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000
AGNT40 KWNM 240235
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
935 PM EST Thu Nov 23 2017

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

Geocolor satellite images show cold clouds over most of the
region with lightning density map showing TSTMS moving northeast
across Florida and may spread into the southern waters. Satellite
images also show fewer clouds across New England and the mid
Atlantic states which is in line with the observed surface
features. At 00Z the NCEP weather map has a cold front to the
east of the region that turns into a nearly stationary front as
it enters and lies across the far southern waters. Inland high
pressure 1020 MB over the Mid Atlantic states has ridges into
the northern and central waters. Pressure gradient is very
relaxed across most of the region except near the frontal
boundary over the waters. Winds are mainly from the west over
the north waters and from the north and northeast over the rest
of the region. Higher winds with a maximum of 35 kt are confined
to the southern waters in the vicinity of the stationary front.

Seas are generally small across the region with a peak at 8 ft
over the Baltimore Canyon. Seas range between 3 and 6 ft over the
western half north of 34N while they range between 6 and 7 ft
over the rest of the area. NWW3 and ECMWFWAVE models agree well
with the observed 00Z seas pattern. The wave models agree in the
short term on keeping seas less than 8 ft across the region but
will be relatively higher over the northern waters. Will not
deviate from the previous wave model choices and so will use a
50/50 blend.

In the upper levels, an upper level ridge extends northeast from
the southwestern states while an upper level trough lies just
west of the forecast waters across the great lakes into the
southeastern states. There is some energy embedded in the upper
level trough that will pass east over the waters in the short
term. Another upper level trough will approach the waters and
bring some energy especially to the northern waters Sunday.

Models GFS/CMC/NOGAPS/UKMETHR/ECMWFHR/NAM have initialized the
00Z surface observations fairly well especially over the forecast
waters. In the short term, models generally agree on low
pressure moving northeast from the Gulf of Mexico into the
southeastern portion of the southern waters but differences are
on the actual track of the low center. The models have difficulty
in locating the center as low pressure moves into the waters.
More models suggest a complex low over the southeastern waters.
The position of the low center will help to determine if winds
will reach gale force over the waters. For this issuance, I plan
to just retain the previous models choice which is using GFS
initially then switch to ECMWF.


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

The current GOES visible satellite imagery indicates a strong
low pres system centered NE of the area over Newfoundland, and
cold air cumulus streaming across the offshore waters in the cold
advection behind the associated cold front now E of the area.
Ascat winds from 15Z this morning indicated up to 20 kt behind
the front over NT1, and current surface observations show up to
15 kt. The 12Z GFS is initialized well when compared with the
data, and indicate the winds will decrease slightly over the nrn
offshore waters as a high pres ridge builds offshore. In
addition, the satellite indicates low pres over the far NE Gulf
of Mexico with a warm front over extending NE from the low into
the srn NT2 offshore waters. The 12Z GFS indicates the low pres
will develop over the srn waters in the vicinity of the Gulf
Stream tonight, and increase the winds near the low center. The
GFS has been showing a small area of gales in srn NT2 for a brief
period tonight into Fri over the past few runs. The ECMWF/UKMET
had been weaker than the GFS in previous runs, but the 12Z runs
of both models trended stronger with the winds. The 12Z ECMWF now
is in good agreement with the GFS, while the 12Z UKMET is just
slightly weaker. The ECMWF had also been a little slower with
the track of the low, but the 12Z run trended a little faster and
is now in decent agreement with the timing of the GFS. As a
result of the improved agreement, confidence in gales developing
is just above average since the models are showing NE flow over
the Gulf Stream which should act to deepen the mixed layer. For
the next forecast, planning on starting out with the 12Z GFS 10m
winds in stable areas and first sigma winds in unstable areas.
Will also add gale warnings to several srn NT2 zones.

On Sun, the 12Z GFS/ECMWF indicate that a region with somewhat
strong cold advection will set up over nrn NT2 and NT1 as the
aforementioned low moves off to the NE. The GFS has been hinting
at gales, but it becomes an outlier solution at that point as the
12Z ECMWF/GEM indicate the stronger cold advection will pass E of
the area faster than indicated by the GFS. The 12Z ECMWF has been
consistent on the timing of this feature, and was preferred in
the previous forecast. At this time am preferring the timing of
the 12Z ECMWF as a result of the marginally better support. Also,
confidence with gales is low since there only the 12Z GFS
indicates winds that strong, so planning on switching to the 12Z
ECMWF by 00Z Sun and capping the winds at 30 in the cold
advection.

In the medium range, the 12Z models all agree that another cold
front will approach from the NW Tue before moving offshore Tue
night. The 12Z GFS is faster than the rest of the 12Z models, and
is also stronger with the winds in the SW flow ahead of the front
over the cold shelf waters in NT1. The GFS indicates gales in the
SW flow, and seems overdone. The 12Z ECMWF seems a bit more
representative of the 12Z guidance with the timing and intensity,
so planning on staying with it through the medium range period.
Also, planning on keeping winds below gale force with this system
as confidence is low from the poor model support.

Seas...The 12Z ECMWF WAM and Wavewatch are initialized well in
the offshore waters, and are in good agreement over the short
range. Will start out with a 50/50 blend, but will adjust higher
with the gales developing over srn NT2 tonight into Fri as the
models are normally too conservative in this type of situation.
Will then transition to the 12Z ECMWF WAM by 00Z Sun to match
the preferred winds of the 12Z ECMWF.

.EXTRATROPICAL STORM SURGE GUIDANCE...n/a.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale tonight into Friday.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale tonight.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale tonight into Friday.

$$

.Forecaster Musonda/Kells. Ocean Prediction Center.


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