379  
AGNT40 KWNM 231326  
MIMATN  
 
MARINE WEATHER DISCUSSION FOR N ATLANTIC OCEAN  
NWS OCEAN PREDICTION CENTER WASHINGTON DC  
826 AM EST THU NOV 23 2017  
   
FORECAST DISCUSSION
 
MAJOR FEATURES/WINDS/SEAS/SIGNIFICANT  
.WEATHER FOR THE NORTH ATLANTIC OCEAN W OF 50W FROM 30N TO 50N.  
 
GOES VISIBLE SATELLITE IMAGERY INDICATES A STRONG LOW PRES SYSTEM  
CENTERED NE OF THE AREA OVER THE GULF OF ST LAWRENCE, AND COLD  
AIR CUMULUS STREAMING ACROSS THE OFFSHORE WATERS IN THE COLD  
ADVECTION BEHIND THE ASSOCIATED COLD FRONT NOW E OF THE AREA.  
THE ASCAT WINDS FROM LAST NIGHT INDICATED GALES IN THE COLD  
ADVECTION OVER THE OFFSHORE WATERS, BUT THE STRONGEST COLD  
ADVECTION HAS MOVED OFF TO THE NE AND CURRENT SURFACE REPORTS  
SHOW UP TO 20 KT IN THE NE ZONES. 06Z GFS 10M WINDS SHOW A FEW  
REMAINING 25 KT WINDS OVER THE EASTERN GULF OF MAINE AT 12Z, BUT  
THE MODELS ALL INDICATE THE WINDS WILL DECREASE EARLY TODAY AS A  
HIGH PRES RIDGE BUILDS OVER THE W ATLC TODAY INTO TONIGHT. THE  
PREVIOUS FORECAST STARTED OUT WITH THE 00Z GFS FOR TODAY WHICH IS  
SUPPORTED BY THE 06Z RUN, SO PLANNING ON STAYING WITH IT IN THE  
UPDATE PACKAGE.  
 
THE SATELLITE ALSO INDICATES LOW PRES OVER THE NE GULF OF MEXICO,  
AND THE 00Z GFS/ECMWF INDICATE THE LOW WILL MOVE NE INTO THE SRN  
NT2 OFFSHORE WATERS LATE THIS AFTERNOON INTO TONIGHT ALONG THE  
TAIL END OF THE AFOREMENTIONED FRONTAL BOUNDARY. THE MODELS ALL  
INDICATE THE WINDS WILL INCREASE N OF THE LOW AS IT MOVES OVER  
THE GULF STREAM, WITH THE GFS SHOWING FEW GALES TONIGHT IN THE  
ENHANCED NE FLOW. THE 00Z ECMWF IS A LITTLE SLOWER WITH THE TRACK  
OF THE LOW, BUT OVERALL AGREES WELL WITH THE GFS ON TAKING IT  
OVER THE GULF STREAM. IT IS SLIGHTLY WEAKER WITH THE WINDS, AND  
INDICATES UP TO 30 KT. HOWEVER, CONFIDENCE WITH THE GALES IS LOW  
AS THE MODELS ALL INDICATE ONLY ABOUT 30 KT AT 925 MB, SO THE  
LIKELIHOOD OF GALES MIXING DOWN IS LOW. ALSO, THERE IS STILL A  
FAIR AMOUNT OF DISAGREEMENT ON THE LOW CENTER ITSELF, WITH THE  
GFS A BIT FASTER THAN THE 00Z ECMWF/UKMET/GEM. THERE IS ALSO A  
STRONG SIGNAL IN THE 06Z GEFS MEMBERS FOR A SLOWER SOLUTION. AS A  
RESULT, PLANNING ON STAYING WITH THE PREVIOUS GRIDS WHICH WERE  
BASED ON THE 00Z ECMWF SOLUTION. WILL ALSO CAP WINDS AT 30 KT IN  
THE NE FLOW.  
 
IN THE MEDIUM RANGE, THE GFS/ECMWF COME INTO BETTER AGREEMENT ON  
THE LOW AS IT MOVES WELL NE OF THE AREA. THE MODELS INDICATE  
STRONG COLD ADVECTION BEHIND THE FRONT BY SUN INTO SUN NIGHT OVER  
NT1 AND NRN NT2. THE GFS HAS BEEN A BIT STRONGER WITH THE WINDS  
IN THE COLD ADVECTION, AND SHOWS A FEW GALES IN THE FIRST SIGMA  
WINDS. HOWEVER, IT IS NOT SUPPORTED WELL BY THE REST OF THE 00Z  
GUIDANCE WITH ONLY THE 00Z GEM INDICATING A BRIEF PERIOD OF  
MARGINAL GALE FORCE WINDS. THE PREVIOUS FORECAST STAYED WITH THE  
00Z ECMWF THROUGH THE MEDIUM RANGE PERIOD, SO PLANNING ON STAYING  
WITH IT AS IT SHOWS A BRIEF PERIOD OF 30 KT IN THE COLD  
ADVECTION. OTHERWISE, NOT PLANNING ON ANY MAJOR CHANGES THROUGH  
THE REMAINDER OF THE FORECAST PERIOD.  
 
-----------------------------------------------------------------  
PREVIOUS DISCUSSION...  
 
THE LATEST GOES SATELLITE IMAGERY, OBSERVATIONS AND PRELIMINARY  
06Z OPC-NCEP SURFACE ANALYSIS SHOWS A DEEPENING 990 MB LOW OVER  
CENTRAL NOVA SCOTIA MOVING NE AWAY FROM THE OFFSHORE WATERS WITH  
HIGH PRESSURE CENTERED OVER THE OHIO VALLEY MOVING E. AN ASCAT  
OVERPASS FROM LAST EVENING INDICATED GALE FORCE WINDS MOVING E  
FROM THE FAR NE NT2 AND EASTERN NT1 WATERS ASSOCIATED WITH THE  
DEEPENING LOW. THE THREAT FOR GALES OVER THE OFFSHORE WATERS IS  
EXPECTED TO REMAIN RATHER LOW FOR THE NEXT 3-5 DAYS.  
 
