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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPBZ 211854
AFDPBZ

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

.SYNOPSIS...
PERIODIC SHOWER AND THUNDERSTORM CHANCES WILL RETURN TODAY...
WITH INCREASING CHANCES WEDNESDAY AND THURSDAY AS A COLD FRONT
APPROACHES AND MOVES THROUGH. HIGH PRESSURE WILL RETURN FRIDAY.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
LATE MORNING TRENDS ARE INDICATING DRAMATIC TEMPERATURE RISES
ACROSS WESTERN PENNSYLVANIA IN PARTICULAR. NEW CASTLE WAS ALREADY
83F AT 11 AM...WITH NEAR 80F COMMON ACROSS THE PITTSBURGH AREA.
TEMPERATURES WERE NUDGED UPWARD FOR THIS AFTERNOON IN THE
AFOREMENTIONED AREA...AND MAINTAINED ELSEWHERE.

PREVIOUS DISCUSSION FOLLOWS...CONVECTIVE DEBRIS CLOUDS ARE
CURRENTLY IN TRANSIT ACROSS THE FORECAST AREA AND CONCURRENTLY
DISSIPATING. TEMPERATURE READINGS ARE SEVERAL DEGREES HIGHER THAN
THIS TIME YESTERDAY AS WELL. WITH LITTLE IN THE WAY TO PREVENT
ABUNDANT INSOLATION ACROSS MUCH OF THE AREA...AND THE FACT THAT
THE LOW LEVEL THERMAL TONGUE WILL BE TRAVERSING THE HEART OF THE
CWA AT MAX HEATING...WITH 850 MB TEMPERATURES NEARING +18C AROUND
21Z OVER PITTSBURGH WITH DECENT BOUNDARY LAYER MIXING...IT WOULD
SEEM TEMPERATURES SHOULD SURPASS THOSE REACHED YESTERDAY. AS
SUCH...HIGHS WERE INCREASED INTO THE UPPER 80S FOR THE SOUTHERN
TWO-THIRDS OF THE CWA...AND THE MID 80S UP ALONG I-80.

WITH INCREASED HEATING AND SIMILAR DEWPOINTS AS COMPARED TO
YESTERDAY IN THE UPPER 60S...CAPE VALUES LOOK TO RUN UP TOWARD
3000 J/KG. GIVEN A WEAKLY SHEARED ENVIRONMENT AND NO SIGNIFICANT
SYNOPTIC OR MESOSCALE DISTURBANCE TO HARNESS THE CAPE...WIDESPREAD
THUNDERSTORM DEVELOPMENT IS NOT ANTICIPATED. HOWEVER...WEAK LOCAL
FORCING SEEMS LIKELY TO ALLOW FOR SCATTERED SHOWERS AND
THUNDERSTORMS TO DEVELOP ACROSS MUCH OF THE AREA BY
AFTERNOON...PARTICULARLY IN AREAS WEST OF PITTSBURGH AS WELL AS IN
THE RIDGES. POPS WERE INCREASED IN THESE AREAS. WITH WEAK SHEAR
AND WEAK MOMENTUM IN THE COLUMN...AS WELL AS A RATHER WARM
PROFILE...UPDRAFTS TO SUSTAIN HAIL DEVELOPMENT WILL BE DIFFICULT
TO MAINTAIN...HOWEVER A WIND GUST OR TWO MAY BE POSSIBLE OUT OF A
FEW OF THE CELLS. FRIES

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/...
BOUNDARY WASHES OUT THIS EVENING...AND WITH THE LOSS OF DAYTIME
HEATING...CONVECTION SHOULD DISSIPATE. AS THE RIDGE CONTINUES TO
WEAKEN...WAVES OF ENERGY...BREAKING AWAY FROM THE MAIN MIDWEST
SYSTEM...WILL RIDE OVER TOP OF WHAT IS LEFT OF THE RIDGE THROUGH
NORTHERN PA. WILL GO WITH LOW CHC POPS ACROSS THE NORTH AND SCHC
IN THE SOUTH. ATMOSPHERE STABILIZES OVERNIGHT AND ANY NOTICEABLE
FORCING IS SPARSE.

EXPECT CONVECTION TO GET GOING AGAIN LATE WEDNESDAY AS THE ENTIRE
SYSTEM CREEPS EVER CLOSER TO THE UPPER OHIO VALLEY. ATMOSPHERE
WILL AGAIN DESTABILIZE AND THE UPPER LEVEL SUPPORT IMPROVES AS THE
RIDGE CONTINUES TO BREAK DOWN. WIND FIELD ALOFT STRENGTHENS
PULLING MORE DEEP MOISTURE INTO THE REGION AND SHEAR VALUES
INCREASE. HAVE DECIDED TO GO WITH LIKELY POPS WED DUE TO THE
AFOREMENTIONED FACTORS. SPC CONTINUES TO KEEP THE AREA IN A SLIGHT
RISK FOR WED...SO WILL INCLUDE WORDING OF POSSIBLE SEVERE AND
MENTION IN HWO.

SLOW MOVING FRONT DRIFTS THROUGH THE AREA ON THURSDAY...SO WILL
CONTINUE WITH LIKELY POPS. THURSDAYS NOT NEARLY AS UNSTABLE AS
TODAY OR WEDNESDAY...SO AMOUNT AND STRENGTH OF CONVECTION STILL IN
QUESTION.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
-- Changed Discussion --
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 BELOE SEASONAL AVERAGES SAT/SUN
BEFORE MODERATING CLOSER TO NORMAL FOR THE REST OF THE PERIOD.
-- End Changed Discussion --

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --
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.
-- End Changed Discussion --

&&

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

&&

$$