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000
AGNT40 KWNM 270041
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
841 PM EDT Wed Apr 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 1007 mb low
centered near 40n73w, with an occluded front extending from the
low across the s of new england waters, and an associated cold
front extending s and sw across the nt2 waters. The analysis also
indicates a strong high pres ridge building w towards the
offshore waters and resulting in a strong pres gradient between
the cold front and the ridge. Latest available ascat hi-res and
ascat passes from this morning show 25 to 35 kt winds just e of
the nrn nt2 waters, with a swath of 10 to 20 kt winds near the
middle atlantic and se coasts. Lightning density product data
at 23z shows and area of isolated showers and tstms over the nrn
nt2 waters e of 1000 fathoms.

Models...The medium range models are in good agreement across the
offshore waters for tonight into Fri night, so will be using the
representative gfs 10m/30m solution with the stability smart
tool for this timeframe. The gfs is faster than the other models
with the warm front Mon and Mon night, and is also stronger than
most of the models with the s to sw flow ahead of the cold front
for late Mon and Mon night. Will continue to use the ecmwf for
late Fri night through the rest of the forecast period, since it
has very good support from the ukmet and the lastest wpc medium
range forecaster guidance. Am not planning to make any
significant changes to the current forecast trend.

Seas...The wna wavewatch and ecmwf wam likely initialized a foot
or so too low in the strong southerly flow in the ern part of the
nt2 waters, so adjusted the seas upward accordingly. The models
are in good agreement for tonight through Sun night, with only
minor differences noted, so a 50/50 blend of the wavewatch/wam
will be used for the sea height grids during this timeframe to
smooth out the differences. The ecmwf wam will be used
exclusively for mon through the rest of the forecast period,
since the ecmwf is the preferred model for this timeframe.

Extratropical Storm Surge Guidance...Observed water levels are
showing that the 18Z ETSS is performing better than the 18Z
ESTOFS along the New England coast with the SE winds in the nt1
waters.

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

The latest NCEP surface analysis indicates strong low pres over
far NW NT2 with a cold front across the entire offshore waters.
The GOES visible satellite imagery along with the lightning
density product shows an area of showers and tstms along the
frontal boundary, and the Ascat wind retrievals from between 1300
UTC and 1530 UTC this morning indicated gale force winds up to
40 kt in the southerly flow ahead of the front with the highest
winds over the nrn outer NT2 waters over the Gulf Stream.
Current surface reports indicate winds to 40 kt in the
vicinity, and the 12Z GFS first sigma winds are initialized well
when compared with the data. The 12Z UKMET/ECMWF are slightly
lower, while the 12Z NAM/GEM are a bit too strong and indicate
widespread gales ahead of the front including over stable areas
north of the Gulf Stream where shallower mixing would be
expected. The models all agree, however, that the system will
weaken tonight as the low moves inland over the New England
coast. The previous forecast had gales over far NE NT2 for
earlier today, and at this time expecting the higher winds to
have shifted east of the offshore waters. The current surface
observations are indicative of this, so planning on dropping gale
warnings for the NT2 waters associated with this system by the
start of the next forecast. Will otherwise use the 12Z GFS 10m
winds for this system except in unstable regions where the first
sigma winds shall be used.

Otherwise, the 12Z models are in reasonably good agreement
throughout the remainder of the forecast period and indicate a
pair of weaker cold fronts will move through the area Fri and
Sat. The 12Z GFS and ECMWF have only minor timing differences on
these features, and the 12Z GFS is a tad slower with the second
front. The 12Z timing of the UKMET/GEM solutions support the 12Z
ECMWF marginally better, although all solutions indicate winds
will remain below gale force with both fronts. As a result,
planning on switching the the 12Z ECMWF by 06Z Sat.

By late Mon, the 12Z models all indicate a strong cold front will
approach the area and increase the winds in the southerly flow
ahead of the front. The 12Z GEM/GFS both indicate gales ahead of
the front, though the GEM looks a bit overdone north of the Gulf
Stream. However, the 12Z ECMWF/UKMET winds are both weaker and
indicate only 30 kt by late Mon night. Also, the GFS and ECMWF
both indicate a strong low level jet will set up just ahead of
the front with 925 mb winds to 40 kt. However, confidence is
just below average with the gales as a result of the longer range
of the forecast. Would also prefer to see a bit more support from
the ECMWF and UKMET, so will hold off on the gales for Mon and
cap winds at 30 kt. Will otherwise remain with the 12Z ECMWF
winds for the remainder of the forecast period.

Seas...The 12Z NWW3 and ECMWF WAM are initialized a tad low in
the strong southerly flow ahead of the front currently over the
offshore waters, so will need to adjust seas accordingly.
Otherwise the models are in good agreement into Sun before the
Wavewatch looks overdone with the system on Mon. As a result,
planning on using a 50/50 blend of the two models before using
the 00Z ECMWF WAM exclusively from Mon onward.

.EXTRATROPICAL STORM SURGE GUIDANCE...Observed water levels
indicate the 12Z ETSS is performing slightly better that the 12Z
ESTOFS along the New England coast with the SE winds in the Gulf
of Maine.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Scovil/Kells. Ocean Prediction Center.


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