314  
AGNT40 KWNM 170134  
MIMATN  
 
MARINE WEATHER DISCUSSION FOR N ATLANTIC OCEAN  
NWS OCEAN PREDICTION CENTER WASHINGTON DC  
0933 PM EDT FRI APR 16 2021  
   
FORECAST DISCUSSION
 
MAJOR FEATURES/WINDS/SEAS/SIGNIFICANT  
.WEATHER FOR THE NORTH ATLANTIC OCEAN W OF 50W FROM 30N TO 50N.  
 
AT 00Z, A 992 MB LOW WAS LOCATED OVER CAPE COD, MASSACHUSETTS  
NEAR 42N70W, WITH AN OCCLUDED FRONT EXTENDED NE FROM THE CENTER  
TO 43N69W OVER THE GULF OF MAINE, THEN E. OVER THE NT2 WATERS, A  
1016 MB HIGH WAS LOCATED OFF THE CAROLINAS NEAR 33N76W, WITH A  
1010 MB LOW TO THE S OF THAT HIGH NEAR 31N77W. A STATIONARY FRONT  
EXTENDED W FROM THE LOW CENTER INTO N GEORGIA, WHILE A WARM FRONT  
EXTENDED E FROM THE LOW TO 31N75W. GOES-EAST SATELLITE IMAGERY  
REVEALED SHOWERS OVER THE NT1 AND N NT2 WATERS, WITH A HEAVIER  
CONCENTRATION OF SHOWERS AND THUNDERSTORMS OVER THE S NT2 WATERS  
IN THE VICINITY OF THE LOW AND ITS FRONTAL BOUNDARIES. HIRES ASCT  
DATA REVEALED 5-20 KT WINDS OVER THE NT2 WATERS FROM 31N TO 39N  
ROUGHLY BETWEEN 71W AND 78W.  
 
I WILL NOT BE MAKING CHANGES TO THE ONGOING GRIDS IN THIS  
INTERMEDIATE FORECAST PACKAGE, GIVEN THE AGREEMENT OF THE 18Z GFS  
WITH THE 12Z SUITE OF GLOBAL MODELS THROUGH THE NEAR TERM. HENCE,  
CURRENT WARNING HEADLINES WILL REMAIN AS IS FOR NOW.  
 
PLEASE REFER TO THE PREVIOUS MARINE WEATHER DISCUSSION BELOW FOR  
ADDITIONAL INFORMATION.  
 
-----------------------------------------------------------------  
PREVIOUS DISCUSSION...  
 
OF NOTE, A NOAA BUOY OBSERVATION OUTAGE BEGAN AT 18Z, AND THE  
OUTAGE IS BEING INVESTIGATED. OBVIOUSLY, AS A RESULT, WE HAVE  
NOT  
HAD ANY RECENT OBSERVATIONS ACROSS THE OFFSHORE AREAS WITH GALE  
WARNINGS THIS AFTERNOON.  
 
