745  
FXUS61 KPHI 232223  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
623 PM EDT MON APR 23 2018  
   
SYNOPSIS
 
 
A LARGE AREA OF HIGH PRESSURE WILL SHIFT SLOWLY EAST OF THE  
REGION OVERNIGHT. A LOW IN THE LOWER MISSISSIPPI VALLEY WILL  
LIFT INTO THE MID ATLANTIC STATES BY WEDNESDAY, WITH WIDESPREAD  
RAIN SPREADING ACROSS THE REGION IN ADVANCE OF THE SYSTEM.  
ANOTHER LOW PRESSURE SYSTEM PASSING TO THE SOUTH OF OUR REGION,  
COULD BRING ANOTHER CHANCE OF RAIN LATE THIS WEEK. BY LATE IN  
THE WEEKEND, HIGH PRESSURE BEGINS TO BUILD SOUTH TOWARDS THE  
REGION.  
 

 
   
NEAR TERM /UNTIL 6 AM TUESDAY MORNING/
 
 
QUIET CONDITIONS WILL CONTINUE TONIGHT ACROSS THE AREA AS HIGH  
PRESSURE REMAINS OFFSHORE OF THE NEW ENGLAND COAST. OUR AREA  
WILL REMAIN UNDER THE INFLUENCE OF SOUTHEAST RETURN FLOW, WHICH  
WILL ALLOW MOISTURE TO INCREASE SOME OVERNIGHT. OVERNIGHT LOWS  
WILL DROP THROUGH THE NIGHT AS WINDS DIMINISH AND MOSTLY CLEAR  
SKIES WILL BEGIN THE NIGHT. HOWEVER, WITH THE FLOW BEING MORE  
FROM THE SOUTHEAST, AND THE FACT SOME CLOUDS MAY BEGIN MOVING IN  
LATE IN THE NIGHT, OVERNIGHT LOWS ARE NOT EXPECTED TO BE AS  
COOL AS SUNDAY NIGHT. MOST PLACES SHOULD DROP CLOSE TO NORMAL,  
EXCEPT PORTIONS OF THE PINE BARRENS WHICH MIGHT DROP A FEW  
DEGREES LOWER AND HAVE SOME PATCHY FROST DEVELOP. WE WILL  
INCLUDE PATCHY FROST IN THE FORECAST, BUT NOT ISSUE A FROST  
ADVISORY YET AS THERE IS STILL UNCERTAINTY AS TO HOW LOW THE  
TEMPERATURES WILL GET.  
 

 
   
SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/
 
 
DRY CONDITIONS WILL BEGIN THE DAY TUESDAY, BEFORE RAIN CHANCES  
INCREASE THROUGH THE DAY. AN OCCLUDING LOW PRESSURE SYSTEM WILL  
CONTINUE MOVING ACROSS THE SOUTHERN APPALACHIANS EARLY IN THE DAY,  
BEFORE SLOWING MOVING INTO THE CAROLINAS BY THE AFTERNOON. OUR AREA  
WILL BE WELL NORTH OF THE LOW PRESSURE SYSTEM AND THE SLOWLY LIFTING  
WARM FRONT THROUGH THE DAYTIME HOURS. HOWEVER, SOUTH-SOUTHWEST FLOW  
ALOFT WILL LEAD TO INCREASING MOISTURE THROUGH THE DAY. A COUPLE OF  
SHORT WAVE/VORTICITY IMPULSES WILL LIFT ACROSS THE AREA THROUGH THE  
DAY AS WELL. RAIN CHANCES INCREASE, ESPECIALLY DURING THE AFTERNOON  
AS THE SECOND OF THE SHORT WAVES MOVES THROUGH AND THE BEST  
MOISTURE/LIFT COMBINATION TAKES PLACE. THERE IS LITTLE TO NO  
INSTABILITY FORECAST, SO NO THUNDERSTORMS ARE EXPECTED AT THIS TIME.  
 
WINDS WILL BECOME GUSTY THROUGH THE MORNING AND INTO THE AFTERNOON  
WITH GUSTS REACHING 20 TO 25 MPH AT TIMES.  
 

 
   
LONG TERM /TUESDAY NIGHT THROUGH MONDAY/
 
 
OVERVIEW: THE MAIN FOCUS THROUGH THIS PERIOD REMAINS ON THE  
TUESDAY NIGHT AND WEDNESDAY TIME FRAME AND THE POTENTIAL FOR  
HEAVY RAIN (THOUGH THIS THREAT APPEARS TO BE DECREASING).  
AFTER THAT, THE NEXT CHANCE FOR RAIN LOOKS TO BE PRIMARILY ON  
SATURDAY AS OUR REGION WILL BE IN BETWEEN A LOW OFF THE COAST  
AND A COLD FRONT APPROACHING FROM THE WEST.  
 
