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North Atlantic Marine Weather Discussion
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AGNT40 KWNM 162005
.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
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.
.NT1 New England Waters...
.NT2 Mid-Atlantic Waters...
.Forecaster Clark. Ocean Prediction Center.