910  
FXUS61 KPHI 100826  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
326 AM EST MON DEC 10 2018  
   
SYNOPSIS
 
 
HIGH PRESSURE WILL NOSE INTO THE REGION TODAY AS LOW PRESSURE  
OFF THE CAROLINA COAST MOVES FARTHER OFFSHORE. A WEAK SURFACE  
LOW WILL MOVE FROM THE GREAT LAKES TONIGHT TO NEW ENGLAND  
TUESDAY NIGHT, BRINGING A COLD FRONT THROUGH THE AREA ON  
WEDNESDAY. HIGH PRESSURE WILL BUILD INTO THE NORTHEAST ON  
THURSDAY. A STRONG SURFACE LOW WILL DEVELOP IN THE SOUTHERN U.S.  
LATE THIS WEEK AND MOVE RAPIDLY NORTHEAST NEAR OR THROUGH THE  
AREA BY EARLY THIS WEEKEND.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
PRECIPITATION IS DIMINISHING AND SHIFTING SOUTH AND EAST OF  
DELMARVA EARLY THIS MORNING AS THE STRONG LOW OFF THE CAROLINA  
COAST BEGINS ITS PUSH OFFSHORE. NO POPS ARE INCLUDED IN THE  
FORECAST FOR THE REST OF THE NIGHT.  
 
AN UPSTREAM PERTURBATION IN THE TENNESSEE VALLEY WILL ALLOW FOR  
THE ELONGATION OF THE SURFACE LOW IN THE WESTERN ATLANTIC TODAY,  
FOLLOWED BY A SECONDARY SURFACE LOW DEVELOPING NEAR THE CAROLINA  
COAST TONIGHT. HOWEVER, LARGE-SCALE LIFT WILL REMAIN SUPPRESSED  
TO OUR SOUTH AS THIS PROCESS OCCURS, AS SUBTLE RIDGING  
DOWNSTREAM OF A NORTHERN-STREAM SHORTWAVE TROUGH WILL PRECLUDE  
SUFFICIENT ASCENT FOR PRECIPITATION IN OUR AREA TODAY AND  
TONIGHT.  
 
THIS FLOW EVOLUTION WILL LEAD TO YET ANOTHER COLD/DRY DAY AND  
NIGHT ACROSS THE AREA, WITH FORECAST HIGHS/LOWS REMAINING  
A FEW (3-8) DEGREES BELOW SEASONAL AVERAGES. MAV MOS HAS  
TRENDED A LITTLE COOLER, AND MET MOS HAS TRENDED A LITTLE  
WARMER; HOWEVER, BOTH HAVE VERIFIED TOO WARM THE PAST FEW DAYS.  
GENERALLY WENT A COUPLE DEGREES BELOW GUIDANCE TODAY, ALTHOUGH  
WITH PERHAPS MORE SUN THAN SUNDAY, THIS MAY BE A LITTLE ON THE  
PESSIMISTIC SIDE.  
 
WITH CLEAR SKIES AND LIGHT WINDS TONIGHT, EXCELLENT RADIATIONAL  
COOLING CONDITIONS WILL EXIST. MAV GUIDANCE LOOKS TOO WARM, SO  
WENT MUCH CLOSER TO MET AND IN-HOUSE BIAS-CORRECTED GUIDANCE FOR  
OVERNIGHT LOWS.  
 

 
   
SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/
 
 
THE COLD/DRY PATTERN LOOKS TO CONTINUE THROUGH MIDWEEK, WITH THE  
MAIN FEATURE OF INTEREST A MIDLEVEL VORT MAX DIGGING  
SOUTHEASTWARD INTO THE NORTHEAST TUESDAY NIGHT. A WEAK SURFACE  
LOW AND ATTENDANT COLD FRONT WILL ACCOMPANY THE UPPER-LEVEL  
TROUGH, BUT MOISTURE WILL BE SEVERELY LACKING THANKS TO THE  
RESIDUAL EFFECTS OF A SURFACE HIGH AFFECTING THE REGION THE PAST  
FEW DAYS. OPERATIONAL MODELS ARE DEPICTING SOME LIGHT SNOW  
SHOWERS TO OUR NORTHWEST, BUT THERE IS LITTLE INDICATION  
SUFFICIENT LIFT WILL OCCUR IN OUR CWA TO GENERATE MEASURABLE  
PRECIPITATION. THE MAIN INFLUENCE, THEN, WILL BE INCREASED  
CLOUDS IN THE NORTHERN CWA TUESDAY NIGHT AND WEDNESDAY MORNING  
(WITH A CAVEAT HERE THAT THE FORECAST MAY NOT BE PESSIMISTIC  
ENOUGH IN THIS REGARD; MODELS TEND TO UNDERFORECAST SKY COVER  
WITH THESE SYSTEMS).  
 
