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000
AGNT40 KWNM 250800
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
400 AM EDT Wed Apr 25 2018

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

Over the short term, no major changes to the previous hazards
and forecast is anticipated for the early morning package. We
will populate grids with a 50/50 blend of the previous grids and
00Z GFS first sigma layer winds as this blend fits well with an
ASCAT overpass from last evening around 0230Z which shows fairly
widespread gales E of Cape Hatteras, extending from roughly 33N
to 37N between 70W and 75W. We will use this blend through Thu.
1006 mb low pressure is centered over N. Carolina early this
morning with a warm front extending SE over the central and
southern NT2 waters and a cold front extending S along 76W or so
per the preliminary 06Z OPC-NCEP surface analysis. GOES-16 IR
satellite imagery shows a large circulation associated with the
low covering nearly all of the eastern U.S. and offshore waters,
with lightning products indicating numerous thunderstorms over
the offshore waters mainly S of 37N or so, or from the Delmarva
southward. This lines up well with the latest SREF thunderstorm
guidance. Thunderstorms will persist over most of the NT2 waters
today, and spread N into southern NT1 or southern New England
waters later today into tonight, before ending from SW to NE
later tonight and early Thu. Caution is advised for local wind
gusts exceeding gale force, and very rough seas, in and near any
of the stronger thunderstorms that cross the waters today into
tonight. Otherwise, the low is still forecast to track from N.
Carolina early this morning, to around Delaware Bay this
afternoon, to near Long Island tonight, to Maine on Thu.
Overall, confidence in the forecast for today into Thu is above
average.

For the medium range, Thu night through Sun night, we will once
again trend the forecast more toward the consistent ECMWF
guidance, and use a 80 percent 00Z ECMWF and 20 percent previous
forecast blend for the early morning package to take into account
relatively minor differences in strength and timing of systems
impacting the waters over the medium range. We will also make a
few manual edits in deference to nearby coastal WFO and TAFB
grids and forecast throughout the medium range. Of note, the 00Z
ECMWF is a little deeper with the next low forecast to cross the
region from SW to NE Thu night into early Sat, but not as strong
as the 00Z GFS which appears to be a too deep outlier at this
time. We will continue to monitor future model cycles to see how
the forecast will need to be adjusted over the next couple of
days. For now, we will not have any hazards associated with this
low as it crosses the region Thu night into early Sat. The cold
front associated with this low is still expected to push E of
the waters Sat night or early Sun, followed by high pressure
building SE over the eastern states toward the offshore waters
late Sun and Sun night, with sub-gale winds forecast in the
moderate cold air advection over the waters in the wake of the
front by later in the weekend. Confidence levels are near
average over the medium range portion of the forecast.

Seas: The 00Z ECMWF WAM appears to have verified and intialized
a little better than the 00Z Wavewatch based on the latest
observations over the waters early this morning, with the WAM
also very close to the previous grids. Sea heights per the 06Z
RA1 OPC sea state analysis graphic range from near 18 feet off
the N. Carolina coast to 2 feet or so E of Cape Cod. For the
early morning package we will use a 50/50 blend of the 00Z WAM
and previous grids through Thu, and then to match the wind
forecast use a 80 percent 00Z WAM, 20 percent previous wave
height grid blend Thu night through Sun night over the region.
Overall, these two blends will result in only minor changes from
the previous OPC forecast over the waters for the next few days.
We will make a few manual edits in deference to nearby coastal
WFO and TAFB grids through the next 5 days.

Extratropical Storm Surge Guidance: The 00Z ESTOFS output remains
slightly higher with surge values than the 00Z ETSS along the
Eastern Seaboard as low pressure lifts NE near the coast from N.
Carolina early this morning. With persistent strong onshore
flow, would tend to favor the higher values that approach about
1 foot in ESTOFS guidance mainly from Cape Hatteras northward
today into Thu. These values may even prove slightly underdone
with GFS 10m winds too low compared to ASCAT winds.

.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ810...South of New England...
     Gale today.
.ANZ815...South of Long Island...
     Gale today.

.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale today into tonight.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale today into tonight.
.ANZ905...The Great South Channel to the Hague Line...
     Gale today into Thursday.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale today into tonight.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale today.

$$

.Forecaster Mills. Ocean Prediction Center.



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