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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KCTP 211900
AFDCTP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
255 PM EDT TUE MAY 21 2013

.SYNOPSIS...
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A WARM AND HUMID...SUMMERTIME AIRMASS WILL CONTINUE TO BRING WELL
ABOVE NORMAL TEMPERATURES TO CENTRAL PENNSYLVANIA LATE TODAY THROUGH
WEDNESDAY. A COLD FRONT WILL CROSS THE REGION ON THURSDAY. A LARGE
AREA OF HIGH PRESSURE WILL BUILD SOUTHEAST FROM CENTRAL CANADA INTO
THE NORTHEAST AND MID-ATLANTIC STATES. THIS HIGH WILL BRING COOLER
AND DRIER AIR TO THE AREA FOR MEMORIAL DAY WEEKEND...THAT COULD
LAST RIGHT THROUGH MONDAY.
-- End Changed Discussion --

&&

.NEAR TERM /THROUGH TONIGHT/...
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EARLY AFTERNOON VISIBLE SATELLITE LOOP SHOWS CU FIELD GRADUALLY
EXPANDING EAST FROM THE ALLEGHENY PLATEAU OF NORTHERN AND WRN PENN
AND BEGINNING TO GROW A BIT VERTICALLY.

THE LIMITING FACTORS TO EXPLOSIVE TSRA DEVELOPMENT SO FAR IN THE
HIGHLY UNSTABLE AIRMASS AT LLVLS HAS BEEN THE WSWRLY AND BROADLY
DIFFLUENT FLOW WITHIN THE 925-850 MB LAYER...BENEATH A LARGE POOL OF
QUITE WARM AIR /+7-8C AT 700 MB/...AND WEAK RIDGING ALOFT. THIS WILL
SERVE TO LIMIT CONVECTION ACROSS THE CWA THROUGH 20-21Z.

HIGH RESOLUTION 12-17Z MODEL GUIDANCE CONTINUES TO DEVELOP VERY
ROBUST...LATE DAY ML CAPE OF OVER 2000 J/KG WITHIN THE AREA OF MOST
ABUNDANT SUNSHINE SO FAR THIS AFTERNOON. AS THE RATHER FLAT RIDGING
SLIDES JUST EAST OF THE AREA LATER TODAY...A FAIRLY WELL-DEFINED LEE
TROUGH SHOULD FORM EAST OF THE ALLEGHENY FRONT AND PROVIDE ENOUGH
LLVL CONVERGENCE /IN CONCERT WITH SLIGHT COOLING ALOFT ARRIVING FROM
THE WEST/ TO SPARK ISOLATED TO SCATTERED STRONG TO POTENTIALLY
TSRA. SINCE THE LAST AFD...SPC HAS GREATLY EXPANDED THEIR DY1 SLIGHT
RISK AREA TO ENCOMPASS ABOUT THE NWRN TWO-THIRDS OF PENN...WITH SVR
TSRA WATCH 203 RECENTLY POSTED ISSUED FOR MUCH OF NEW YORK...RIGHT
AGAINST THE ENTIRE PA/NY BORDER.

SREF PROP OF CAPE EXCEEDING 1200 J/KG IS PEGGED BETWEEN AT 100
PERCENT FOR ALL BUT THE EXTREME SERN ZONES BETWEEN 20-23Z TODAY.

NAM HAS BEEN WOBBLING EAST AND WEST A BIT WITH IT/S DEPICTION OF THE
AXIS OF RELATIVELY HIGH EHI. 00-06Z RUNS WERE A BIT FURTHER EAST
ACROSS THE SUSQ VALLEY WITH VALUES AROUND 1.5 M2/S2...WHILE THE
FRESH 12Z RUN IS PAINTING A WIDE STRIPE OF MORE OMINOUS 2-2.5 MS/S2
EHI CENTERED ON A LINE FROM NEAR KELM...TO KUNV AND KAOO BY 00Z WED.

THIS TYPE OF EHI...WITH PRECEDING ABUNDANT INSOLATION IS QUITE
INFREQUENT IN THIS NECK OF THE WOODS. TYPICALLY...MORE PRONOUNCED
SHEAR AND FORCING HERE IN CENTRAL PENN USUALLY RESULTS IN MORE EARLY
DAY CONVECTION /OR STRATIFORM PRECIP/ WHICH LATER LIMITS THE DEGREE
OF LLVL-BASED INSTABILITY.

THE STRENGTHENING SWRLY FLOW AT 850 MB /TO AROUND 30 KTS LATE TODAY/
IS THE MOST CONCERNING FOR THE SEVERE THREAT ACROSS THE CENTRAL
RIDGE AND VALLEY REGION. IF WE CAN GET CONVECTION TO INITIATE WITHIN
THIS HIGHLY PRIMED/UNSTABLE ATMOS...STORMS COULD EASILY BECOME VERY
TALL AND LOCALLY SEVERE.

