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North Atlantic Marine Weather Discussion

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AGNT40 KWNM 180717
MIMATN

.Marine Weather Discussion for N Atlantic Ocean 
.NWS Ocean Prediction Center Washington DC 
215 AM EST MON 18 DEC 2017 

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

The 01 UTC Ascat still returned some northwest winds to 25 kt 
across the far outer ANZ905 and ANZ910 zones, while the 
subsequent pass even showed some 20 kt winds south of the 
stationary boundary as far west as about 80 to 150 nm southest 
of Cape Charles, over the Gulf Stream. The 00 UTC GFS first 
sigma level winds were the best initialized with these higher 
winds offshore. The 00 UTC models are consistent that relatively 
weak gradients and surface winds will occur across the offshore 
waters at least through the early part of today as the high 
pressure ridge is forecast to shift east of the offshore waters. 
Also, the models share a similar timing in with a developing 
surface low along the stationary front across the northern NT2 
waters today. The developing low should pull the weak front 
south to near Cape Fear Mon before it again stalls and 
dissipates tonight/Tue. Based on the slightly higher winds seen 
in the Ascat, will be blending in some of the higher 00 UTC GFS 
first sigma level winds with the previous OPC wind grids, which 
were based on the ECMWF, through tonight. 

The latest models then share a similar timing with another cold 
front forecast to move off the northern mid-Atlantic and New 
England coasts Tue night. The GFS and ECMWF have become more 
consistent, and slightly stronger compared to their respective 
runs yesterday, with the post-frontal west to northwest winds 
across the eastern NT1 and northeastern NT2 zones Tue night into 
Wed. As a result, we have increasing forecast confidence that 
there will be a brief period of gales across the Gulf of 
Maine/Georges Bank and the outer ANZ905 waters at that time. For 
these winds, we again used an even blend of the GFS first sigma 
level winds and the ECMWF 10m winds. The gale hazards will 
remain consistent with the previous offshore forecasts. 

The 00 UTC models are then consistent that a southern stream low 
will develop and move off the southern Mid Atlantic coast late 
Wed and Wed night, with the 00 UTC GFS/ECMWF/UKMET all 
advertising associated winds as high as 30 kt across the waters 
south of Cape Hatteras. Of note, the 00Z GFS even trended 
stronger than previous runs, and now shows some marginal gales 
associated with the low Wed night. We are favoring the weaker 
model consensus here, with winds only as high as 30 kt, close to 
the more consistent ECMWF. Once the low passes east of the 
offshore waters early Thu, the models are then consistent that 
an inverted low pressure trough will linger along the Carolina 
coast Thu night and Fri. At the same time, a strong high 
pressure ridge will pass east across the NT1 and northern NT2 
waters. 

The models then all show another cold front will approach the 
eastern seaboard late Fri and Fri night. However, the 00 UTC 
models vary as to the timing of the front, and vary widely with 
how amplified the supporting upper level trough is forecast. 
Versus its previous few runs, the 00 UTC GFS trended 
significantly faster, and also is a strong outlier with the 
pre-frontal winds Fri night into the weekend. OPC preference 
lies with the weaker slower non-GFS model consensus, which is 
also supported by the latest WPC medium range guidance.      

.Seas: The 00 UTC Wavewatch III and 00 UTC ECMWF WAM are equally 
well initialized with the wave heights across the west Atlantic 
this morning. Will be updating the previously preferred 3:1 
ECMWF WAM and WW3 blend for the wave height grids through Thu 
night. Then, with the GFS being the strong outlier with the late 
Fri and Fri night winds, we will transition solely to the 00Z 
ECMWF WAM beginning Fri. 

Extratropical Storm Surge Guidance...N/A. 

.WARNINGS...Preliminary. 

.NT1 New England Waters...
.ANZ800...Gulf of Maine...
     Gale Tuesday night.
     Gale Possible Wednesday.
.ANZ805...Georges Bank west of 68W...
     Gale Tuesday night.
.ANZ900...Georges Bank east of 68W...
     Gale Tuesday night.
     Gale Possible Wednesday.

.NT2 Mid-Atlantic Waters...
.ANZ905...The Great South Channel to the Hague Line...
     Gale Tuesday night.
     Gale Possible Wednesday.

$$ 

.Forecaster Clark. Ocean Prediction Center.