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000
AGNT40 KWNM 251243
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
843 AM EDT Tue Jul 25 2017

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

As of 12UTC, the axis of 850mb winds at or above 35kt had moved
just east of the offshore zones. A review of ASCAT data overnight
showed very isolated gale conditions could have occurred, but
likely with deep convection based on IR satellite loops and
lightning plots, east of the Virginia and North Carolina coasts.
Other isolated near-gale conditions were observed off the south
coast of Maine. With the axis of 850mb winds moving east and
sub-35kt 850mb winds over the offshore waters into Friday,
hazards will be associated with deep convection in frontal zones.
For today and into tonight, this should be along and south of a
line from the eastern part of ANZ905 through ANZ833 off of the
Carolina coast. Aside from isolated or widely scattered showers
north of that line today, the better chances of any showers or
thunderstorms until overnight Thursday into Friday will be along
and south of the aforementioned line.

Main forecast challenge comes Friday into Saturday. The 00UTC
ECMWF has maintained its trend of a compact, sub-1000mb low
moving off of the coast near Massachusetts Friday into Saturday.
The low is not as deep on the latest ECMWF as on previous runs,
but adding confidence, the GFS has trended toward the ECMWF as
both now have a closed 500mb low over southeast Canada by
Saturday morning. The Canadian model has the same basic low track
as the ECMWF and GFS but slower, and now the formerly-favored
UKMET appears to be a slow, north outlier.

Leaning toward a combination of the GFS and ECMWF, the challenge
now becomes the potential for gale-force winds in parts of the
mid-Atlantic and northern waters Friday into Saturday. 850mb
winds on both the GFS and ECMWF reach near 50kt, but the details
differ in terms of location. The ECMWF would suggest winds in the
vicinity of lower-end gales more over northern waters, while the
GFS would suggest such winds farther east and closer to the Gulf
Stream. Will await an update to model guidance this afternoon,
but if trends continue and at least the GFS and ECMWF model
guidance remains in at least modestly good agreement, there
currently appears a good chance that winds around 35kt will
appear in some offshore zones by Friday night by the late
afternoon forecast issuance.

-----------------------------------------------------------------
PREVIOUS DISCUSSION...

Summary...The 00z ncep surface analysis shows a 1007 mb low
centered just E of Cape Cod, with a warm front extending E from
the low across the Georges Bank waters, and a cold front
extending SW from the low to the Delaware coast. The analysis
also indicates a high pres ridge building E into the central and
southern nt2 waters, with a low pres trough approaching the mid
Atlantic and Carolina coasts. Latest available ascat hi-res and
ascat passes from a few hours show 25 to 30 kt winds in the nt1
waters N of the low, and 25 to 30 kt winds in SW flow over the
central and northern nt2 waters E of the 1000 fathoms curve.
Lightning density product data at 0720z indicates scattered
showers and tstms in the outer zones of the nt2 area ahead of the
cold front, and also over the inner zones near the mid Atlantic
coast.

Models...The 00z medium range models are in good overall
agreement across the offshore waters for today through Thu night,
and the 00z ecwmf appears to be the most representative model
for this timeframe and will be used for the wind grids. The 00z
ukmet is a strong and slow outlyer solution with the low moving
across the nrn nt2 and nt1 waters on Fri and Sat, while the 00z
ecmwf and 00z gfs 10m have come into good agreement for that
period. A 50/50 blend of the 00z ecmwf/gfs 10m will be used for
Fri through the end of the forecast period.

Seas...The 00z wna wavewatch and 00z ecmwf wam both initialized
well over the offshore waters. Since am favoring the 00z ecmwf
through Thu night, will use the 00z ecmwf wam for the sea height
grids over this timeframe in order to be consistent. Then will
switch to using a 50/50 blend of the 00z wna wavewatch/ecmwf wam
for Fri through the end of the forecast period for consistency
purposes.

.Extratropical Storm Surge Guidance...N/A.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Figurskey/Scovil. Ocean Prediction Center.



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