH
DECENT MID-LEVEL SUPPORT AND SHEAR/INSTABILITY...DAMAGING WINDS
WILL BE POSSIBLE WITH ANY MULTI-CELLULAR DEVELOPMENT THAT DOES
OCCUR FROM LATE MORNING ONWARD IN OHIO...WITH A LESSER CHANCE
FARTHER EAST.

AS THE UPPER LEVEL WAVE TRACKS FARTHER EAST WEDNESDAY
EVENING...THE RIGHT ENTRANCE REGION OF THE UPPER JET TRACKS
DIRECTLY OVER THE CWA. WITH IS IN CONCERT WITH THE MAIN SYNOPTIC
WAVE PASSAGE ROUGHLY 06Z/2AM THROUGH PITTSBURGH WEDNESDAY NIGHT.
BOTH NAM AND ECMWF SOLUTIONS ARE SUGGESTIVE OF LINEAR DEVELOPMENT
ALONG THE FRONTAL BOUNDARY AS IT PASSES THROUGH THE AREA LATE
WEDNESDAY EVENING/NIGHT...RESULTING IN THE POSSIBILITY OF A SQUALL
LINE MANAGING TO PERSIST WELL PAST THE NORMAL DIURNALLY FAVORED
HOURS OWING TO LARGE SCALE SUPPORT FOR ASCENT. CHANCES OF SEVERE
THUNDERSTORMS WITH DAMAGING WINDS WERE THUS ALLOWED TO SPREAD
ACROSS THE REST OF THE CWA FROM ROUGHLY 8 PM TO 3 AM AS THIS UPPER
WAVE PASSES THROUGH.

COLDER AIR FLOODS INTO THE AREA ON THURSDAY...HOWEVER WITH THE
MID-LEVELS COOLING MUCH QUICKER THAN THE LOW LEVELS...SHOWERS AND
THUNDERSTORMS WILL AGAIN BY POSSIBLE ACROSS THE AREA WITH SURFACE
TEMPERATURES MODERATING TOWARD 70F.

THE DUMP OF MUCH COLD AIR IS HELD BACK FOR A SECONDARY FRONTAL
PASSAGE ON FRIDAY WITH A WIDE SPREAD IN GUIDANCE FOR AFTERNOON
TEMPERATURES. ECMWF AND GFS 850 MB TEMPERATURES IN THE AFTERNOON
HOURS STRADDLE 0C...WHICH WOULD SUGGEST TEMPERATURES WOULD
STRUGGLE TO REACH 50F IF SUNSHINE FAILED TO MATERIALIZE. AT THE
MOMENT...IT WOULD SEEM MORE REASONABLE TO EXPECT AT LEAST SOME SUN
ON FRIDAY...SO HIGH TEMPERATURES WERE GENERALLY ADJUSTED FOR THE
MID 50S TO MID 60S ACROSS THE AREA. AS CONDITIONS CLEAR BY THE
AFTERNOON AND EVENING FRIDAY...COLD AIR ADVECTING INTO THE AREA
WILL LIKELY ALL FOR A VERY RAPID COOL OFF GOING INTO THE EVENING
AND POSSIBLY FROST BY THE OVERNIGHT. FRIES
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&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
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THE MODELS ARE IN GOOD AGREEMENT ON THE OVERALL UPPER LEVEL PATTERN
THROUGH THE EXTENDED WITH AN UPPER LEVEL RIDGING OF THE CENTRAL
CONUS AND TROUGHING IN BOTH THE WEST AND EAST. WHILE THIS WOULD
GENERALLY LEAD TO A MUCH LESS THAN CONFIDENT FORECAST IN THE
SHORTWAVES WITHIN THE FLOW...THIS DOESN`T SEEM TO BE THE CASE AS
BOTH THE GFS AND ECMWF SUGGEST THAT MUCH OF THE PRECIPITATION WILL
BE SUPPRESSED SOUTH OF THE REGION THROUGH MUCH OF THE PERIOD.
THUS...OPTED TO TAKE A BLEND OF THE MODELS AND HPC
GUIDANCE. ALSO...HEDGED A BIT COOLER WITH TEMPERATURES WITH
NORTHWEST FLOW PRESENT ESPECIALLY FOR THE FIRST HALF OF THE
PERIOD. THIS KEPT TEMPERATURES BELOW SEASONAL AVERAGES SAT/SUN
BEFORE MODERATING CLOSER TO NORMAL FOR THE REST OF THE PERIOD.
-- End Changed Discussion --

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
GENERAL VFR IS EXPECTED THROUGH THE AFTERNOON. HOWEVER...SINCE WE
REMAIN IN THE WARM SECTOR...PLENTY OF INSTABILITY IS PRESENT FOR
THE DEVELOPMENT OF THUNDERSTORMS...WHICH IS PROGGED TO OCCUR
AROUND 21-23Z ACROSS THE REGION BASED ON HI RES MODEL OUTPUT.
CURRENT THINKING IS THAT A WEAK BOUNDARY...INDICATED BY A WIND
SHIFT OVER OH COULD BECOME THE FOCUS FOR DEVELOPMENT AS A WAVE
APPROACHES. SINCE THUNDERSTORMS HAVE YET TO DEVELOP AND CONFIDENCE
THAT ANY ONE SITE WILL BE IMPACTED REMAINING LOW...DECIDED TO
MAINTAIN THE VCTS MENTION FOR APPROX 4-5 HOURS THROUGH THE EARLY
EVENING. THERE COULD BE TEMPORARY RESTRICTIONS TO VISIBILITY IN
ANY SHOWERS THAT DO DEVELOP...BUT ALSO OPTED TO KEEP THIS OUT AT
THIS TIME.

OVERNIGHT FOG COULD ONCE AGAIN DEVELOP AS THE LOW LEVELS REMAIN
MOIST...ESPECIALLY IF ANY AREAS RECEIVE RAINFALL.

S-SW WIND WILL CONTINUE AT 5-10KTS TODAY AND OVERNIGHT.

.OUTLOOK.../WEDNESDAY THROUGH SATURDAY/...
SHOWERS AND THUNDERSTORMS WILL INCREASE IN COVERAGE WITH POSSIBLE
RESTRICTIONS FROM WEDNESDAY INTO THURSDAY EVENING AS A COLD FRONT
AND UPPER TROUGH APPROACHES AND CROSSES THE REGION. NORTHWEST FLOW
MAY CONTINUE THE CHANCE FOR SHOWERS AND SOME RESTRICTIONS FRIDAY BEFORE
BUILDING HIGH PRESSURE BRINGS THE RETURN OF VFR FRIDAY NIGHT AND
SATURDAY.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...NONE.
OH...NONE.
PA...NONE.
WV...NONE.

&&

$$