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359
AGNT40 KWNM 210029
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
829 PM EDT Thu Jul 20 2017

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

Update...The 18z ncep surface analysis shows a weak high pres
ridge over the offshore waters, with a low pres trough extending
roughly NE to SW across the Eastern Seaboard. Latest available
ascat and ascat hi-res passes from this morning show 5 to 15 kt
winds over the offshore waters. Lightning density product data at
2310z shows scattered showers near the New Hampshire coast, and
also along the South Carolina and Georgia coasts.

Models...The medium range models are in very good overall
agreement across the offshore waters for tonight through Sat, so
the representative gfs 10m solution will be used for the wind
grids over this timeframe. After Sat, the gfs is generally slow
with the timing of the main weather features, and also looks a
bit overdone with the SW flow north of the Gulf Stream on Sun.
Since the ecmwf appears to be the best median model solution
regarding timing and strenght for Sat Night through Tue Night,
it will be used for the wind grids during this period. Am not
planning to make any significant changes to the current forecast
trend.

Seas...The wna wavewatch and ecmwf wam both initialized well over
the offshore waters, and are in good overall agreement through
Sat with only minor differences noted. So will go with a 50/50
blend of the wna wavewatch/ecmwf wam for tonight through Sat to
smooth out the model differences. Then the ecmwf wam will be used
exclusively for the sea height grids for Sat Night through Tue
Night, in order to be consistent with the preferred ecmwf model.

Extratropical Storm Surge Guidance...N/A.

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

At 18Z a weak frontal boundary extended along the Maine coast
and further inland across New England, while a persistent high
pressure ridge was oriented east to west just south of the
southern NT2 waters. The 12Z models are in general agreement
across the offshore waters over the next couple days, but there
are differences with how the models are handling the series of
weak surface lows which are forecast to pass east along the
persistent frontal boundary which will linger off the New
England and northern Mid Atlantic coasts tonight into early next
week. The 12Z GFS and 12Z ECMWF both move the first low
northeast near the northern Gulf of Maine tonight sharing a
similar timing. The associated weak cold front is forecast to
move offshore tonight into early Friday, but should quickly
dissipate across the NT2 waters. The 12Z models then also share
a similar timing in moving a stronger cold front through the NT1
waters Friday night, and stalling near 1000 fathoms Saturday.
With the models only having minor differences with the offshore
winds through Saturday, will continue to populate the OPC wind
grids with the 12Z GFS.

The model disparity then increases late Saturday/Sunday as
models are having difficulty with the timing of a series of lows
forecast to track east along the front, as was noted above. Also
the 12Z ECMWF/UKMET both begin to lift the front back north as a
warm front already by Saturday night, while the 12Z GFS is
slower. The guidance is in agreement that the southwest return
flow south of the front will increase Saturday night/Sunday.
However the 12Z GFS/UKMET seem slightly overdone with winds to
30 kt given the stable low level lapse rates across the colder
waters north of the Gulf Stream. Beginning Saturday night, OPC
preference transitions to the weaker 12Z ECMWF. By early next
week, the 12Z GFS remained a strong and slow outlier with the
upper level trough moving into the Northeastern US. The 12Z
UKMET is slightly more progressive than the 12Z ECMWF, so
through Tuesday night OPC preference will continue with the 12Z
ECMWF which offers a compromised timing. Like the 12Z ECMWF, the
12Z UKMET also indicates that a high pressure ridge will build
toward the New England coast Tuesday night.

Seas...Both the 12Z Wavewatch III and 12Z ECMWF WAM were well
initialized with wave heights in the W Atlantic this afternoon,
which are currently 4 ft or less in the offshore waters. The
models are generally within a ft or so of each other through the
weekend. As a result, will continue using a 50/50 blend of the
two solutions through Sunday night as a compromise to the minor
differences. Then, with the 12Z ECMWF and 12Z GFS diverging
Mon/Tue and OPC preference with the 12Z ECMWF, transitioned
solely to the 12Z ECMWF WAM at that time.

.Extratropical Storm Surge Guidance: n/a.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Scovil/Clark. Ocean Prediction Center.



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