306  
FXUS61 KCTP 292004  
AFDCTP  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE STATE COLLEGE PA  
404 PM EDT MON MAY 29 2017  
   
SYNOPSIS
 
 
A TROUGH OF LOW PRESSURE WILL STALL OUT OVER CENTRAL AND  
EASTERN PA LATE TODAY, BEFORE RETREATING BACK INTO THE CENTRAL  
MTNS TONIGHT. ANOTHER COLD FRONT WILL PUSH TOWARD THE REGION  
TUESDAY AND COMBINE WITH THE LINGERING BOUNDARY TO PRODUCE  
NUMEROUS SHOWERS AND SCATTERED AFTERNOON THUNDERSTORMS, SOME OF  
WHICH COULD CONTAIN GUSTY WINDS AND HAIL. LOW PRESSURE AT THE  
SURFACE AND ALOFT WILL STAY CENTERED NEAR JAMES BAY CANADA  
THROUGH THE REST OF THE WEEK. THIS WEATHER FEATURE WILL SUPPLY A  
PERIOD OF SLIGHTLY COOLER THAN NORMAL TEMPERATURES, AND PUSH A  
FEW ADDITIONAL WEAK COLD FRONTS THROUGH THE COMMONWEALTH  
ACCOMPANIED BY SCATTERED SHOWERS AND PERHAPS SOME ISOLATED  
AFTERNOON AND EARLY EVENING THUNDERSTORMS.  
 

 
   
NEAR TERM /UNTIL 6 AM TUESDAY MORNING/
 
 
WEAK SURFACE FRONT REMAINS STALLED OVER THE SUSQUEHANNA VALLEY  
LATE THIS AFTERNOON WITH PARTLY TO MOSTLY SUNNY SKIES AND A  
LIGHT TO MDT WESTERLY BREEZE ACROSS THE CENTRAL AND WESTERN  
MTNS OF PA.  
 
TO THE EAST OF THE BOUNDARY, COOL AND CLOUDY AIRMASS WITH LOW  
STRATUS BELOW 1000 FT AGL WILL PREVAIL WITH TEMPS ONLY IN THE  
LOW-MID 60S. MAXES ELSEWHERE WILL BE MAINLY IN THE 70S, WITH  
MID-UPPER 70S LIKELY IN THE CENTRAL AND SOUTHERN VALLEYS TO THE  
SE OF A KAOO TO KUNV LINE WHERE THE LLVL WESTERLY DOWNSLOPE  
FLOW AND HEATING WILL BE MAXIMIZED.  
 
DIFFERENTIAL HEATING ALONG THE WESTERN EDGE OF THE CLOUD  
BOUNDARY ACROSS THE SUSQ VALLEY WILL LEAD TO A NARROW RIBBON OF  
ENHANCED CAPE AND MOISTURE CONVERGENCE WHICH WILL HELP TO  
PRODUCE ISOLATED-SCATTERED SHOWERS AND ISOLATED LOW-TOPPED TSRA  
PER THE LATEST 18Z HRRR. HOWEVER, THE MODEL APPEARS TO BE  
OVERESTIMATING LLVL CAPE AND CONVECTION AT THIS TIME, THANKS TO  
THE PRESENCE OF AT LEAST WEAK LARGER SCALE SUBSIDENCE ALOFT -  
BENEATH THE RIGHT EXIT REGION OF A 105 KT UPPER JET. ANY  
CONVECTION WILL DRIFT TO THE EAST AND SHOULD GREATLY WEAKEN  
WITHIN 20-30 MILES EAST OF THE STALLED OUT LLVL BOUNDARY.  
 

 
   
SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/
 
 
BRIEF RIDGING WILL ALLOW ANY POPCORN SPRINKLES AND LIGHT SHOWERS  
TO CLEAR OVER THE WESTERN TWO-THIRDS OF THE CWA THIS EVENING,  
AND RESIDUAL MOISTURE MAY LEAD TO AREAS OF FOG, ESPECIALLY IN  
THE CENTRAL VALLEYS. THE LOW LEVEL MOISTURE AND LOW STRATUS DECK  
AOB 1000 FT AGL WILL PUSH GRADUALLY WESTWARD ACROSS THE CENTRAL  
MTNS AND WEST BR SUSQ VALLEY AND STAY INTACT THROUGH EARLY  
TUESDAY. MINS TONIGHT WILL DIP INTO THE 50S FOR MOST, AS THE  
DEWPOINTS WILL BE THE BOTTOM LIMIT.  
 
A SEPARATE, WEAK SFC TROUGH WILL BE PUSHED INTO THE AREA FROM  
THE NORTHWEST TONIGHT. 8H TEMPS FALL A BIT OVER THE NW. HEIGHTS  
FALL JUST A LITTLE, TOO, SO SOME PRECIP IS POSSIBLE BEFORE  
SUNRISE TUESDAY OVER THE NORTHWESTERN THIRD OF THE AREA, BUT IT  
SHOULD BE LIGHT.  
 
