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000
AGNT40 KWNM 281204
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
804 AM EDT Fri Jul 28 2017

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

As a weak cold front near the New England and N mid Atlantic
coasts drifts SE, the SWly gradient in its advance across the
waters is supporting max winds at most in the 15-20 kt range. Max
seas in this gradient are likely only in the 4-6 ft range,
highest in the vicinity of the gulf stream across the central NT2
waters, which are being handled fine by the 06Z Wavewatch III and
00Z ECMWF WAM at the moment.

The latest models forecast a complicated pattern to develop. In
response to a strong upper level short wave trough forecast to
dive towards the mid Atlantic coast tonight/Sat, the latest
models generally agree that an associated surface low will
develop on the front near the mid Atlantic coast tonight, then
move slowly offshore into the NT2 waters Sat into Sun with
multiple centers developing. Versus its previous 12Z run, the 00Z
ECMWF forecasts a significantly more progressive and weaker
solution than the other global models. The previous offshore
forecast package was based on the more amplified and slower 12Z
ECMWF solution. For now to maintain some degree of continuity,
will continue to use this solution through Sat night with
additional and at times significant edits in deference to the 00Z
ECMWF and the 06Z/00Z GFS. Then Sun and beyond, since its more in
line with the latest WPC medium range guidance, would favor the less
progressive 06Z/00Z GFS and 00Z UKMET solutions, which take the
complex surface low with multiple centers more slowly E across
the NT2 waters than forecast by the 00Z ECMWF. Therefore as a
compromise, from Sun and beyond for the next offshore forecast
package will repopulate with our smart tool that will place
stronger 06Z GFS first sigma level winds in unstable areas and
weaker 06Z GFS 10m winds in stable areas, which will result in
significant changes to the previously forecasted gale warnings.
So since the winds will be repopulated, will then follow suit
and repopulate with 06Z Wavewatch III seas from Sun and beyond.




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

Gale warnings starting Saturday will be retained as strong low
pressure will move east across the central waters. Geocolor
satellite images still show a series of cloud bands inland
approaching the region from the northwest with just few clouds
over most of the central and southern regions. Lightning density
map show dense lightning east of the region moving southeast. The
SREF model has PROB 20 for TSTMS threat over the far southern
waters. Highest winds are still 20 kt over maine waters while 5
to 15 kt winds have been observed elsewhere. The 06Z NCEP weather
map has low pressure 1010 mb just east of the Baltimore Canyon
with a stationary front now just southeast of the southern
waters. Weak high pressure 1018 mb near 44N47W extends a very
weak ridge into the northeastern waters. An inland front
stretches southwest from low pressure over Canada and passes just
northwest of mid Atlantic as it curves southwest to pass across
the upper/lower Mississippi. Pressure gradient is fairly relaxed.

In the upper levels, models GFS/CMC/ECMWFHR/UKMETHR show a
trough with some energy to the northwest of the region that will
move across the north waters. A pocket of energy over Canada just
north of the great lakes will move southeast and when it gets
near the waters will spread as its associated trough stretches
southwest and eventually pass east across the waters. Models have
very slight differences on the timing of this system as it moves
across the waters. Models have also minor differences on the
extent of the trough. GFS ends the trough farther south than the
other models. Otherwise, the models generally agree on
strengthening the trough and that will result in a tight pressure
gradient over the north and central waters.

The models GFS/CMC/NOGAPS/ECMWFHR/UKMETHR/NAM have initialized
fairly well the 06Z surface observations. Models are generally in
good agreement in the short term with just minor differences
mainly on the timing of the low that will impact the waters
beginning late tonight. For this issuance will not deviate from
the previous forecast and so will go with ECMWFHR for winds.

.SEAS...are relatively small with a peak at only 5 ft over the
eastern Baltimore Canyon and they range between 2 and 5 ft with
smallest over the western portion. Wave models NWW3 and ECMWFWAVE
fit well the observed seas pattern and continue to be in good
agreement on building seas to 15 ft in areas of gale warnings.
Will stick to a blend of both models. In the short term, seas
will remain below 8 ft then will build to peak at 15 ft.


.EXTRATROPICAL STORM SURGE GUIDANCE...



.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ810...South of New England...
     Gale Saturday into Saturday night.
.ANZ815...South of Long Island...
     Gale Saturday into Saturday night.
     Gale Possible Sunday.

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale Saturday into Saturday night.
     Gale Possible Sunday.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Saturday into Saturday night.
     Gale Possible Sunday into Sunday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Saturday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale Saturday night.
     Gale Possible Sunday.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Saturday into Saturday night.
     Gale Possible Sunday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Saturday into Saturday night.
     Gale Possible Sunday.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale Possible Sunday.

$$

.Forecaster Vukits/Musonda. Ocean Prediction Center.



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