473  
FXUS61 KPHI 210852  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
352 AM EST TUE NOV 21 2017  
   
SYNOPSIS
 
 
A COLD FRONT WILL CROSS THE AREA TUESDAY NIGHT AND EARLY WEDNESDAY,  
FOLLOWED BY HIGH PRESSURE INTO SATURDAY. ANOTHER COLD FRONT IS  
EXPECTED TO CROSS THE AREA SATURDAY NIGHT, WITH HIGH PRESSURE  
ANTICIPATED FOR THE FIRST PART OF NEXT WEEK.  

OUTLOOK...  
 
WEDNESDAY...LULL IN SCA CONDITIONS POSSIBLE WEDNESDAY MORNING.  
OTHERWISE, WINDS SHIFT TO THE NW AND INCREASE TO 15-20 KT WITH 25-30  
KT GUSTS.  
 
THURSDAY THROUGH FRIDAY...SUB-SCA CONDITIONS EXPECTED.  
 
SATURDAY AND SUNDAY...SCA CONDITIONS ARE POSSIBLE WITH THE PASSAGE  
OF A STRONG COLD FRONT.  
 
   
NEAR TERM /UNTIL 6 PM THIS EVENING/
 
 
DIGGING VORT MAX IN THE UPPER MIDWEST WILL CONTINUE SOUTHEASTWARD  
THROUGH THE GREAT LAKES REGION TODAY. DOWNSTREAM, MIDLEVEL RIDGING  
WILL AMPLIFY IN RESPONSE TO LOW-LEVEL WARM-AIR ADVECTION EXPANDING  
THE ATMOSPHERIC COLUMN. PREDECESSOR PERTURBATIONS IN THE FLOW WILL  
PROGRESS THROUGH THE EAST TODAY, THOUGH THESE SHOULD HAVE GENERALLY  
MINIMAL IMPACTS ON SENSIBLE WEATHER OTHER THAN INCREASING HIGH  
CLOUDS, ESPECIALLY THIS AFTERNOON. AS THE SURFACE RIDGE TO OUR SOUTH  
CONTINUES OFFSHORE TODAY, SOUTHWESTERLY FLOW WILL CONTINUE, GETTING  
A BOOST FROM DIABATIC HEATING AND A STRONG SURFACE LOW MOVING  
THROUGH ONTARIO (AIDING IN AN INCREASED SURFACE PRESSURE GRADIENT).  
AS SUCH, EXPECT BREEZY CONDITIONS AGAIN TODAY, ESPECIALLY  
ALONG/SOUTHEAST OF THE URBAN CORRIDOR, AS AREAS TO THE NORTHWEST  
WILL BE IN SOMEWHAT OF A SURFACE COL BY AFTERNOON.  
 
STATISTICAL GUIDANCE SHOWS GOOD AGREEMENT, BUT I SUSPECT THERE IS A  
BIT OF A COOL BIAS WITH TEMPERATURES TODAY GIVEN THE WARMING  
PROFILES AND THE AMPLIFIED RIDGING. HARD TO GO AGAINST GOOD  
AGREEMENT AMONG THE GUIDANCE, BUT I DID NUDGE UP TEMPERATURES A  
DEGREE OR TWO IN SPOTS FROM THE INHERITED FORECAST. IT IS QUITE  
POSSIBLE THAT PHILLY REACHES 60 TODAY, WITH OVERALL MAX TEMPERATURES  
ABOUT 2-7 DEGREES ABOVE SEASONAL AVERAGES.  

 
   
SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/
 
 
A PERTURBATION OVER THE SOUTHEAST WILL LIFT NORTHEASTWARD DURING THE  
DAY, LIKELY IN VICINITY OF THE CAROLINAS BY 21Z. PRECIPITATION  
SHOULD BREAK OUT NEAR THIS VORT MAX, WITH A SURFACE WAVE LIKELY  
DEVELOPING DURING THE DAY NEAR OR OFF THE COAST OF FLORIDA. THIS  
SOUTHERN-STREAM SYSTEM WILL BE AFFECTED STRONGLY BY THE NORTHERN-  
STREAM VORT MAX, WITH SOME PHASING LIKELY TO OCCUR DURING THE  
OVERNIGHT PERIOD, AND RAPID DEVELOPMENT/NORTHEASTWARD PROGRESSION OF  
THE SURFACE LOW NEAR/OFF THE CAROLINA COAST. HOWEVER, DESPITE THE  
OVERALL EXCELLENT AGREEMENT IN THE LARGE-SCALE PATTERN, THE NAM/GFS  
DEPICTIONS OF THE SOUTHERN-STREAM LOW LEAVE SOME QUESTION MARKS. THE  
GFS LOOKS A LITTLE ON THE FAST/PROGRESSIVE SIDE, WHICH IS A TYPICAL  
BIAS IT EXHIBITS IN THE SHORT TERM. COMPARISON TO THE RGEM SUGGESTS  
IT MIGHT BE ON THE ORDER OF 3 HOURS TOO FAST, AND THIS DIFFERENCE  
MAY BE ENOUGH TO KEEP THE GFS TOO DRY TONIGHT. IT WILL BE CRITICAL  
TO WATCH THE EVOLUTION OF THE SOUTHERN-STREAM SYSTEM TODAY (AND THE  
PRECIPITATION/UPSCALE FEEDBACK PROCESSES) TO DETERMINE IF THE GFS IS  
KEEPING US TOO DRY TONIGHT.  
 