AS MENTIONED A COUPLE DAYS AGO, THERE IS A SURFACE TROUGH THAT  
DEVELOPS UPSTREAM FROM THE SECONDARY LOW THAT DEVELOPS OFF THE  
CAROLINA COAST MONDAY NIGHT. AS IT MOVES OFFSHORE TUESDAY AND  
TUESDAY NIGHT, THIS TROUGH LIFTS NORTHWARD TOWARD THE SOUTHERN  
NEW ENGLAND COAST. HOWEVER, INTERACTION WITH THE UPSTREAM VORT  
MAX LOOKS LIMITED AT BEST, AND MODELS DO NOT INDICATE SUFFICIENT  
LIFT WILL BE PRESENT TO GENERATE PRECIPITATION FAR NORTHWEST OF  
THE OFFSHORE LOW. THE 00Z ECMWF SEEMS MOST AGGRESSIVE WITH THIS,  
BUT KEEPS PRECIP WELL OFFSHORE. AS SUCH, NO POPS ARE IN THE  
FORECAST THROUGH THE PERIOD.  
 
TEMPERATURES WILL REMAIN ON THE CHILLY SIDE THROUGH THE PERIOD,  
AND WITH (ALBEIT WEAK) COLD ADVECTION UPSTREAM OF A COLD FRONT  
PASSING THROUGH THE AREA ON WEDNESDAY, IT MAY END UP BEING A  
LITTLE BIT COOLER THAN ON TUESDAY.  
 

 
   
LONG TERM /THURSDAY THROUGH SUNDAY/
 
 
THE MEDIUM-RANGE PERIOD LOOKS ACTIVE, WITH A STRONG SYSTEM  
AFFECTING THE REGION FRIDAY AND SATURDAY AND A PREDECESSOR VORT  
MAX POSSIBLY AFFECTING THE AREA ON THURSDAY.  
 
THE 00Z OPERATIONAL MODELS ARE IN VERY POOR AGREEMENT WITH A  
VORT MAX MOVING INTO THE NORTHEAST ON THURSDAY. THE GFS IS WELL  
TO THE NORTHWEST OF THE CMC AND ECMWF, WITH THE ECMWF FARTHEST  
SOUTH AND SPEEDIEST WITH THE PERTURBATION. THIS DISAGREEMENT  
APPEARS TO BE TIED TO THE UPSTREAM/DIGGING SOUTHERN-STREAM VORT  
MAX REACHING THE SOUTHERN PLAINS BY THURSDAY EVENING, WITH THE  
ECMWF CONSIDERABLY SLOWER/FARTHER WEST THAN THE NORTHEAST-  
DISPLACED GFS/CMC. NOTABLY, THE GFS HAS TRENDED MUCH FASTER WITH  
THE PERTURBATION AFFECTING THE AREA ON THURSDAY, WHICH APPEARS  
TIED TO A SOMEWHAT FASTER PROGRESSION OF THE VORT MAX MOVING  
THROUGH THE NORTHEAST TUESDAY NIGHT/WEDNESDAY. THE GFS LIFTS THE  
THURSDAY VORT MAX NORTHEAST AS A RESULT OF SOME PHASING WITH A  
CANADA TROUGH. IN SHORT, THERE ARE SOME COMPLEX INTERACTIONS AND  
SENSITIVITIES HERE THAT SUGGEST A LOW-PREDICTABILITY PERIOD OF  
THE FORECAST. A BLENDED APPROACH APPEARS REASONABLE, AS THE  
OUTLIER POSITIONING OF THE GFS/ECMWF WITH THE THURSDAY VORT MAX  
AND THE SOUTHERN PLAINS SYSTEM AT THESE TIMES SUGGESTS NEITHER  
SOLUTION IS PROBABLE.  
 
WITH THE 00Z ECMWF INDICATING A MORE SOUTHERN TRACK WITH THE  
THURSDAY SYSTEM (I.E., ONE WITH MORE DIRECT IMPACTS ON THE CWA),  
I WAS INCLINED TO INTRODUCE SOME SLIGHT-CHANCE POPS DURING THE  
DAY FOR THE AREA. TEMPERATURES WOULD LIKELY BE COLD ENOUGH FOR  
SOME SNOW SHOWERS, ESPECIALLY IN THE MORNING. QPF/SNOW AMOUNTS  
WOULD BE QUITE LIGHT WITH SUCH A SYSTEM (IF ANY IMPACTS OCCUR  
IN OUR AREA AT ALL, AND THAT IS A BIG IF), BUT THIS DEFINITELY  
NEEDS TO BE MONITORED GIVEN THE LARGE UNCERTAINTY IN THE TRACK  
OF THE VORT MAX AND THE POTENTIAL FOR THERMAL PROFILES COLD  
ENOUGH FOR SNOW. I ALSO LOWERED TEMPERATURES ON THURSDAY GIVEN  
THE POTENTIAL FOR AT LEAST SOME INFLUENCE FROM THIS VORT MAX.  
 
