North Atlantic Marine Weather Discussion

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AGNT40 KWNM 162005
MIMATN

.Marine Weather Discussion for N Atlantic Ocean 
.NWS Ocean Prediction Center Washington DC 
405 PM EDT THU 16 AUG 2018 

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

The 1410Z Ascat overpass returned winds to 25 kt associated with 
the thunderstorms which were located about 60 to 90 nm southeast 
of Cape Lookout. Recent GOES-16 imagery and lightning density 
indicate that the system became outflow dominant, cloud tops 
warmed, and these cells have since dissipated over the last 
couple hours. There continues to be some scattered thunderstorms 
across the outer NT2 waters generally along a line from 31N 78W 
to 33N 72.5W, as well as more isolated area in the vicinity of 
35N 71W. Scattered thunderstorms are expected to continue to 
develop across the waters south of Hatteras Canyon and will 
expand northward across the Baltimore to Hatteras Canyons waters 
late tonight and early Fri. The latest HREF suggests that any 
thunderstorms which develop this evening and tonight will have 
tough time supporting any gale force winds at the surface with 
NAM/RAP soundings also indicating stabilizing mid levels. We 
will likely drop the mention of any winds in thunderstorms in 
this afternoons NT2 text forecast. 

The 12Z models are generally in good agreement across the west 
Atlantic for the next couple days. The cold front near the Maine 
coast will drop south tonight, stall near Cape Cod Fri, and then 
lift back north as a warm front late Fri and Fri night. Winds 
associated with the front should remain 15 kt or less through 
Fri, then begin to increase Fri night as low pressure moves 
northeast along the Maine coast and a stronger cold front 
approaches the coast. Meanwhile, high pressure will persist 
southeast of the NT2 waters into the weekend. Through Fri night 
we will use an even blend of the 12Z HRW-NMM and 12Z NAMNest for 
the wind grids.    

The 12Z models have not come into any better agreement with this 
weekends forecast, That is, with the cold front expected to move 
south across the waters late Sat into early Mon and then the 
developing surface low along the stalled front across the NT2 
waters Sun/Mon. The 12Z GFS and 12Z UKMET are flatter and more 
progressive than the 00Z/12Z ECMWF with the low. The 12Z ECMWF 
tracks the low northeast to near Cape Cod late Mon and toward 
southwestern Nova Scotia Mon night/Tue, while the GFS/UKMET both 
show high pressure along the New England coast at least into 
early Tue. We have some concern that there will be more 
widespread 25 kt winds north of the front Sat night through Mon. 
Given the disparity among the models, we are again choosing to 
use a blend for the wind grids. We used an even blend of the 12Z 
GFS and 12Z UKMET, then increased the northeast winds north of 
the front by 15 percent or so Sat night through Mon night. These 
edits basically just expanded the areas of 25 kt winds somewhat. 
By Tue/Tue night, the models are generally consistent that 
another cold front will approach the New England and 
Mid-Atlantic coasts. We prefer ECMWF/UKMET which are more 
amplified with the upper level trough through the eastern US 
Tue/Wed.    

Seas: The wave height grids were mostly based on the 12Z 
Wavewatch. We did make some edits to increase wave heights by 
about 15 percent or so across the areas with the stronger 
northeast winds over the northern waters late Sat through Mon.  

Extratropical Storm Surge Guidance: No significant storm surge 
is expected through Sat. Late in the weekend, there could be 
minor surge that develops across the New England and northern 
Mid Atlantic coasts poleward of the stalled front in east to 
northeast onshore flow. This, of course, depends on how strong 
the developing low and associated easterly gradient become, as 
well as how persistent any stronger winds are. At the moment, 
given the model variability, our confidence in any one given 
forecast scenario is low. 

.WARNINGS...Preliminary. 

.NT1 New England Waters... 
     None. 

.NT2 Mid-Atlantic Waters... 
     None. 

$$ 

.Forecaster Clark. Ocean Prediction Center.