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000
AGNT40 KWNM 220046
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
846 PM EDT Sat Oct 21 2017

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

Current Conditions...The 18z ncep surface analysis shows a weak
low centered over the southern Maine coast, with an associated
low pres trough extending NE from the low along the coast.
Otherwise, the analysis shows a high pres ridge over the offshore
waters. Latest available ascat hi-res and ascat passes from this
morning show 5 to 15 kt winds over the offshore waters.

Models/Forecast...The medium range models are in good agreement
over the offshore waters for tonight through Mon, so the
representative gfs 10m/30m solution with the stability smart tool
will be used for the wind grids during this timeframe. Since the
gfs may be somewhat overdone with the S flow ahead of the cold
front for Mon night into Thu, the weaker ecmwf will be preferred
for the wind grids for Mon night through the rest of the forecast
period. While we are still indicating gales over the Georges Bank
waters, confidence is higher with the gales in the warmer
northern nt2 waters for Wed and Wed night.

Seas...The wna wavewatch and ecmwf wam both initialized well
across the offshore waters, and are in good agreement with one
another for tonight through Mon night. By Tue through Wed night,
with the models differing somewhat with the strengthening S
flow, the wavewatch is 3 to 6 ft higher than the ecmwf wam, and
generally looks overdone with the S flow ahead of the cold front.
So will go with a 50/50 blend of the wavewatch/ecmwf wam for the
wave height grids for tonight through Mon night, then use the
ecmwf wam exclusively for Tue through the rest of the forecast
period in order to be consistent with the preferred ecmwf wind
grids.

Extratropical Storm Surge Guidance...N/A.

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

At 18Z, a high pressure ridge extended south southwest from the
New England offshore waters and along the Mid Atlantic coast.
The 12Z models are in good agreement that as the ridge drifts
eastward the offshore winds will remain 15 kt or less through
Sun evening. The only exception is across the far southern
portion of the offshore waters south of Cape Fear tonight
through Mon, where winds will reach 20 kt or so as a series of
low pressure troughs pass west toward the Florida coast. Also,
the latest guidance is consistent that a weak surface low will
develop along a stationary front near the New Hampshire/Maine
coast later today, and move east across the Gulf of Maine
tonight before dissipating early Sun. Winds associated with the
low will be limited to 10 or 15 kt. Will populate the winds
grids through Mon with the 12Z GFS, but will use the stability
smart tool which populates the GFS first sigma level winds
across areas with negative static stabilites.

Once the high pressure ridge shifts east of the offshore water
Sun night, a strong cold front will slowly approach the Mid
Atlantic and New England coasts Mon through Tue, and will be the
weather feature which will have the most impact on the offshore
waters through the forecast period. Whereas previous runs of the
GFS were faster than the ECMWF/UKMET/GEFS with the front, the
12Z GFS has trended slower and is now in excellent agreement
with the model consensus. Versus previous runs, the 06Z/12Z GFS
is also weaker with the pre frontal offshore winds late Tue
through Wed night. The weaker 12Z GFS is also in better
agreement with the ECWMF/UKMET/NAM. The main forecast challenge
with this fropa is how widespread (or limited) any pre frontal
gales may be, specifically  determining how much of the 45 to 55
kt low level jet (as shown by the 12Z GFS and 12Z ECMWF) will
mix to the surface. Similar to the 12Z GFS and compared to its
respective previous run, the 12Z ECMWF also backed off with the
strength of the southerly winds Wed/Wed night and only shows
very limited gales across the outer waters east of about 69W. In
addition, versus its previous run, the 12Z ECMWF is also slower
with the front. Based on these trends we will be blending in 50
percent of the 12Z ECMWF into the existing wind grids and then
making some edits to make the areas of gales more limited than
previously forecast. Will drop the gales across the ANZ925
waters, but will maintain the mention of gales across the other
zones as was done previously, albeit somewhat more limited in
area and duration. We have much more confidence with the pre
frontal southerly gales forecast across the warmer northern NT2
waters Wed/Wed night than those across the colder waters near
Georges Bank at that time.

.Seas...Both the 12Z Wavewatch III and 12Z ECMWF WAM are equally
well initialized with the west Atlantic wave heights this
afternoon, and also are in good agreement through Mon. By Tue
through Wed night, with the models differing slighly with the
increasing southerly winds, the 12Z WW3 is anywhere from 3 to 6
ft higher than the 12Z ECMWF WAM, and generally looks too high
in this type of southerly flow regime. Will be populating with
an even blend of the WW3/ECMWF WAM through Mon night before
transitioning solely to the lower ECMWF WAM Tue through the
remainder of the forecast period.

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

.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ805...Georges Bank west of 68W...
     Gale Possible Wednesday night.
.ANZ900...Georges Bank east of 68W...
     Gale Possible Wednesday night.

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

$$

.Forecaster Scovil/Clark. Ocean Prediction Center.


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