MEANWHILE, THE NAM LOOKS VERY SIMILAR TO THE RGEM IN THE OVERALL  
EVOLUTION OF THE SYSTEM TONIGHT, BUT THE NAM IS SIMPLY DRIER THAN  
THE RGEM (ANOTHER BIAS OBSERVED ON MULTIPLE OCCASIONS THIS SEASON).  
BASED ON THIS (AND THE GFS BIAS), I WEIGHTED GUIDANCE THAT WAS  
SLIGHTLY SLOWER/WETTER (E.G., THE NAM NEST AND THE WRF-ARW, ALONG  
WITH THE RGEM) SOMEWHAT HIGHER. ALSO INCLUDED SOME OF THE 00Z ECMWF  
WITH THE QPF GRIDS TONIGHT, DESPITE ITS COARSER RESOLUTION, AS IT  
SEEMS TO BE DOING A GOOD JOB THIS FALL DEPICTING THE CUTOFF OF THE  
POLEWARD SIDE OF THE PRECIPITATION SOMEWHAT BETTER THAN THE NAM/GFS  
(LIKELY BECAUSE OF ITS GENERAL SLOWER/DEEPER DEPICTIONS OF THESE  
PHASING SYSTEMS).  
 
THE NORTHERN-STREAM UPPER-LEVEL JET STREAK WILL BE IN A FAVORABLE  
POSITION BY NIGHT'S END FOR RATHER DEEP ASCENT, BUT BY THIS POINT,  
THE STRONGER LIFT WILL PROBABLY BE OFFSHORE. NEVERTHELESS, MESOSCALE  
BANDING OF THE PRECIPITATION IS CLEARLY EVIDENT IN MOST OF THE MODEL  
SIMULATIONS TONIGHT, AND THIS DOES OCCUR AS FAR WEST AS I-95 IN MOST  
OF THESE SIMULATIONS. THIS MAKES THE QPF A CHALLENGE, SINCE THESE  
SETUPS CAN BE UNDERDONE IN THE MODELS (BOTH WITH PRECIPITATION RATES  
AND THE SLOW PROGRESSION OF THESE PHENOMENA COMPARED TO THE SPEED OF  
DEEP-LAYER FLOW). THIS WAS ANOTHER REASON I UPPED THE QPF FROM THE  
PREVIOUS FORECAST, BUT TEMPERED TO SOME DEGREE VERSUS THE WETTEST  
GUIDANCE (SREF/ECMWF).  
 
WITH THE ABOVE IN MIND, BROUGHT POPS UP TO CHANCE GENERALLY URBAN  
CORRIDOR SOUTHEASTWARD AND TO LIKELY SOUTHEAST OF A DOVER, DE, TO  
SANDY HOOK, NJ, LINE. THE TIMING OF THE PRECIPITATION LOOKS TO BE  
MAINLY AFTER MIDNIGHT, BUT THERE IS SOME UNCERTAINTY HERE GIVEN THE  
DISCREPANCIES MENTIONED EARLIER. AMOUNTS GENERALLY IN THE TENTH TO  
QUARTER INCH RANGE ALONG/SOUTHEAST OF I-95 TO POTENTIALLY A HALF  
INCH NEAR THE COAST, BUT CONFIDENCE IN THESE VALUES IS LOW BECAUSE  
OF THE UNCERTAINTY WITH THE EXACT TRACK OF THE PRECIPITATION. ALSO,  
THE CUTOFF OF THE PRECIPITATION WILL LIKELY BE SHARPER THAN I HAVE  
DEPICTED IN THE GRIDS (AN INEVITABLE RESULT OF PRECIPITATION  
FORECAST UNCERTAINTY). BOTTOM LINE: LARGE ERRORS IN THE POPS/QPF ARE  
POSSIBLE TONIGHT GIVEN THE REMAINING MODEL DISCREPANCIES.  
 