ASCAT OVERPASSES FROM THIS MORNING CONFIRMED WIDESPREAD GALE  
FORCE WINDS TO 42 KT ACROSS THE GULF OF MAINE AND EXTENDING JUST  
INTO THE NORTHERN PORTIONS OF GEORGES BANK. GALES WERE ALSO  
OBSERVED ACROSS THE SOUTH QUADRANT OF THE SURFACE LOW ACROSS THE  
ANZ810 OFFSHORE ZONE SOUTH OF NANTUCKET. THESE ASCAT WINDS WERE  
GENERALLY HIGHER THAN THE 12Z MODELS INITIALIZED THIS MORNING,  
ESPECIALLY WITHIN THE WESTERLY WINDS SOUTH OF NEW ENGLAND. THE  
12Z GFS FIRST SIGMA LEVEL WINDS APPEARED THE BEST INITIALIZED  
HERE. BASED ON THE ASCAT AND THE 12Z GFS, WE OPTED TO EXTEND THE  
GALE WARNINGS OVER ANZ810 THROUGH EARLY SATURDAY MORNING, AND  
ALSO FOR ANZ805 AND ANZ900, GEORGES BANK, THROUGH 18Z SATURDAY.  
WE DID NOT CHANGE THE TIMING OF THE WARNING EXPIRATION FOR THE  
GULF OF MAINE, STILL THROUGH 00Z. THE 12Z MODELS ALSO GAVE US  
HIGHER CONFIDENCE WITH THE GALE WARNINGS ACROSS THE FOUR  
NORTHERN  
OUTER NT2 ZONES TONIGHT THROUGH SATURDAY AFTERNOON OR EVENING.  
WE  
UPDATED THE WINDS GRIDS THROUGH 06Z SUNDAY WITH A 2:1 BLEND OF  
THE PREVIOUS OPC GRIDS AND THE 12Z GFS FIRST SIGMA LEVEL WINDS  
WITH THE CHANGES IN THE HAZARD GRIDS ALREADY HIGHLIGHTED ABOVE.  
THE SURFACE LOW NEAR CAPE COD WAS ANALYZED WITH A CENTRAL  
PRESSURE OF 989 MB AT 18Z, AND IS EXPECTED TO DRIFT EAST TONIGHT  
AND SATURDAY. WE CONTINUE TO HAVE HIGH CONFIDENCE WITH THE GALE  
WARNINGS AS PREVIOUSLY DESCRIBED.  
 
ZONAL PROGRESSIVE UPPER FLOW, GENERALLY WEAK, SHOULD THEN  
PERSIST  
ACROSS THE FAR WESTERN ATLANTIC THROUGH THE REMAINDER OF THE  
WEEKEND. A SERIES OF WEAK MID-LEVEL DISTURBANCES IS EXPECTED TO  
SUPPORT WEAK SURFACE LOW OR LOW PRESSURE TROUGHS ACROSS THE  
SOUTHERN AND CENTRAL MID-ATLANTIC OFFSHORE WATERS OVER THE  
WEEKEND. WINDS ASSOCIATED WITH THESE WEAK FEATURES ARE GENERALLY  
EXPECTED 20 KT OR LESS SATURDAY AND SUNDAY, WITH ANY 20 KT WINDS  
GENERALLY CONFINED TO THE WATERS SOUTH OF CAPE FEAR. HOWEVER, WE  
CANNOT RULE OUT SOME HIGHER WINDS ASSOCIATED WITH THE CONVECTION  
EXPECTED ACROSS THESE FAR SOUTHERN ZONES SATURDAY NIGHT INTO  
SUNDAY NIGHT. THE 12Z GFS THEN REMAINED CONSISTENT AND WERE  
AGAIN SLIGHTLY MORE AMPLIFIED WITH AN MID/UPPER LEVEL SHORTWAVE  
TROUGH MOVING TO THE MID-ATLANTIC COAST MONDAY. THE 12Z UKMET  
ALSO TRENDED TOWARD THE MORE AMPLIFIED GFS, WITH BOTH MODELS NOW  
 
ADVERTISING WINDS ASSOCIATED THE SURFACE LOW TO 25 KT OR 30 KT  
ACROSS THE CENTRAL MID-ATLANTIC ZONES MONDAY AND MONDAY NIGHT.  
AS A COMPROMISE, WE ARE USING AN EVEN BLEND OF THE 12Z GFS AND  
THE WEAKER 12Z ECMWF FOR THE WIND  
GRIDS SUNDAY THROUGH MONDAY NIGHT, WHICH WILL GENERALLY LIMIT  
THE  
MAXIMUM WINDS 20 KT.  
 
