194
AGNT40 KWNM 211936
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
336 PM EDT Tue Aug 21 2018

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

GOES-East visible imagery reveals widespread convection firing
along a stationary boundary draped across the central waters this
afternoon. Unfortunately, we are still without lightning data at
the moment due to ongoing network problems that began about 0530
UTC, however based on overshooting and cooling tops, deepest
convection currently firing across zones 828, 830, and 925. Based
on NAM and SREF probabilistic guidance, showers and
thunderstorms should increase in coverage through the evening and
overnight across all offshore zones as the stationary front
lifts north as a warm front, and a strong cold front approaches
the coast. Caution to mariners for locally very strong winds and
very rough seas in or near any of the thunderstorms.

Overall, not too much change seen in the 12z suite of model
guidance this afternoon. Through the near term portion of the
forecast Thu morning, plan to re-populate the grids with a
boosted version of the 12z ECMWF winds to fine tune the timing
and location of strongest winds ahead of the approaching cold
front. This includes widespread gales developing across Baltimore
Canyon just after 12z Wed (zones 905, 915, 920, and 910), then
persisting into about 06z Thu as winds weaken ahead of the front.
While not much change in the guidance, forecast confidence is
about average. Thu onward, will trend the grids towards the 12z
GFS. Cold front weakens and stalls across the central and
southern waters, as high pressure builds to the coast and the W
Atlantic. The high slides across the northern portions Fri
through Sat night, while the stationary front begins to drift NW
toward the coast. The stationary front dissipates Sat night into
Sun, as a high pressure ridge builds W into the waters. A cold
front approaches the far northern waters Sun night.

Seas: similar to the wind grids, plan to use the 12z WAM into
Thu, then transition forecast toward the WW3. Expect seas up to
about 12 feet during periods of gales Wed and Wed night.

Extratropical Storm Surge Guidance: Minor positive surge remains
in the forecast through tonight (highest values up to about a
half foot from the 12z ESTOFS) across the Mid Atlantic coast.
Otherwise, no significant surge events are expected during the
next couple of days.


.WARNINGS...Preliminary.

.NT1 New England Waters...
None.

.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
Gale Wednesday.
.ANZ920...Baltimore Canyon to the Great South Channel...
Gale Wednesday.
.ANZ905...The Great South Channel to the Hague Line...
Gale Wednesday into Wednesday night.
.ANZ910...East of the Great South Channel and south of 39N...
Gale Wednesday into Wednesday night.

$$

.Forecaster Collins. Ocean Prediction Center.