DETAILS: THE LOW CONTINUES TO VERY SLOWLY PROGRESS NORTHEAST,  
REACHING THE MID ATLANTIC BY WEDNESDAY. THE WINDOW FOR HEAVY  
RAIN HAS NARROWED A BIT, NOW LOOKS TO BE CONFINED MAINLY LATE  
TUESDAY NIGHT, THOUGH LINGERING LIGHT RAIN IS POSSIBLE INTO  
WEDNESDAY EVENING.  
 
AS FOR THE HEAVY RAIN THREAT, PRECIPITABLE WATER VALUES ON  
TUESDAY NIGHT ARE STILL EXPECTED TO BE WELL ABOVE NORMAL,  
POSSIBLY IN THE 90TH PERCENTILE FOR THIS TIME OF YEAR. HOWEVER,  
THERE ARE A FEW FACTORS WORKING AGAINST THE HEAVY RAIN THREAT.  
THE WARM CLOUD LAYER IS RELATIVELY SHALLOW (GENERALLY 8000 TO  
9000 FT), THANKS TO CONTINUED BELOW NORMAL TEMPERATURES.  
ADDITIONALLY, STORM MOTIONS AREN'T EXPECTED TO BE PARTICULARLY  
SLOW, AROUND 20 MPH, ALTHOUGH TRAINING STORMS COULD MAKE THIS  
POINT MOOT. IN GENERAL, IT LOOKS LIKE THE HIGHEST THREAT FOR  
HEAVY RAIN WILL BE OVER THE COASTAL PLAINS OF DELMARVA AND FAR  
SOUTHERN NJ.  
 
THERE IS SOME VERY LIMITED ELEVATED INSTABILITY TUESDAY NIGHT,  
BUT IT IS VERY ELEVATED (GENERALLY FOR PARCELS AT OR ABOVE 850  
MB). THUS, HAVE NOT MENTIONED THUNDERSTORMS IN THE FORECAST FOR  
NOW, BUT WILL CONTINUE TO MONITOR THIS POTENTIAL AS WE GET  
CLOSER.  
 
FOR THE LATE WEEK PERIOD, THERE REMAINS SOME UNCERTAINTY IN HOW  
TWO LOW PRESSURE SYSTEMS, ONE LIFTING OUT OF THE SOUTHEAST AND  
BECOMING A COASTAL LOW AND ONE DIGGING OUT OF THE NORTH CENTRAL  
U.S., WILL INTERACT AND WHAT THE IMPACT WILL BE FOR OUR REGION.  
HOWEVER, THE MODELS TODAY ARE IN BETTER AGREEMENT THAN WHAT WE  
HAVE SEEN THE LAST FEW DAYS, WITH A BRIEF CHANCE FOR RAIN ON THE  
BACK SIDE OF THE COASTAL LOW ON FRIDAY NIGHT OR SATURDAY, AND A  
LIMITED IMPACT FROM THE COLD FRONT MOVING IN FROM THE WEST.  
 

 
   
AVIATION /22Z MONDAY THROUGH SATURDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
TONIGHT...VFR CONDITIONS WILL CONTINUE THROUGH TONIGHT. WINDS WILL  
BE FROM THE SOUTH-SOUTHEAST AROUND 5-10 KNOTS THROUGH THIS EVENING,  
BUT WILL BECOME LIGHT AND VARIABLE MOST PLACES OVERNIGHT.  
 
TUESDAY...VFR CONDITIONS WILL BEGIN THE DAY ACROSS THE TAF SITES  
THROUGH AT LEAST MIDDAY TUESDAY. HOWEVER, CLOUDS WILL THICKEN AND  
LOWER THROUGH THE AFTERNOON, AND COULD EVENTUALLY REACH MVFR BY LATE  
IN THE AFTERNOON OR EARLY EVENING. RAIN WILL ALSO BE MOVING FROM  
THE SOUTH DURING THE AFTERNOON HOURS.  
 
OUTLOOK...  
 
TUESDAY NIGHT AND WEDNESDAY...MVFR AND EVEN LOCALIZED IFR  
CONDITIONS WILL BE POSSIBLE ESPECIALLY THROUGH WEDNESDAY  
MORNING AS PERIODS OF RAIN ARE LIKELY ACROSS THE REGION.  
EASTERLY WIND GUSTS UP TO 20 KT ARE POSSIBLE. MODERATE  
CONFIDENCE ON FLIGHT CATEGORIES, BUT LOW CONFIDENCE ON THE  
TIMING OF ANY FLIGHT CATEGORY CHANGES.  
 
