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000
AGNT40 KWNM 270046
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
846 PM EDT Wed Jul 26 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 low pres
center at 33N78W near Cape Romain, with a warm front extending E
from the low across zone 835, becoming a cold front and extending
NE over zones 935 and 930, before continuing NE over the high
seas area. The analysis also shows a high pres ridge over the
offshore waters N of the fronts. Latest available ascat hi-res
and ascat passes from this morning show 15 to 20 kt winds in NE
flow over Georges Bank and the NE part of the nt2 area, but
otherwise indicate 5 to 15 kt winds over the offshore waters.
Lightning density product data at 2250z shows isolated showers
and tstms over the southern nt2 zones.

Models...The gem is too fast/progressive over the entire forecast
period, and is considered to be an outlyer solution. Otherwise,
the medium range models are in good overall agreement across the
offshore waters for tonight through Fri night, and the ecmwf will
be used for the wind grids over this timeframe because it has
shown the best run to run consistency of any of the models. The
models are showing fairly significant timing differences for Sat
through the rest of the forecast period, but the gfs 10m guidance
looks like a good median model solution and will be used starting
Sat. The gfs 10m winds will be boosted somewhat over the SW nt1
and NW nt2 waters for early Sat into early Sun, in deference to
the stronger gem and ukmet models and also to maintain existing
gale warnings. 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. Used the ecmwf wam for the sea height grids
for tonight through Fri night, since it builds up the seas more
quickly over the inner zones of the nt2 area as a developing low
approaches the mid Atlantic coast. Then went with the wna
wavewatch for Sat through the rest of the forecast period, in
order to be consistent with the preferred gfs 10m solution over
that timeframe. The wna wavewatch guidance will be adjusted a bit
upwards for Sat into Sun, in deference to the more robust ecmwf
wam.

Extratropical Storm Surge Guidance...N/A.

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

Through Friday, the low-level gradient should be fairly slack
with winds well below hazards. Outside of any showers and
thunderstorms, mainly over southern waters into Thursday night
near the stalled, dying front off of the Carolina coast, areas
of more limited visibility should be in the northern waters of
ANZ800, ANZ805, and ANZ900 through Friday. Friday, ahead of the
next system and east of low pressure that will be developing
over the mid-Atlantic, there should be an increased threat of
showers and thunderstorms especially off of the mid-Atlantic and
near the Carolina coasts.

Focus of forecast attention continues to be late Friday night
into Sunday, with low pressure moving off of the mid-Atlantic
coast Saturday and then heading northeast. General model
consensus is good, with differences in the fine details, but the
upshot of this forecast will be to maintain gales at least as
already oriented in time and space. 850mb winds off of the mid-
Atlantic coast easily reach above 35kt in many locations,
particularly across ANZ810 southwest through ANZ815 by early
Saturday morning, and by afternoon across ANZ910 southwest to
ANZ930. GFS and NAM boundary layer winds actually have increased
to 40 to 45kt in the tight gradient northwest of the surface
low, between the low and the land, Saturday morning. Certainly
not confident enough in any storm conditions and currently think
this is a low-end alternative, but the potential is non-zero.
For this forecast, with the slower UKMET providing for gales as
far south as ANZ825, and GFS boundary layer winds noting gales
that far south, plan to introduce that potential for Saturday
there. Not confident enough to add them to ANZ828 at present.
Given the positively-tilted orientation of the upper trough, and
the fact that the low aloft closes for a time moving southeast,
will lean a little slower and keep gales out of late Friday
night, but a reasonable alternative is for the addition of gales
late Friday night in later forecasts.

In addition, despite being a little north of the Gulf Stream,
close enough conditions in the far southeast part of ANZ905 for
gale-force winds exist to add them there as well. After the low
pulls away, some guidance suggests the continued threat for gale-
force conditions into Sunday morning in the immediate vicinity
of the Gulf Stream off of the mid-Atlantic in the eastern
offshores. For now, will limit the gales to Saturday night
temporally, but it is possible these could be needed from ANZ915
and ANZ920 east early Sunday.

This will be especially true if the ECMWF verifies. The ECMWF has
an interesting triple-low orientation as the low emerges off of
the mid-Atlantic coast which at present may be a result of the
model trying to handle deep convection. For now, will lean more
toward a GFS/UKMET blend with the wind forecast, or keep things,
again, fairly similar in their orientation as with the earlier
forecast but with the aforementioned adjustments.

After Sunday, winds should be below hazards. A wave developing
along the front over the southern waters should initiate winds
in the 20s knots with gusts around 30kt off the Carolina coast
Monday. Thanks again to WFO BOX for the good collaboration on
this forecast.

.SEAS...Leaned toward the ECMWF WAM guidance for this forecast
through much of Friday night. As with the winds, focused on the
gale-force winds areas, of course, for highest waves and the
greatest adjustments, with waves in the lower to middle teens
feet in the tight western gradient of the low and in the good
southwesterly flow ahead of it. From 12Z Sunday onward, leaned
toward the Wavewatch model but with a general foot upward
adjustment.

.EXTRATROPICAL STORM SURGE GUIDANCE...A little early to get into
the details with regard to surge, but in spots from the MA coast
southwest toward Delmarva, both the ETSS and ESTOFS would suggest
isolated areas to around a foot.

.WARNINGS...Preliminary.

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

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale Possible Saturday into Saturday night.
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale Possible Saturday into Saturday night.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Possible Saturday into Saturday night.
.ANZ905...The Great South Channel to the Hague Line...
     Gale Possible Saturday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Possible Saturday into Saturday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale Possible Saturday.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Possible Saturday.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Possible Saturday.

$$

.Forecaster Scovil/Figurskey. Ocean Prediction Center.



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