ALL OF THIS IS A PRECURSOR TO THE MAIN EVENT IN THE MEDIUM  
RANGE. THE AFOREMENTIONED SOUTHERN-STREAM VORT MAX MAKES THE  
MOVE INTO THE SOUTHEAST BY FRIDAY NIGHT, WITH AN UPPER-LEVEL  
TROUGH ACQUIRING A SLIGHT NEGATIVE TILT UPSTREAM OF A STRONG  
250-MB JET STREAK IN NEW ENGLAND. A SURFACE LOW SHOULD READILY  
DEVELOP IN THIS REGIME, BUT WHERE IT DEVELOPS/TRACKS IS A BIG  
QUESTION MARK. INCREASINGLY, SO IS THE LOW'S INTENSITY. THE  
ECMWF HAS SHOWN LARGE VARIABILITY RUN-TO-RUN, WITH THE NEW 00Z  
SIMULATION CONSIDERABLY SLOWER AND SOMEWHAT FARTHER SOUTH THAN  
PRIOR RUNS. THE FARTHER SOUTH TRACK IS PLAUSIBLE GIVEN THIS PAST  
WEEKEND'S SOUTHERN-TRACK SYSTEM, BUT THE SLOWER TREND LOOKS  
SUSPICIOUS GIVEN THE PROGRESSIVE NATURE OF THE MID- AND UPPER-  
LEVEL FLOW OF LATE. THE 00Z GFS LOOKS RATHER DISORGANIZED WITH  
THE SURFACE PHENOMENA OVERALL, WHICH SEEMS UNLIKELY GIVEN THE  
FAVORABLE UPPER-LEVEL PATTERN FOR STRONG CYCLOGENESIS. THE CMC  
IS WAY (WAY) FARTHER NORTH/WEST WITH THE SURFACE LOW TRACK (BY  
HUNDREDS OF MILES) AND APPEARS MOSTLY TO BE AN OUTLIER BY THIS  
POINT IN THE FORECAST.  
 
PRECIPITATION LOOKS LIKELY IN OUR AREA LATE FRIDAY THROUGH EARLY  
SATURDAY NO MATTER WHAT THE SOLUTION (AND NONE OF THOSE  
DESCRIBED ABOVE WILL LIKELY END UP CORRECT), AND TEMPERATURES  
LOOK WARM ENOUGH FOR THIS TO BE A MOSTLY-RAIN EVENT.  
NEVERTHELESS, FINE-TUNING THE DETAILS IS A FRUITLESS ENDEAVOR AT  
THIS POINT OWING TO THE LARGE VARIABILITY IN MODEL OUTPUT. THE  
MAIN QUESTION MARKS SENSIBLE-WEATHER-WISE WILL BE (1) HOW SOON  
PRECIPITATION REACHES THE AREA, WITH A FASTER ARRIVAL IMPLYING  
SOME PRECIPITATION TYPE CONCERNS EARLY ON (I.E., THURSDAY NIGHT/FRIDAY  
MORNING), (2) HOW CLOSE THE SYSTEM WILL BE TO THE AREA, WITH A  
CLOSER TRACK LIKELY GENERATING STRONGER WINDS AND HIGHER QPF,  
AND (3) HOW SLOW THE SYSTEM WILL BE, WITH RESULTANT IMPACTS ON  
POTENTIAL FOR HEAVY RAIN AND FLOODING AND A FINAL TURNOVER TO  
SNOW IN THE NORTH/WEST SATURDAY NIGHT.  
 
TEMPERATURES WILL BE WARM FRIDAY AND SATURDAY AS MIDLEVEL  
RIDGING DOWNSTREAM OF THE SOUTHERN-STREAM SYSTEM SHOULD LEAD TO  
SUBSTANTIAL WARM/MOIST ADVECTION DURING THIS PERIOD.  
 
CONDITIONS SHOULD DRY OUT AS NORTHWEST FLOW COMMENCES ON SUNDAY,  
BUT THIS MAY LEAD TO A FEW WEAK CLIPPER-TYPE SYSTEMS EARLY NEXT  
WEEK.  
 

