439
AGNT40 KWNM 250224
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1024 PM EDT Wed Apr 24 2019

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

The latest NCEP surface analysis shows a cold front across the
nrn outer NT2 offshore waters this evening extending to the SW to
near Cape Hatteras. A very recent Ascat pass from Metop-B
indicates 25 to 30 ahead of the front on the edges of the
offshore waters with the highest winds across the Gulf Stream.
The 12Z/18Z model guidance is initialized low when compared with
the data, so will need to boost winds for early tonight since the
previous wind grids which were based on the 12Z guidance were
also slightly low. The models indicate that the front will pass E
of the nrn waters early tonight while taking the strongest winds
with it, so will make only minimal short term adjustments to
account for the current data. In addition, the infrared satellite
imagery and the lightning density product show showers and tstms
along the front across the Gulf Stream in nrn NT2. The models
indicate that the stronger tstms will pass E of the area early
tonight, so will adjust weather grids accordingly. However, while
the convection is still present, winds and seas will be locally
higher near it.

The models then show another cold front approaching the area on
Fri, before it moves across the offshore waters Fri night into
Sat. The GFS/ECMWF both indicate a strong low level jet setting
up ahead of the front, with 925 mb winds increasing to 60 kt Fri
night over NT2 in the vicinity of the Gulf Stream. The models are
also showing unstable vertical lapse rates, so it is no surprise
that the GFS and ECMWF both indicate gales developing ahead of
the front. The models then indicate strong cold air advection
behind the front, with a second area of gales over nrn NT2. This
again seems reasonable since the system seems strong, so
confidence in previous gale warnings is above average at this
time. As a result, will only be making slight adjustments to
warning headlines. The previous forecast also used a blend of
the GFS and ECMWF as a compromise to the fairly small timing and
intensity differences between the two models. This still seems
reasonable, so planning on staying along those lines into Sun.
Beyond that, the models indicate another developing low will pass
across the area on Sun, and the GFS is again much stronger than
the rest of the guidance. Since there is still quite a bit of
uncertainty as a result of the intensity differences, am planning
on staying near continuity and keeping winds below warning
criteria through the remainder of the forecast period.

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PREVIOUS DISCUSSION...

GOES-E visible imagery indicates weak low pressure centered
across the Gulf of Maine this afternoon, with a cold front
draped SW across the northern waters to the Delmarva. Lightning
data suggests convection beginning to fire along the boundary,
with an earlier ASCAT overpasses returning highest winds to 25 kt
directly atop the Gulf Stream. There were a few sparse flags to
30 kt, which matched the ongoing wind grids fairly well. As this
low and front clears E of the offshore waters tonight and Thu,
will use a blend between the NAM nest and ARW for the
representative wind grids, with further edits to ensure small
localized pockets of 30 kt remain across ANZ910 and ANZ920.

Then for the Fri into Sat system, in surprising contrast to its
previous run, the 12Z ECMWF now phases a pair of upper troughs
across the Upper Mississippi and Ohio Valleys, developing a
significant surface low and cold front across the W Atlantic.
This southern stream phasing trend bucks previous ensemble
guidance, however now matches very well the ongoing deterministic
runs first seen in the 00Z GFS last night, and supported from
the 12Z GFS, GEM, and UKMET. In fact the guidance is similar
enough in timing and strength that confidence in the forecast is
above average, and for the upcoming grids, plan to use a blend
with the GFS (however using the first sigma layer winds in
unstable areas) and the ECMWF, with widespread gales now in the
forecast across most of the southern zones ahead of the cold
front late Fri into Fri night, spreading quickly northeast across
Baltimore Canyon, Georges Bank, and into the Gulf of Maine early
Sat morning. Front clears E of the area quickly, with a second
round of gales forecast across western Baltimore Canyon in the
W-NW cold air advection Sat.

And toward the end of the medium range, again the 12Z ECMWF
appears to be playing catch-up with the other global models, and
now indicates another low pressure and cold front moving
offshore Long Island Sun night, then passing through the northern
waters while pulling another cold front through the waters. The
previous two ECMWF cycles had a 1026 mb high in this vicinity.
Aside from being much weaker, the timing and location is in good
agreement with the 12Z GFS/UKMET/GEM. And with the guidance so
well clustered, here again will use a blend between the 12Z
GFS/ECMWF, which has the advantage of limiting winds to 32 kt or
less and below gale hazard for now.

Seas: Flat seas generally 8 ft or less expected into Fri midday,
and will base the seas on the WW3 in the near term. Then for the
remainder of the forecast, in similar manner as the wind grids,
plan to run a 1:1 blend between WW3 and ECMWF WAM for the
forecast seas.

Extratropical Storm Surge Guidance: Both the 12Z ESTOFS and ETSS
indicate positive surge values from .5 to 1 ft from Delaware Bay
Fri, spreading NE along the New England coast Fri night into Sat
as a strong low pressure and cold front move through the area.
With global atmospheric models trending toward and some stronger
than the GFS, these values seem reasonable if not slightly too
low.

.WARNINGS...Preliminary.

.NT1 New England Waters...
None.

.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
Gale Possible Friday night into Saturday.
.ANZ920...Baltimore Canyon to the Great South Channel...
Gale Possible Friday night into Saturday.
.ANZ905...The Great South Channel to the Hague Line...
Gale Possible Friday night into Saturday.
.ANZ910...East of the Great South Channel and south of 39N...
Gale Possible Friday night into Saturday.
.ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light...
Gale Possible Friday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
Gale Possible Friday night.
.ANZ830...Inner Waters from Currituck Beach Light to Cape Hatteras...
Gale Possible Friday night.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
Gale Possible Friday night.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
Gale Possible Friday night.

$$

.Forecaster Kells/Collins. Ocean Prediction Center.