WEAK HT FALLS WILL DEVELOP IN ASSOCIATION WITH A S/W CRESTING THE
UPPER RIDGE OVER THE UPPER GRT LKS...AND NOSE OF STG LLJ WILL MOVE
NE INTO THE THE UPPER OH VLY/NW PA BY 00Z WED. THE MODELS SHOW A
MCS WITH THIS FEATURE MOVG EWD ACRS THE ERN GRT LKS/ST. LAWRENCE VLY
INTO NRN NEW ENGLAND. HIGHEST POPS OVERNIGHT ARE ACRS THE NRN
TIER...WITH NWD DISPLACEMENT MDL BIAS ARGUING FOR A MORE SRN MCS
TRACK. IN ADDITION...A LINGERING...NEARLY NORTH/SOUTH RIBBON OF HIGH
ML CAPE ACROSS THE SUSQ VALLEY COULD SUPPORT CONTINUED SCATTERED
SHRA/TSRA THERE THROUGH ABOUT 06Z WED.

THE SOUTHWEST LLVL FLOW WILL CONTINUE TO ADVECT VERY WARM AND MOIST
AIR INTO THE REGION WITH MID TO LATE AFTERNOON TEMPS TOPPING OUT
WELL INTO THE 80S ACROSS THE VALLEYS OF CENTRAL AND SRN PENN...AND
SFC DEWPOINTS IN THE LOWER TO MID 60S. MAX TEMPS ACROSS THE MTNS
WILL BE 80-84F.

CONVECTION WILL GRADUALLY DIMINISH LATE TONIGHT WITH WIND BECOMING
LIGHT IN THE MUGGY AIRMASS. PATCHY 1-2SM FOG WILL FORM AFTER
07Z...AND COULD BE LOCALLY DENSE IN THE VALLEYS WHERE RAINFALL
OCCURS.
-- End Changed Discussion --

&&

.SHORT TERM /WEDNESDAY/...
-- Changed Discussion --
A S/W MOVG EWD ACRS THE UPPER MS VLY INVOF IA WILL PHASE WITH A
CANADIAN UPPER LOW/TROUGH DROPPING SWD ACRS HUDSON/JAMES BAY DURING
THE DAY WEDNESDAY...REACHING THE MIDWEST BY F48. AT THE SFC...A
COLD FRONT PUSHING EWD THRU THE OH VLY WILL BE THE MAIN FOCUS FOR
TSTM DEVELOPMENT/SVR POTENTIAL THIS PERIOD.

AM CLOUDS WITH SOME SPOTTY SHOWERS AND AREAS OF 1-2SM FIG WILL
COMBINE WITH WEAK RIDGING WITHIN THE PREDOMINANT SWRLY FLOW
ALOFT...TO LIMIT DIURNAL HEATING TO SOME EXTENT ACRS THE DAY 2 SPC
SLGT RISK AREA - WHICH COVERS NEARLY ALL OF THE CWA WITH THE
EXCEPTION OF THE FAR SERN ZONES. ALTHOUGH LOW TO MID LEVEL WINDS
WILL BE STRONGEST ACROSS THE NW HALF OF PENN...LATEST SREF AND SHORT
RANGE MODEL GUIDANCE PAINTS THE GREATEST PROBABILITY FOR CAPE
GREATER THAN 1200 J/KG ACROSS THE LOWER SUSQ VALLEY LATE WEDNESDAY
AFTERNOON.

DESPITE THE TEMPERED DESTABILIZATION...EXPECT AN AFTN INCREASE IN
TSTM COVERAGE /WITH LOWER CLOUD BASES COMPARED TO TUESDAY`S/ ALONG
WITH SOME PICK-UP IN INTENSITY AS BELT OF STRONGER FLOW ALOFT SHIFTS
EWD AHEAD OF THE MID-UPPER TROUGH. BANDS OF TSTMS CAPABLE OF
PRODUCING DMGG WNDS/MARGINAL HAIL WILL BE POSSIBLE AND THUS WILL
CONTINUE TO HIGHLIGHT AN ELEVATED SVR WX THREAT IN THE HWO FOR
WEDNESDAY. BETTER FORCING PORTENDS HIGHER POPS RANGING FROM THE
MED-HIGH CHC RANGE IN THE SE TO LOW LKLYS IN THE NW. SHOWERS/TSTMS
MAY PERSIST AFTER DARK INTO WED NGT.