TEMPERATURES WERE GENERALLY A BLEND OF MET/MAV/ECS MOS, THOUGH I DID  
NUDGE TEMPERATURES UP A LITTLE, IN GENERAL GIVEN THE INCREASED  
CLOUDS AND REMAINING LOW-LEVEL SOUTH/SOUTHWEST FETCH. LOOKS LIKE  
TEMPERATURES WILL BE TOO WARM FOR NON-LIQUID, NON-FREEZING  
PRECIPITATION ANYWHERE IN OUR CWA THROUGH DAYBREAK.  
 
ONE FINAL NOTE: GIVEN THE STRENGTHENING DYNAMICS ALOFT, STRONG LIFT  
WILL AID IN MIDLEVEL COOLING POSSIBLY ENOUGH TO GENERATE SOME  
MARGINAL INSTABILITY. NOT TOTALLY OUT OF THE QUESTION TO SEE A  
COUPLE LIGHTNING STRIKES, THOUGH THE BEST CHANCES ARE OFFSHORE.  

 
   
LONG TERM /WEDNESDAY THROUGH MONDAY/
 
 
THE EXTENDED PERIOD STARTS OFF WITH A COLD FRONT NEAR OR ALONG THE  
COAST WEDNESDAY MORNING, MOVING EAST. CHILLIER AIR, NORMAL FOR THIS  
TIME OF YEAR, WILL MOVE IN BEHIND THIS FRONT. WITH RIDGING ALOFT  
OVER THE WESTERN U.S. AND TROUGHINESS IN THE EAST, A SEASONABLY COOL  
BUT MOSTLY DRY PATTERN IS EXPECTED THROUGH MOST OF THE LONG TERM  
PERIOD, WITH A CHANCE OF PRECIPITATION LATER SATURDAY AND SUNDAY AS  
ANOTHER COLD FRONT AND ASSOCIATE DEEP UPPER LEVEL TROUGH CROSSES THE  
MID-ATLANTIC REGION.  
 
THE FROPA EARLY WEDNESDAY WILL TRIGGER A FEW SHOWERS N/W OF PHL. A  
SFC LOW PRESSURE IS ALSO FORECAST TO DEVELOP OFF THE NC/SC COASTS  
WITH MOISTURE SPREADING NORTH AHEAD OF IT ALONG THE DE/NJ COASTAL  
AREAS. THE HIGHEST POPS CONTINUE OVER SOUTHERN NJ AND DELMARVA,  
COUNTER TO CLIMO. OVERNIGHT TEMPS SHOULD REMAIN WARM ENOUGH SO THAT  
ALL PRECIP WILL BE IN LIQUID FORM. HOWEVER A FEW SHOWERS MAY LINGER  
PAST SUNRISE, AND WITH COLD ADVECTION BEHIND THE FRONT A FEW SNOW  
SHOWERS OR FLURRIES ARE POSSIBLE IN THE POCONOS WEDNESDAY MORNING.  
 
THE WEATHER FOR WED AFTERNOON THROUGH FRIDAY NIGHT WILL BE DOMINATED  
BY HIGH PRESSURE MOVING ACROSS THE MID-ATLANTIC REGION. TEMPERATURES  
WILL BE A FEW DEGREES BELOW NORMAL ON THURSDAY BUT SHOULD RETURN TO  
NEAR NORMAL BY FRIDAY AS THE HIGH SHIFTS OFFSHORE AND A MORE SWLY  
FLOW DEVELOPS. A COLD FRONT MAY PUSH SOUTH IN TO NY STATE AND NEW  
ENGLAND ON FRIDAY BUT PROBABLY WILL NOT AFFECT OUR AREA TO THE SOUTH,  
 
ANOTHER FAST-MOVING FRONTAL SYSTEM IS FORECAST TO PUSH THROUGH OUR  
AREA ON SATURDAY. IN PART BECAUSE OF ITS FAST MOVEMENT IT WILL NOT  
BE ABLE TO PICK UP MUCH MOISTURE. THE CURRENT FORECAST CONTINUES  
CHANCE POPS, HIGHER N/W OF PHL, BUT EVEN THIS MAY BE OVERDONE.  
AGAIN, TEMPS ON SATURDAY APPEAR TO BE WARM ENOUGH TO AVOID ANY  
FROZEN PRECIP. HOWEVER, THE AIRMASS BEHIND THIS FRONT SEEMS TO BE  
COLDER THAN THE ONE MID-WEEK, WITH STRONG LOW LEVEL COLD ADVECTION  
BEGINNING SATURDAY NIGHT AND CONTINUING THROUGH THE REST OF THE  
WEEKEND. THIS PATTERN OF NW FLOW WILL FAVOR LAKE EFFECT SNOW WITH  
SOME STREAMERS POSSIBLY REACHING THE POCONOS AND VICINITY.  
 

