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000
AGNT40 KWNM 171343
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
943 AM EDT Thu Aug 17 2017

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

The latest water vapor satellite imagery shows a compact upper
low moving east over the Canadian Maritimes, with a mid-level
shortwave poised to move over the far northern waters during the
day. The surface gradient will be tight at least through early
afternoon, relaxing during the afternoon as high pressure builds
in from the west. In the entire offshore waters, outside of deep
convection, the highest winds should be located over the northern
part of ANZ800, where in the tight gradient and with the passage
of the mid-level shortwave, winds should increase for a time to
15 to 25 kt. These winds should diminish quickly during the
evening and through the overnight hours as the surface gradient
really relaxes.

Any deep convection should remain south of a stationary front off
of the Carolina coast, and where a weak surface wave will be
moving slowly east along the front today through Friday. A few
showers were noted on regional radar loops moving southeast
across Virginia and these could affect especially ANZ825 this
morning. With convergence being weak and heights increasing
aloft, coverage of deep convection after this morning should
become more isolated by the overnight hours.

Through the period, the potential for wind hazards is low. On
Saturday, there are some differences between the GFS and the
ECMWF, with the former having a tighter surface gradient between
a 999mb surface low toward James Bay and an elongated high south
from the Canadian Maritimes, while the latter has weaker low
pressure toward the northwest. The GFS would try to get winds
between 25 and 30kt in parts of ANZ800 by early Saturday morning,
while the ECMWF is 5 to 10kt less. The NAM is closer to the GFS
in this regard. For now will maintain the winds in the official
forecast around 20kt and evaluate guidance during the afternoon.
Given a period of a tight gradient, a later guidance compromise
may be prudent. After that, stronger, but still sub-gale, winds
do not appear again as a possibility under Tuesday into Wednesday
next week, mainly over northern waters.

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

The infrared satellite imagery indicates Hurricane Gert now well E
of the area with center now east of 60W. Ascat overpasses from
0045 UTC and 0230 UTC indicated winds up to 20 in the far NE NT2
offshore waters, though the limited surface observations from
0600 UTC indicated slightly less than 20 kt as high pres builds
over the area. The 00Z GFS/ECMWF model winds are initialized well
when compared with the data, and both models agree well on the
initial surface pattern when compared with the current NCEP
surface analysis. In addition to Gert, the analysis indicates a
pair of weak frontal boundaries across the offshore waters this
morning - a cold front across the NT1 offshore waters and a
stationary boundary over the central NT2 waters with a weak low
along the front. The 00Z models all agree that the northern front
will weaken and pass E of the area today, while the srn boundary
is expected to continue over the area in its weakened state for
the next couple of days. The 00Z models are all in decent
agreement in the short range on these features with the only
noticeable difference being the weak low that passes E of the
offshore waters this afternoon. All solutions indicate it will
remain fairly weak, though the 00Z NAM is slightly stronger than
the rest of the guidance. The NAM seems a bit overdone, however,
as there is not much upper level support for this feature. As a
result, favoring the weaker 00Z GFS in the short range, so will
start out with it and populate the wind grids with the GFS 10m
winds.

In the medium range, the 00Z GFS/ECMWF agree somewhat well on the
timing of the next cold front that moves SE over the offshore
waters on Sat and Sat night before stalling over the central NT2
offshore waters on Sun. The GFS, however, seems a little strong
with the southerly flow ahead of the front over the cooler shelf
waters in the Gulf of Maine and Georges Bank where SSTs are
currently in the mid to upper 60 degree Fahrenheit range. With
the boundary layer modifying colder over this area, am expecting
a low level inversion and less mixing in this region. Also, the
front is expected to remain weak, so thinking it will probably
not force deeper mixing. As a result, favoring weaker S to SW
flow ahead of the front. The 00Z GFS 10m winds indicate 25 kt in
that area with a few 30 kt barbs, and the model seems a bit too
strong. However, it is in good overall agreement on the timing
with the rest of the 00Z models, so will continue through the
medium range period with the 00Z GFS 10m winds. However, will cap
them at 20 kt which agrees well with the intensity of the more
conservative 00Z ECMWF/UKMET solutions.

Seas...Seas associated with Hurricane Gert are on the very
eastern fringes of the offshore waters this morning with the
highest values up to about 8 ft. The 00Z Wavewatch and ECMWF WAM
are initialized within a foot of each other over the offshore
waters, and agree on the wave heights remaining fairly light with
the weather pattern becoming weaker after Gert moves well east
of the area. There is slight difference on the timing of the seas
building in NT1 with the aforementioned cold front on Sat with
the ECMWF WAM a little east of the Wavewatch. Otherwise, the
models are in good agreement over the forecast period. As a
result, planning on populating wave height grids with the 00Z
Wavewatch as it is well supported by 00Z ECMWF WAM. However, will
cap seas at 7 ft in NT1 on Sat to account for the 00Z GFS winds
being a little overdone.

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


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Figurskey/Kells. Ocean Prediction Center.


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