LATER IN THE FORECAST PERIOD THE MODEL GUIDANCE SEEMS TO BE  
CONVERGING ON A STRONGER COLD FRONT EXPECTED TO REACH THE  
MID-ATLANTIC AND NEW ENGLAND COASTS WEDNESDAY, AND RAPIDLY PASS  
EAST ACROSS THE WATERS WEDNESDAY NIGHT. THE 12Z MODELS HAVE ALSO  
COME INTO MUCH BETTER AGREEMENT WITH THE TIMING OF THE FRONT.  
CHANCES ARE INCREASING THAT GALE HEADLINES MAY AGAIN BE NEEDED  
ACROSS THE NORTHERN NT2 AND NT1 WATERS IN THE COLD AIR ADVECTION  
WEST OF THIS FRONT DURING THE WEDNESDAY NIGHT INTO THURSDAY  
TIMEFRAME. FOR NOW WE WILL HOLD OFF ON ADDING ANY HAZARDS TO THE  
FORECAST AT DAYS 5 AND 6. A BLEND OF THE PAST TWO RUNS OF THE  
ECMWF WAS USED FOR THE WIND GRIDS TUESDAY THROUGH THURSDAY  
NIGHT.  
 
.SEAS: BEFORE THE BUOY OBSERVATION OUTAGE BEGAN AT 18Z, THERE  
WERE TWO BUOYS WHICH OBSERVED 17 FT AT 17Z ACROSS THE GULF OF  
MAINE. THE 12Z WAVE GUIDANCE DID COME IN HIGHER THAN PREVIOUS  
RUNS, AND WERE REASONABLY WELL INITIALIZED WITH THESE WAVE  
HEIGHTS WITH THE ECMWF WAM SLIGHTLY HIGHER THAN THE GFS WAVE.  
THROUGH THE NEAR TERM WE RELIED ON THE LATTER WITH SOME  
ADDITIONAL MINOR UPWARD ADJUSTMENTS. A BLEND OF THE 12Z GFS WAVE  
AND 12Z ECMWF WAM WAS USED FOR THE WAVE HEIGHT GRIDS SATURDAY  
NIGHT THROUGH MONDAY NIGHT. THEN, CONSISTENT WITH THE WINDS, THE  
WAVE HEIGHT GRIDS WERE BASED ON A BLEND OF THE  
PAST TWO RUNS OF THE ECMWF WAM.  
   
EXTRATROPICAL STORM SURGE GUIDANCE
 
THE 12Z ESTOFS AND 12Z  
ETSS CONTINUE TO HAVE A DECENT HANDLE ON THE ELEVATED WATER  
LEVELS  
ALONG THE NEW ENGLAND COAST. AS ALWAYS, PLEASE MONITOR PRODUCTS  
FROM COASTAL NATIONAL WEATHER SERVICE OFFICES FOR DETAILED WATER  
LEVEL INFORMATION.  
   
WARNINGS
 
PRELIMINARY.  
   
NT1 NEW ENGLAND WATERS
 
   
ANZ805...GEORGES BANK WEST OF 68W
 
 
GALE SATURDAY.   
ANZ900...GEORGES BANK EAST OF 68W
 
 
GALE SATURDAY.   
ANZ810...SOUTH OF NEW ENGLAND
 
 
GALE SATURDAY.  
   
NT2 MID-ATLANTIC WATERS
 
   
ANZ915...HUDSON CANYON TO THE GREAT SOUTH CHANNEL
 
 
GALE TONIGHT INTO SATURDAY.    
ANZ920...BALTIMORE CANYON TO THE GREAT SOUTH CHANNEL
 
 
GALE TONIGHT INTO SATURDAY.    
ANZ905...THE GREAT SOUTH CHANNEL TO THE HAGUE LINE
 
 
GALE TONIGHT INTO SATURDAY.    
ANZ910...EAST OF THE GREAT SOUTH CHANNEL AND SOUTH OF 39N
 
 
GALE TONIGHT INTO SATURDAY.  
 

 
 
.FORECASTER HOLLEY/CLARK. OCEAN PREDICTION CENTER.  
 
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