000
FXUS61 KCTP 212130
AFDCTP

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

.SYNOPSIS...
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.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
THE UPPER RIDGE WILL BREAK DOWN AS A TROUGH DEEPENS INTO THE GREAT
LAKES REGION BY THURSDAY AFTERNOON. THIS WILL CORRESPOND WITH A
FRONT MOVING THROUGH THE NORTHEAST THURSDAY. TIMING FOR THIS FRONT
IS STILL VARYING BUT CURRENT EC BRINGS THE FRONT THROUGH MIDDAY
THURSDAY. COUPLE THIS FRONT WITH DAYTIME HEATING AND STRONG
SOUTHWEST FLOW...THURSDAY COULD BE THE BEST DAY FOR DAYTIME
CONVECTION THIS WEEK. BOTH THE OPERATIONAL GFS AND THE ECMWF CONTINUE
TO OUTPUT THE POSSIBILITY OF A LOW LEVEL JET THURSDAY
AFTERNOON...WHICH WITH 35-40KTS OF DEEP-LAYER SHEAR WILL ONLY
INCREASE PROBABILITIES FOR PRECIPITATION THURSDAY AFTERNOON. THIS
EASTERN NORTH AMERICA TROUGH AXIS EXTENDING FROM JAMES BAY SWD
THRU THE GRT LKS/MID- SOUTH TO THE CENTRAL GULF COAST WILL PUSH
SLOWLY EWD AND SHOULD REACH THE EAST COAST BY 00Z SAT. THIS WILL
ALLOW FOR PERSISTENT INSTABILITY OVER THE AREA THROUGH THE END OF
THE WEEK...WITH COLD NORTHWESTERLY FLOW BEHIND THE FRONT. 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.

BEHIND THE DEPARTING COLD FRONT...A STG AND LARGE ANTICYCLONE
SHOULD DOMINATE PATTERN IN THE FRI-MON TIMEFRAME...BUILDING SEWD
FROM NUNAVUT CANADA ACROSS THE GRT LKS INTO THE NORTHEAST. AN
UNSEASONABLY COLD/DRY AIRMASS WILL ACCOMPANY THE HIGH RESULTING IN
BELOW 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. HOWEVER CURRENT LOW TEMPS REMAIN ABOVE 40
FOR MOST OF THE REGION WITH THE EXCEPTION BEING ALONG THE NEW YORK
BORDER. WINDS SHOULD REMAIN GUSTY SATURDAY MORNING. THE BETTER
CHANCE FOR FROST WILL BE SUNDAY MORNING...TEMPERATURES WILL BE
SLIGHTLY WARMER BUT THE WINDS SHOULD BE LIGHT TO CALM. THERE
SHOULD BE A MODERATION INTO THE MIDDLE OF NEXT WEEK WITH TEMPS
GETTING BACK TO NEAR NORMAL AROUND 5/28.

&&

.AVIATION /22Z TUESDAY THROUGH SUNDAY/...
DID ADJUST 21Z TAF PACKAGE SOME TO CUT BACK THE MENTION OF 
THUNDER. MAIN CHANCE WILL BE NEAR THE NY BORDER THIS EVENING...
FURTHER SOUTH IT IS WARMER ALOFT WITH LOWER DEWPOINTS. 

WILL CONTINUE TO ADJUST THIS EVENING AS NEEDED.

EARLIER DISCUSSION BELOW.

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.

&&

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

&&

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