 
   
AVIATION /08Z MONDAY THROUGH FRIDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
TODAY...MAINLY VFR, THOUGH THERE IS A SLIGHT CHANCE FOR A BRIEF  
PERIOD OF MVFR AT RDG/ABE THIS MORNING. AFTER LIGHT/VARIABLE  
WINDS EARLY THIS MORNING, NORTHERLY FLOW AROUND 10 KTS SHOULD  
DEVELOP BY LATE MORNING INTO THE AFTERNOON. MODERATE CONFIDENCE.  
 
TONIGHT...VFR WITH LIGHT NORTH OR NORTHEAST WINDS. HIGH  
CONFIDENCE.  
 
OUTLOOK...  
 
TUESDAY THROUGH WEDNESDAY: MAINLY VFR WITH LIGHT WEST WINDS  
TUESDAY AND TUESDAY NIGHT, BECOMING NORTHWEST AND INCREASING TO  
7 TO 15 KTS ON WEDNESDAY (WITH HIGHER GUSTS POSSIBLE). MODERATE  
CONFIDENCE.  
 
WEDNESDAY NIGHT AND THURSDAY: LOW-CONFIDENCE FORECAST AS A WEAK  
SYSTEM MAY AFFECT THE REGION DURING THIS PERIOD (ESPECIALLY  
THURSDAY). BEST CHANCES ARE FOR VFR, BUT CANNOT RULE OUT A  
PERIOD OF SUB-VFR IF THE SYSTEM MORE DIRECTLY IMPACTS THE AREA.  
WINDS BECOMING EAST GENERALLY NEAR OR BELOW 10 KTS.  
 
THURSDAY NIGHT AND FRIDAY: DETERIORATING CONDITIONS EXPECTED  
THROUGH THE PERIOD, WITH SUB-VFR CONDITIONS LIKELY BY FRIDAY  
AFTERNOON. INCREASING CHANCES OF RAIN. SOUTHEAST WINDS  
INCREASING TO 7 TO 15 KTS ON FRIDAY, WITH HIGHER GUSTS POSSIBLE,  
ESPECIALLY NEAR THE COAST. MODERATE CONFIDENCE.  
 

 
   
MARINE
 
 
WINDS HAVE BEEN SLOW TO REACH ADVISORY LEVELS ACROSS THE  
ATLANTIC WATERS OF SOUTHERN NEW JERSEY AND DELAWARE AND ON LOWER  
PORTIONS OF DELAWARE BAY OVERNIGHT. GUSTS OF 20-25 KTS HAVE  
BEEN OBSERVED AT DEWEY BEACH AND LEWES, BUT THE TREND HAS BEEN  
RATHER STAGNANT RECENTLY. HOWEVER, SEAS HAVE REACHED 5-6 FEET AT  
BUOY 44009 AND SHOULD STAY THERE THROUGH THE DAY. WINDS,  
THOUGH, WILL LIKELY REMAIN SUB-ADVISORY LEVEL AFTER DAYBREAK.  
FOR NOW, KEPT THE ADVISORY AS IS, WITH LOWER DELAWARE BAY SET TO  
EXPIRE AT 6 AM. I DID NOT EXTEND OR CONTRACT THE ADVISORY FOR  
THE ATLANTIC WATERS, AS CONFIDENCE IS TOO LOW REGARDING WAVE  
HEIGHTS THIS AFTERNOON TO MAKE ANY CHANGES.  
 
SUB-ADVISORY WINDS/SEAS ARE EXPECTED ON UPPER PORTIONS OF  
DELAWARE BAY AND OFF THE NORTHERN/CENTRAL NJ COAST THROUGH THE  
PERIOD.  
 
OUTLOOK...  
TUESDAY: SUB-ADVISORY WINDS/SEAS AND FAIR WEATHER EXPECTED.  
 
TUESDAY NIGHT: A BRIEF PERIOD OF ADVISORY-LEVEL WINDS IS  
POSSIBLE AS A COLD FRONT APPROACHES AND NORTHWEST FLOW BECOMES  
ESTABLISHED.  
 
WEDNESDAY THROUGH THURSDAY NIGHT: SUB-ADVISORY WINDS/SEAS  
EXPECTED.  
 
FRIDAY: SOUTHEAST WINDS WILL INCREASE THROUGH THE DAY AND MAY  
APPROACH ADVISORY LEVELS LATE. INCREASING CHANCES FOR RAIN.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EST EARLY THIS MORNING FOR  
ANZ431.  
SMALL CRAFT ADVISORY UNTIL 6 PM EST THIS EVENING FOR  
ANZ453>455.  
 

 
 

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