 
   
AVIATION /09Z TUESDAY THROUGH SATURDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
TODAY...VFR WITH SOUTHWEST WINDS 5 TO 15 KTS WITH OCCASIONAL GUSTS  
TO 20 KTS OR SO, ESPECIALLY FROM THE I-95 CORRIDOR SOUTHEASTWARD.  
HIGH CONFIDENCE.  
 
TONIGHT...RAPIDLY INCREASING CLOUDS, LIKELY TO SUB-VFR NEAR THE  
COASTS (KMIV AND KACY MOST LIKELY TO SEE THIS). SHOWERS SHOULD  
SPREAD FROM SOUTHWEST TO NORTHEAST THROUGH THE AREA AFTER 06Z  
WEDNESDAY, MAINLY CONFINED TO THE URBAN CORRIDOR SOUTHEASTWARD. SUB-  
VFR VSBYS ARE LIKELY WITH THE HEAVIER SHOWERS, BUT CONFIDENCE ON HOW  
FAR INLAND THE PRECIPITATION REACHES IS VERY LOW. WINDS GENERALLY  
SOUTHWEST AROUND OR BELOW 10 KTS, WITH SOME VEERING TO WESTERLY  
EXPECTED BY DAYBREAK. MEDIUM CONFIDENCE WITH WINDS; LOW CONFIDENCE  
WITH CIGS/VSBYS.  
 
OUTLOOK...  
 
WEDNESDAY...RAIN POSSIBLE SOUTH AND EAST OF I-95 EARLY WEDNESDAY  
MORNING WITH MVFR OR LOWER CONDITIONS...THEN BECOMING VFR. SW  
WINDS 5-10 KT BECOME NW WEDNESDAY AFTERNOON AND INCREASE TO  
10-20 KT.  
 
WEDNESDAY NIGHT AND THURSDAY...VFR. NW WINDS GRADUALLY DIMINISHING.  
A STORM SYSTEM IS CURRENTLY EXPECTED TO REMAIN OFFSHORE. FORECAST  
CONFIDENCE: MEDIUM TO HIGH.  
 
FRIDAY...VFR. NW WINDS 5-10 KT.  
 
SATURDAY AND SATURDAY NIGHT...VFR EARLY...THEN MVFR OR LOWER  
CONDITIONS POSSIBLE IN RAIN SHOWERS LATER IN THE DAY AND AT NIGHT.  
SW WINDS 10-15 KT.  

 
   
MARINE
 
 
SMALL CRAFT ADVISORY IN EFFECT FOR DELAWARE BAY AND THE DELAWARE  
ATLANTIC COASTAL WATERS THROUGH TODAY AND FOR THE NEW JERSEY COASTAL  
WATERS THROUGH THIS EVENING AS STRONG SOUTHWEST WINDS  
CONTINUE/INCREASE TODAY. WINDS SHOULD DIMINISH OVERNIGHT, ESPECIALLY  
OVER DELAWARE BAY AND THE ADJACENT ATLANTIC WATERS, THOUGH  
CONFIDENCE IS NOT PARTICULARLY HIGH GIVEN THE POTENTIALLY BRIEF  
DURATION OF THESE LIGHTER WINDS.  
 
SHOWERS SHOULD SPREAD NORTHEASTWARD OVERNIGHT (GENERALLY AFTER  
MIDNIGHT), WITH LOCALIZED RESTRICTIONS POSSIBLE. EXPECT ERRATIC  
GUSTS AND LOCALLY VERY CHOPPY SEAS IN VICINITY TO SHOWERS. A  
LIGHTNING STRIKE OR TWO IS NOT OUT OF THE QUESTION, THOUGH THE  
PROBABILITY IS TOO LOW FOR INCLUSION IN THE FORECAST AT THIS POINT.  
 
OUTLOOK...  
 
WEDNESDAY...LULL IN SCA CONDITIONS POSSIBLE WEDNESDAY MORNING.  
OTHERWISE, WINDS SHIFT TO THE NW AND INCREASE TO 15-20 KT WITH 25-30  
KT GUSTS.  
 
THURSDAY THROUGH FRIDAY...SUB-SCA CONDITIONS EXPECTED.  
 
SATURDAY AND SUNDAY...SCA CONDITIONS ARE POSSIBLE WITH THE PASSAGE  
OF A STRONG COLD FRONT.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 1 AM EST WEDNESDAY FOR ANZ450>453.  
SMALL CRAFT ADVISORY UNTIL 7 PM EST THIS EVENING FOR ANZ454-  
455.  
SMALL CRAFT ADVISORY UNTIL 7 PM EST THIS EVENING FOR ANZ430-  
431.  
 

 
 

 
SYNOPSIS...MIKETTA  
NEAR TERM...CMS  
SHORT TERM...CMS  
LONG TERM...MIKETTA  
AVIATION...CMS/MIKETTA  
MARINE...CMS/MIKETTA  
 
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