OVER THE SHORT TERM, THE 00Z MODELS ARE IN VERY GOOD AGREEMENT  
OVER THE REGION TODAY WITH A HIGH PRESSURE AREA BUILDING E INTO  
THE OFFSHORE WATERS AS THE LOW MENTIONED ABOVE TRACKS NE AWAY  
FROM THE REGION. A LOW CURRENTLY OVER THE NE GULF OF MEXICO WILL  
MOVE E AND NE OVER THE NEXT FEW DAYS, AND ALLOW CONDITIONS TO  
DETERIORATE OVER SOUTHERN NT2 WATERS BY TONIGHT. FOR TODAY, WE  
WILL STAY CLOSE TO THE 00Z GFS GUIDANCE, AND USE THE SMART TOOL  
WHICH PLACES THE HIGHER FIRST SIGMA LAYER WINDS OVER THE  
UNSTABLE AREAS, AND SLIGHTLY LOWER 10 METER WINDS OVER THE  
STABLE LOCATIONS. FOR TONIGHT THROUGH SUN, WE WILL THEN TREND  
THE FORECAST MORE TOWARD THE 00Z ECMWF GUIDANCE AS IT IS QUITE  
SIMILAR TO THE 12Z WED ECMWF THAT WAS USED FOR THE PREVIOUS  
PACKAGE. THIS WILL ALSO MAINTAIN CONTINUITY AS THE GFS REMAINS  
RATHER UNSTABLE AT THIS TIME, BEYOND TODAY. THEREFORE, WE STILL  
EXPECT LOW PRESSURE TO MOVE INTO THE FAR SOUTHERN NT2 WATERS  
LATER TONIGHT INTO FRI, CONTINUE ORGANIZING OVER THE SE NT2  
WATERS FRI NIGHT, AND THEN PASS JUST E OF THE CENTRAL AND  
NORTHERN NT2 WATERS SAT. A MODERATE TO STRONG COLD FRONT WILL  
APPROACH THE WATERS FROM THE W AND NW SAT. ANY GALES THAT  
DEVELOP WITH THE LOW ARE EXPECTED TO REMAIN E OF THE OFFSHORE  
WATERS AT THIS TIME, WITH CHANCES FOR GALES OVER THE OFFSHORE  
WATERS NOT HIGH ENOUGH TO ADD TO THE FORECAST AT THIS TIME.  
THERE WILL BE AN INCREASING THREAT FOR THUNDERSTORMS MAINLY OVER  
SOUTHERN NT2 WATERS TONIGHT INTO FRI NIGHT, WITH LOCAL WIND  
GUSTS NEAR OR EXCEEDING GALE FORCE AND ROUGH SEAS POSSIBLE IN  
AND NEAR OF THE HEAVIER THUNDERSTORMS. THE LOW WILL THEN LIFT NE  
AWAY FROM THE WATERS SAT NIGHT INTO SUN AS THE COLD FRONT  
CROSSES THE WATERS FROM NW TO SE, WITH COLD AIR ADVECTION  
BEGINNING OVER THE WATERS AT THAT TIME. HIGH PRESSURE WILL THEN  
BEGIN TO SLOWLY BUILD E TOWARD THE AREA DURING SUN.  
 
OVER THE MEDIUM RANGE, WE SEE NO NEED TO DEVIATE TOO MUCH FROM  
THE 00Z ECMWF GUIDANCE FOR SUN NIGHT THROUGH MON NIGHT OVER THE  
REGION, WITH THE 00Z ECMWF REMAINING QUITE CONSISTENT. THE HIGH  
WILL SLOWLY OVERSPREAD THE WATERS FROM W TO E SUN NIGHT THROUGH  
MON NIGHT WITH IMPROVING CONDITIONS LIKELY OVER THE REGION. ANY  
GALES ASSOCIATED WITH THE RATHER STRONG COLD AIR ADVECTION WILL  
LIKELY REMAIN E OF THE NT1, AND NORTHERN NT2, WATERS SUN NIGHT  
INTO MON EVENING.  
 
SEAS...ONCE AGAIN THE 00Z ECMWF WAM APPEARS TO HAVE INITIALIZED  
SLIGHTLY BETTER WHEN COMPARED TO THE CURRENT DATA OVER THE  
OFFSHORE WATERS. IN ORDER TO FIT THE SEA HEIGHTS FORECAST WITH  
THE WIND FORECAST WILL WE STAY FAIRLY CLOSE TO THE 00Z ECMWF WAM  
THROUGH MON NIGHT OVER THE REGION, WHICH WILL TEND TO RESULT IN  
LITTLE CHANGE FROM THE PAST FEW FORECASTS. WE WILL MAKE ONLY A  
FEW MANUAL ADJUSTMENTS TO FIT SEA HEIGHT GRIDS TO INITIAL  
CONDITIONS, AND TO FIT WITH NEARBY COASTAL WFO AND TAFB GRIDS.  
   
EXTRATROPICAL STORM SURGE GUIDANCE
 
N/A.  
 
   
WARNINGS
 
PRELIMINARY.  
   
NT1 NEW ENGLAND WATERS
 
 
NONE.  
   
NT2 MID-ATLANTIC WATERS
 
 
NONE.  
 

 
 
.FORECASTER KELLS/MILLS. OCEAN PREDICTION CENTER.  
 
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