MAX TEMPS WILL LIKELY BE 3-5F LOWER THAN TUESDAY...BUT STILL AROUND
12F ABOVE NORMAL.
-- End Changed Discussion --

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
ERN NOAM TROUGH AXIS EXTENDING FROM JAMES BAY SWD THRU THE GRT
LKS/MID-SOUTH TO THE CENTRAL GULF COAST AT THE START OF THE PERIOD
WILL PUSH SLOWLY EWD AND SHOULD REACH THE EAST COAST BY 00Z SAT.
THE GUIDANCE IS SHOWING SOME NOTABLE DIFFS WITH THE H5 SYSTEM. THE
NAM/GFS/GEFS ARE SLOWER AND SHARPER WITH TROUGH AS IT CROSSES THE
OH VLY INTO THE MID-ATLC. THE 00Z GFS EVEN CLOSES OFF THE H5 LOW
OVER NRN VA FRI EVE. THE LAST TWO RUNS OF THE CMC HAVE BEEN TRYING
TO GENERATE A TROPICAL CYCLONE-LIKE SYSTEM NEAR THE OUTER BANKS.
THE KNOWN SLOW/TROPICAL BIASES OF THE NAM/CMC AND STRONG/CLOSED
SOLN OF THE GFS LEADS TO A UKMET/ECMWF PREFERRED SOLN WHICH FAVORS
MORE EWD PROGRESSION WITH THE TROUGH AND ITS ASSOCD COLD FRONT.

THE COLD FRONT WILL CROSS THE CWA ON THURS ACCOMPANIED BY SCT TO
NMRS SHOWERS AND TSTMS. MUCH LIKE ON DAY 2...THE PRE-FRONTAL WARM
SECTOR /LOCATED OVER THE ERN 1/2 OF PA/ WILL BE CHARACTERIZED BY
35-40KTS OF DEEP-LAYER SHEAR WITH WEAK INSTABILITY. THEREFORE THE
OVERALL SVR POTENTIAL APPEARS RELATIVELY LOW ATTM. THE SPC DAY 3
OUTLOOK DOES INCLUDE ERN PA IN THE SEE TEXT/5% SVR FOR ISOLATED
HAIL AND WIND THREAT.

BEHIND THE DEPARTING COLD FRONT...A STG AND LARGE ANTICYCLONE
SHOULD DOMINATE PATTERN IN THE FRI-MON TIMEFRAME...BUILDING SEWD
FROM NUNAVUT ACRS THE GRT LKS INTO THE NORTHEAST. AN UNSEASONABLY
COLD/DRY AIRMASS WILL ACCOMPANY THE HIGH RESULTING IN BLW NORMAL
TEMPS FOR MEMORIAL DAY WEEKEND. ECENS MOS DATA SHOWS TEMPS ON
FRI/SAT STRUGGLING TO CRACK THE 60F MARK LOCALLY...WITH SEVERAL
OPPORTUNITIES FOR A LATE-SEASON FROST OVER THE NRN MTNS SAT/SUN/MON
MORNINGS. THERE SHOULD BE A MODERATION INTO THE MIDDLE OF NEXT
WEEK WITH TEMPS GETTING BACK TO NEAR NORMAL AROUND 5/28.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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ALL CENTRAL PA TAF SITES WILL STAY PRIMARILY VFR THROUGH AT LEAST
EARLY THIS EVENING.

HOWEVER...SCT LATE PM TSRA WILL BE AN AVIATION CONCERN. THE BEST CHC
OF TSRA AND ACCOMPANYING BRIEF VSBY REDUCTION/GUSTY WINDS WILL BE
ACROSS THE CENTRAL RIDGE AND VALLEY REGION AND POINTS NORTH TWD THE
NEW YORK BORDER. LATEST MDL DATA CONTINUES TO INDICATE SOME
POSSIBILITY OF A BKN LINE OF TSRA DEVELOPING SOMEWHERE NEAR THE
I-99/RT 220 CORRIDOR FROM KJST/KAOO...NE TO KIPT...THEN SWEEPING SE
ACROSS CENTRAL PENN AND TWD THE SUSQ VALLEY THIS EVENING.

CONVECTIVE SHOWERS AND ISOLATED-SCT TSRA WILL DIMINISH AFTER 06Z
WITH MVFR HAZE LOWERING TO AREAS OF IFR STRATUS AND FOG THROUGH 12
OR 13Z WED. MAINLY VFR TO HIGH END MVFR LATER WEDNESDAY MORNING INTO
THE EARLY AFTERNOON BEFORE MORE SHOWERS AND TSRA DEVELOP ACROSS
CENTRAL PENN IN THE 18Z-23Z WINDOW.

.OUTLOOK...
THU...SCT THUNDERSTORM IMPACTS POSSIBLE...MAINLY IN THE PM.
FRI-MON...NO SIG WX EXPECTED.
-- End Changed Discussion --

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$
SYNOPSIS...LAMBERT/STEINBUGL
NEAR TERM...LAMBERT
SHORT TERM...LAMBERT/STEINBUGL
LONG TERM...STEINBUGL
AVIATION...LAMBERT