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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KCTP 211518
AFDCTP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
1000 AM EDT TUE MAY 21 2013

.SYNOPSIS...
A WARM AND HUMID...SUMMERTIME AIRMASS WILL CONTINUE TO BRING ABOVE
NORMAL TEMPERATURES TO CENTRAL PENNSYLVANIA THROUGH WEDNESDAY. A
COLD FRONT WILL CROSS THE REGION ON THURSDAY. A LARGE AREA OF HIGH
PRESSURE WILL BUILD SOUTH AND EAST FROM CENTRAL CANADA INTO THE
NORTHEAST AND MID-ATLANTIC STATES...BRINGING COOLER AND DRIER AIR
TO THE AREA FOR MEMORIAL DAY WEEKEND.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
LATE MORNING VISIBLE SATELLITE LOOP SHOWS QUICKLY ERODING FOG/LOW
STRATUS ACROSS THE LOWER SUSQ VALLEY...AND SLOW EASTWARD MIGRATION
OF A BKN...AND GENERALLY SHALLOW STRATOCU FIELD ACROSS THE ALLEGHENY
PLATEAU.

BROAD...WESTERLY DIFFLUENT FLOW WITHIN THE 925-850 MB LAYER...
BENEATH AN AREA OF QUITE WARM AIR /+7C AT 700 MB/...AND WEAK RIDGING
ALOFT WILL SERVE TO LIMIT CONVECTION THROUGH THE EARLY-MID AFTERNOON
HOURS.

HIGH RESOLUTION MODEL GUIDANCE DOES DEVELOP VERY ROBUST ML CAPE OF
OVER 2000 J/KG WITHIN THE AREA OF ABUNDANT SUNSHINE THIS AFTERNOON.
AS THE RATHER FLAT RIDGING SLIDES JUST EAST OF THE AREA LATER
TODAY...A FAIRLY WELL-DEFINED LEAD TROUGH SHOULD FORM EAST OF THE
ALLEGHENY FRONT AND SHOULD PROVIDE ENOUGH LLVL CONVERGENCE /IN
CONCERT WITH SLIGHT COOLING ALOFT/ TO SPARK ISOLATED TO SCATTERED
TSRA.

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.

AS NOTED BY THE PREVIOUS SHIFT`S DISCUSSION...THE SPC CATG OUTLOOK
HIGHLIGHTS NRN PA IN THE SEE TEXT WITH 5% PROB OF DMGG WINDS. THIS
AREA MAY BE INFLUENCED BY WEAK HT FALLS ASSOCD WITH S/W CRESTING THE
UPPER RIDGE OVER THE UPPER GRT LKS...AND NOSE OF STG LLJ IN THE
UPPER OH VLY/NW PA BY 00Z. 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.

THE SOUTHWEST LLVL FLOW WILL CONTINUE TO ADVECT VERY WARM AND MOIST
AIR INTO THE REGION WITH AFTERNOON TEMPS IN THE 80S AND SFC
DEWPOINTS IN THE LOWER TO MID 60S.
-- End Changed Discussion --
&&

.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
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 F72. 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 PCPN IS EXPECTED 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.
DESPITE THE TEMPERED DESTABILIZATION...EXPECT AN AFTN INCREASE IN
TSTM COVERAGE 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.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
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 /14Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
MID MORNING FOG HAS BURNED OFF...WITH ALL CENTRAL PA TAF SITES NOW
VFR.

HOWEVER...SCT PM TSRA WILL AGAIN BE AN AVIATION CONCERN. THE BEST
CHC OF AN AFTN TSRA AND ACCOMPANYING BRIEF VSBY REDUCTION WILL BE
ACROSS THE CENTRAL RIDGE AND VALLEY REGION AND POINTS WEST OVER THE
ALLEGHENY PLATEAU. HOWEVER...LATEST MDL DATA INDICATES SOME
POSSIBILITY OF A 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.

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

&&

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

&&

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