THURSDAY...VFR CONDITIONS EXPECTED WITH LIGHT WESTERLY WINDS.  
HIGH CONFIDENCE.  
 
FRIDAY AND SATURDAY...MOSTLY VFR CONDITIONS ARE LIKELY, THOUGH  
THERE IS A CHANCE THAT RAIN AND LOW CLOUDS COULD LOWER  
CONDITIONS TO MVFR AT TIMES. WIND IS FORECAST TO BE WESTERLY OR  
SOUTHWESTERLY NEAR OR BELOW 10 KT. MODERATE CONFIDENCE ON  
FLIGHT CATEGORIES AND WIND.  
 

 
   
MARINE
 
 
CONDITIONS WILL REMAIN BELOW ADVISORY LEVELS TONIGHT AND INTO THE  
FIRST HALF OF TUESDAY. HOWEVER, WINDS COULD BEGIN GUSTING AROUND 25  
KNOTS BY THE AFTERNOON, WHILE SEAS ARE EXPECTED TO BUILD TO 5 FEET  
OR HIGHER BY THE AFTERNOON AS WELL. THEREFORE THE SMALL CRAFT  
ADVISORY REMAIN IN PLACE STARTING NOON TUESDAY.  
 
OUTLOOK...  
 
TUESDAY NIGHT AND WEDNESDAY...EASTERLY AND SOUTHEASTERLY WINDS  
INCREASE TO 15-20 KT WITH GUSTS UP TO 30 KT, AND SEAS BUILD UP  
TO 10 FEET ON THE ATLANTIC COASTAL WATERS. WIND GUSTS NEAR 25 KT  
ARE ALSO EXPECTED ON THE DELAWARE BAY PRIMARILY TUESDAY NIGHT.  
 
WEDNESDAY NIGHT AND THURSDAY...WINDS SHIFT TO NORTHWESTERLY AND  
SHOULD DECREASE. THE MAIN QUESTION WILL BE HOW LONG SEAS REMAIN  
ABOVE 5 FEET ON THE COASTAL WATERS. CURRENT FORECAST IS FOR  
ELEVATED SEAS INTO LATE THURSDAY, BUT THERE IS STILL  
CONSIDERABLE UNCERTAINTY WITH THIS.  
 
FRIDAY AND SATURDAY...WINDS AND SEAS ARE CURRENTLY FORECAST TO  
STAY BELOW SCA CRITERIA ON ALL WATERS. HOWEVER, THERE REMAINS  
SOME UNCERTAINTY WITH IF A COASTAL LOW DEVELOPS AND INTENSIFIES.  
IF THIS HAPPENS, THERE IS A SLIGHT CHANCE FOR SCA CRITERIA,  
PRIMARILY ON SATURDAY.  
 

 
   
FIRE WEATHER
 
 
RELATIVE HUMIDITY VALUES DROPPED ONCE AGAIN TODAY, SIMILARLY TO  
SUNDAY, INTO THE 20 AND EVEN TEENS IN SOME AREAS. HOWEVER, WINDS  
HAVE NOT BEEN VERY WINDY EXCEPT OCCASIONAL GUSTS IN THE TEENS TO  
AROUND 20 MPH IN A FEW SPOTS. THE LOW HUMIDITY VALUES WILL CONTINUE  
INTO SUNSET, WHILE WINDS MAY INCREASE SLIGHTLY. HOWEVER, SINCE WINDS  
ARE NOT GUSTING FREQUENTLY OR WIDESPREAD, WE ARE NOT EXPECTING ANY  
ENHANCED STATEMENTS AT THIS TIME. WHILE IT WILL BE A BIT MORE BREEZY  
AND GUSTY TUESDAY, RELATIVE HUMIDITY VALUES WILL BE HIGHER ON  
TUESDAY THAN TODAY, AND RAIN WILL OVERSPREAD THE AREA BY  
AFTERNOON. SO NO ENHANCED STATEMENTS ARE EXPECTED ON TUESDAY.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY FROM NOON TUESDAY TO 6 PM EDT WEDNESDAY  
FOR ANZ450>455.  
SMALL CRAFT ADVISORY FROM NOON TUESDAY TO 2 AM EDT WEDNESDAY  
FOR ANZ430-431.  
 

 
 

 
SYNOPSIS...JOHNSON/99  
NEAR TERM...ROBERTSON/99  
SHORT TERM...ROBERTSON  
LONG TERM...JOHNSON  
AVIATION...JOHNSON/ROBERTSON/99  
MARINE...JOHNSON/ROBERTSON  
FIRE WEATHER...ROBERTSON  
 
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