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000
AGNT40 KWNM 291154
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
754 AM EDT Sat Jul 29 2017

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

A low pressure center near the Delmarva Peninsula with a
stationary front from center NE to near Georges Bank is poised
to move offshore. Highest associated winds (up to 20-30 kt) are
in the NE gradient N of the front across the far SW NT1 and far
NW NT2 waters and in the SW gradient S of the front across the
waters near Hatteras Canyon. Highest associated seas are in the
SW gradient and are likely in the 6-9 ft range, which are in line
with the 06Z Wavewatch III and 00Z ECMWF WAM models at the
moment.

Over the short term, the latest models in general agree that the
surface low will track slowly E across the NT2 waters today into
Sun night while splitting into multiple centers. The models
continue to have run-to-run continuity problems in regards to
both their forecast timing and forecast strength of these surface
lows (with the 00Z ECMWF now forecasting the strongest solution),
so continue to have low forecast confidence in any one
particular model solution. But if one instead focuses on just the
associated forecast gradients, by this afternoon all the latest
models forecast gale force associated boundary layer winds to
develop. Therefore, forecast confidence in the previously
forecasted gale warnings remains relatively high. So through the
short term, due to the poor run-to-run model continuity, will
continue to use the previously populated wind and sea grids and
will just make some minor additional edits mainly in deference to
the 00Z GFS/ECMWF.

In the long range, on Mon into Tue the 06Z GEFS mean indicates
that the 06Z GFS is likely both too slow and too strong with its
final frontal wave crossing the NT2 waters. Therefore, plan on
favoring the more progressive and weaker 00Z GFS solution then.
Then by Wed/Wed night as all of the latest global models
(disregarding the over amplified 00Z GEM solution) forecast weak
gradients with light winds to develop throughout all of the
coastal/offshore waters, the previously populated 00Z GFS
solution still looks representative. So do not plan on making any
major long range changes in the next offshore forecast package.





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

The latest NCEP surface analysis indicates low pres inland just W
of the Delmarva peninsula with a front extending to the east
from the low center. Ascat winds retrievals from 02Z last night
indicate S winds up to 30 kt ahead of the low just offshore in
tstms, though the strongest winds were in a small area to the
east of the coast as a majority of the area had no more than 20
kt. The 00Z GFS/ECMWF/UKMET all indicate the low will move
offshore today though the VA Capes as the associated shortwave
trough, currently S of the Great Lakes region as indicated by the
water vapor imagery, moves SE toward the area. There are some
disagreements between the model solutions on the development of
multiple lows along the front as the models all indicate that the
consolidated shortwave will become sheared and split into 2
separate and weaker pieces. The 00Z GFS/GEM take the low a little
further S than the 00Z ECMWF/UKMET solutions, but the 00Z GFS has
trended faster from previous runs and now lags only slightly
behind the ECMWF. In fact, it is now a decent compromise to the
differences between the ECMWF and GEM, so preferring it at this
time. As far as wind intensity is concerned, the GFS indicates
gales will develop over NT2 later today as the SW develops over
the warmer waters. The more conservative ECMWF also indicates a
brief period of gales, though the 00Z UKMET does not. However,
the 00Z GFS indicates a brief period with winds up to 50 kt just
E of the offshore waters, and seems overdone as the upper level
pattern does not suggest such strong development and could be the
result of a feedback problem. For the forecast, planning on
starting out with previous grids to maintain continuity and
warnings. Will then transition on Sun to the 00Z GFS 10m winds
over stable areas and first sigma winds over unstable areas over
the short range.

In the medium range, the 00Z models agree fairly well on the
pattern becoming much weaker once the aforementioned frontal
system moves east of the area. The models all indicate that a
weak low will develop off the SE coast on Tue, though the 00Z GEM
is a strong outlier and indicates much stronger development. The
00Z GFS seems representative of the overall model consensus on
this low, so planning on continuing with it through the remainder
of the forecast period.

.SEAS...The 00Z Wavewatch and ECMWF WAM have both initialized
the current seas within a ft of the current data and agree well
over the next 24 hours so will start out with a 50/50 blend. Will
then transition to the Wavewatch exclusively on 12Z Sun as the
ECMWF WAM has been a bit inconsistent with building seas along
the coast.

.EXTRATROPICAL STORM SURGE GUIDANCE...The 00Z ETSS and ESTOFS
seem representative with a 1 ft surge along the Mid Atlc coast
in the NE flow today into Sun.


.WARNINGS...Preliminary.

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

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     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 tonight.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale today into tonight.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale today into tonight.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale today into Sunday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale today into Sunday.

$$

.Forecaster Vukits/Kells. Ocean Prediction Center.


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