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North Atlantic Marine Weather Discussion

AGNT40 KWNM 212018
MIMATN

Marine Weather Discussion for North Atlantic Ocean 
NWS Ocean Prediction Center Washington DC 
415 PM EDT SUN 21 JUL 2019 

.FORECAST DISCUSSION...Major features/winds/seas/significant 
.weather for the North Atlantic Ocean W of 50W from 30N to 50N. 

This mornings Ascat overpasses over the offshore waters returned 
15 to 20 kt across most of the NT2 zones north of Cape Fear. The 
previous OPC wind grids which were largely based on the 00z 
HiResW-ARW agreed well with the Ascat winds. At 18z a cold front 
extended just offshore across the northern Gulf of Maine. As the 
front sinks south tonight, thunderstorms are forecast to develop 
over the New England and northern Mid-Atlantic offshore zones. 
The 12Z HREF guidance shows a chance, or about a 30 to 40 
percent probability, that some of these thunderstorms could 
produce gale force winds mainly over the South of New England 
zones this evening through about 04z. Thunderstorms are expected 
to be much more limited over the central and southern NT2 waters 
at least through early Mon. The 12z models are consistent that 
the front will stall near 40N early Mon. By late Mon and Mon 
night, the guidance all indicates that the southwest winds will 
abruptly increase from south to north as low pressure 
consolidates and tracks northeast from near Long Island. The 12z 
GFS and 12z ECMWF have come into excellent agreement with this 
first frontal wave, and both show winds up to 30 kt expanding 
northward from the central NT2 zones Mon evening, and the 
northern NT2 and southern NT1 waters Mon night into Tue evening. 
There continues to be a slight chance for winds to increase to 
gale force Mon night and Tue. With the guidance depicting the 
stronger winds coincident with the zones where sea surface 
temperatures are cooler, there should be very little vertical 
mixing of the 40 to 50 kt low level jet. Therefore, chances 
remain too low to add gales to the forecast, at least at this 
time. Forecast confidence for tonight through Tue is slightly 
above average.      

During the medium range, we will trend the forecast more toward 
the ECMWF which will keep close continuity with the previous OPC 
forecasts/grids. Versus previous runs, the 00z ECMWF trended 
deeper and stronger with the winds associated with the next set 
of developing surface lows expected to track northeast across 
the central/northern NT2 waters Tue night into early Thu. The 
12z ECMWF was generally consistent with this stronger solution. 
We will use a blend of the 12z ECMWF and the previous grids, 
which will maintain some winds to 30 kt over the northern outer 
NT2 zones Wed into Wed night. As high pressure builds to the New 
England coast toward the end of the week, the front is forecast 
to stall and weaken over the central and southern NT2 waters. As 
more frontal waves track northeast over these areas, winds are 
expected to increase to 20 or 25 kt. Forecast confidence during 
the medium range is about average.  

.Seas: Wave heights across the Gulf of Maine are about 1 ft or 
so lower than the Wavewatch and ECMWF WAM guidance this 
afternoon. We will use an even blend of previous wave height 
grids and the 12z ECMWF WAM through the period. However, the 12z 
WAM and 12z Wavewatch are in very good agreement across the 
offshore waters. 

Extratropical Storm Surge Guidance: No significant positive 
surge events appear likely during the next few days. For more 
information please refer to the latest coastal National Weather 
Service forecast. 

.WARNINGS...Preliminary. 

.NT1 New England Waters... 
     None. 

.NT2 Mid-Atlantic Waters... 
     None. 

$$ 

.Forecaster Clark. Ocean Prediction Center.

 

 



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