THE TROUGH BEGINS TO CATCH THE STALLED TROUGH OVER THE EASTERN  
COUNTIES. THE SOUTHERLY OR SOUTHEASTERLY WIND WILL YIELD GOOD  
CONVERGENCE AND WILL CRANK UP SOME SHOWERS AND THUNDERSTORMS.  
HAVE PAINTED HIGH POPS FOR THE AREA, BUT THE AMOUNT/SEVERITY OF  
DESTABILIZATION IS IN QUESTION WITH THE CLOUDS IN THE NE EARLY  
IN THE DAY. NAM CRANKS OUT 1500JOULES OF CAPE AND LI DROPS TO -2  
OR SO IN THE MID-DAY AND AFTN ON TUES. SPC MARGINAL RISK OF SVR  
WX FOR DAY 2 (TUES) IS PAINTED ACROSS OUR EASTERN HALF, FOCUSED  
ON THE PEAK HEATING TIME. WILL CONTINUE TO MENTION THIS IN THE  
HWO.  
 
MAX TEMPS ACROSS THE REGION WILL RANGE THROUGH THE 70S, WITH THE  
WARMEST READINGS ONCE AGAIN IN THE SCENT MTNS WHERE 78-80F  
READINGS SHOULD OCCUR.  
 

 
   
LONG TERM /TUESDAY NIGHT THROUGH MONDAY/
 
 
THE LONG TERM PERIOD BEGINS WITH A GOOD CONSENSUS ON THE  
LOCATION AND SHAPE OF THE LARGE UPPER LEVEL LOW OVER SOUTHERN  
CANADA. THAT LOW WILL DOMINATE THE WEATHER PATTERN THIS WEEK  
INTO NEXT. AS THE LOW ROTATES IT WILL BRING SUCCESSIVE SHORT AND  
LONG WAVE TROUGHS THROUGH THE MID ATLANTIC REGION. THESE  
TROUGHS WILL BRING THE POSSIBILITY OF PRECIPITATION, MAINLY  
WEDNESDAY AND FRIDAY AFTERNOON. THIS UPPER LEVEL LOW WILL ALSO  
BRING COOLER NORTHWESTERLY FLOW INTO CENTRAL PA. THAT FLOW  
SHOULD KEEP TEMPERATURES AROUND, TO SLIGHTLY BELOW, NORMAL.  
 
THE NEXT CHANCE FOR SIGNIFICANT PRECIPITATION WILL BE FRIDAY NIGHT  
INTO SATURDAY AS THAT UPPER LEVEL LOW PROGRESSES EASTWARD. THE  
MODELS BEGIN TO DIVERGE ON THE POSITION, TIMING AND STRENGTH OF  
THE SYSTEM. THIS DECREASES CONFIDENCE. HOWEVER, ALL MODELS SHOW  
A BOUNDARY THAT SHOULD SET UP THROUGH THE KEYSTONE STATE. THE  
MAIN QUESTION IS WHEN WILL IT PROGRESS THROUGH AND HOW MUCH  
MOISTURE WILL BE AVAILABLE FOR QPF WHEN THE CORRESPONDING COLD  
FRONT MOVES THROUGH SATURDAY.  
CURRENT GFS BRINGS PRECIPITATION THROUGH SATURDAY WHERE THE EC  
BRINGS LARGEST QPF SUNDAY.  
 

 
   
AVIATION /20Z MONDAY THROUGH SATURDAY/
 
 
WIDESPREAD SUB VFR CIGS WILL BE DIFFICULT TO CLEAR EAST OF A  
LINE FROM KIPT TO KSEG AND KMDT LATE THIS AFTERNOON, AND IF SO,  
IT WILL BE ONLY FOR A BRIEF FEW-SVRL HOUR PERIOD. LLVL EASTERLY  
FLOW OF 5-10 KTS WILL LIKELY PERSIST OVER, AND TO THE EAST OF  
THE SUSQ VALLEY WITH MVFR TO IFR CIGS AND VFR TO MVFR VSBYS IN  
LIGHT FOG/HZ.  
 
SCT-BKN VFR STRATOCU AND CU WILL OCCUR ACROSS THE WESTERN TWO  
THIRDS OF THE STATE THANKS TO A DRYING, LIGHT TO MODERATE  
WESTERLY LLVL WIND.  
 
DEPENDING ON HOW MUCH CLEARING OCCURS/REMAINS THIS EVENING,  
WIDESPREAD FOG FORMATION IS POSSIBLE REGION-WIDE TONIGHT. LATER  
ON, EXPECT THE STALLED OUT BOUNDARY ACROSS THE SUSQ VALLEY TO  
DRIFT WESTWARD BACK TO THE ALLEGHENY FRONT (KAOO, KUNV AND KIPT)  
BY...OR SHORTLY AFTER 04Z TUESDAY.  
 
   
OUTLOOK
 
 
TUE...MORNING IFR STRATUS LIKELY ACROSS MUCH OF CENTRAL AND SE  
PA, THEN BREAKS IN/LIFTING OF THE CLOUD DECK LEADING TO MVFR TO  
LOW-END VFR CONDITIONS WITH SHOWERS BECOMING MORE NUMEROUS  
DURING THE LATE MORNING AND AFTERNOON HOURS. ALSO, SCATTERED  
AFTERNOON TSRA WITH BRIEF, LOCALIZED GREATER REDUCTIONS.  
 
WED...CHANCE OF SHOWERS.  
 
THU...NO SIG WX EXPECTED.  
 
FRI...CHANCE OF SHOWERS AND THUNDERSTORMS.  
 

 
   
CTP WATCHES/WARNINGS/ADVISORIES
 
 
NONE.  
 

 
 

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