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AGNT40 KWNM 240035
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
835 PM EDT Sun Jul 23 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 over the far NE part of the nt2 area, with a cold front
extending roughly E from the low across the northern nt2 waters
and into the Delaware coast. The analysis also indicates a high
pres ridge over the central and southern nt2 waters, with a
weaker ridge across the nt1 area. Latest available ascat hi-res
and ascat passes from this morning show 20 to 25 kt winds in SW
flow over the central and southern nt2 waters, with 10 to 15 kt
winds indicated elsewhere over the offshore waters. Lightning
density product data at 2250z shows a line of scattered showers
and tstms extending across the northern and central nt2 waters,
mainly near the cold front.

Models...The medium range models are in very good overall
agreement across the offshore waters for tonight into Mon Night,
so the representative gfs 10m solution will be used for the wind
grids over this timeframe. The models are showing a lack of
agreement regarding the strength of NE flow over the nt1 waters
for the late Mon Night through Tue night timeframe, so as a
comprimise solution will go with a 50/50 blend of the weaker
ecmwf and stronger gfs. For Wed through Fri Night, the gfs
appears to be somewhat stronger than the other models with waves
moving along a warm front over the central and northern nt2
waters. Also the gfs is showing an extraneous low over the
eastern nt2 waters that is not being reflected in the other
medium range models. So the more representative ecmwf will be
used exclusively for Wed through the rest of the forecast period.
Am not planning on making any significant changes to the current
forecast trend.

Seas...The wna wavewatch and ecmwf wam both initialized well over
the offshore waters. In order to be consistent with the preferred
gfs model, the wna wavewatch will be used for the sea height
grids for tonight into Mon Night. Then will go with a 50/50 blend
of the wna wavewatch/ecmwf wam for late Mon Night through Tue
Night, since a comprimise solution will be used for the wind
grids. The ecmwf wam will be used exclusively for Wed through Fri
Night, in order to be consistent with the preferred ecmwf model.

Extratropical Storm Surge Guidance...N/A.

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

Over the short term, the new 12Z models in general agree that the
stationary front now across the far northern NT2 waters will
waffle northward into the southern most NT1 waters by late
tonight in response to a surface low on the front moving off the
New Jersey coast and then tracking ENE across the southern NT1
waters Sun. In regards to this frontal low, the 12Z models have
converged towards similar tracks while the 12Z GFS/UKMET/ECMWF
have trended somewhat stronger than their previous respective
runs. With the 12Z GEFS Mean and 12Z NAM/ECMWF supporting this
stronger trend, will accept the representative 12Z GFS solution
and will populate our forecast winds grids with 12Z GFS 10m winds
for tonight through Mon night, time shifted 3 hours slower in
deference to the 12Z UKMET/ECMWF timing and then with some minor
additional edits in deference mainly to the 12Z ECMWF. So the
biggest impact of these winds will be to bump up the previously
forecasted winds (from up to 25 to 30 kt) in the ENE gradient
immediately N of the low.

Then later in the short term, as all of the 12Z models forecast
the surface low to eject off the E late Mon night into Tue night
they forecast the front to push slowly S as a cold front across
the N and central NT2 waters as high pressure slowly builds into
New England overall supporting a weak NEly gradient (max winds
generally in the 15-20 kt range) N of the front. Would favor a
compromise 12Z GFS/ECMWF solution for this NE gradient. So will
populate with a 50/50 blend of the 12Z GFS 10m and 12Z ECMWF
boundary layer (BL) winds for Tue and Tue night.

In the long range, the 12Z global models agree that the front
will become stationary across the S central NT2 waters Wed/Wed
night with its associated gradients weakening further, Versus the
12Z GFS, would favor the somewhat more suppressed 12Z
GEM/UKMET/ECMWF frontal position Wed/Wed night. Then with some
timing differences the 12Z global models all forecast the front
to return NE as a weak warm front Thu/Thu night. With a lack of
support from the 12Z GEFS, suspect the 12Z GFS may be having some
convective feedback problems with the fairly strong frontal
waves it forecasts to develop along its warm front. Therefore,
would favor the more consistent and weaker 12Z ECMWF solution for
Wed through Thu night. Then Fri/Fri night, the disparity
increases between the 12Z global models in regards to the next
cold front moving offshore. The 12Z GEM/UKMET solutions both look
too amplified with their associated attendant surface lows
looking too strong, while the 12Z GFS continues to forecast what
looks like spurious lows. So since it remains more consistent and
also in better agreement with the latest WPC medium range
guidance, will continue to populate with 12Z ECMWF BL winds on
Fri/Fri night, but due to the model disparity will cap these
winds at 20-25 kt.

.SEAS...Overall, the 12Z Wavewatch and 12Z ECMWF WAM have both
initialized the current seas equally well. Since the 12Z GFS
will be favored, will populate our forecast wave grids with the
12Z Wavewatch III seas for tonight through Mon night, with the
same 3 hour time shift as was done for the winds. Then since a
compromise 12Z GFS/ECMWF solution will be used, will populate
with a 50/50 blend of the two wave models on Tue/Tue night. Then
since the 12Z ECMWF solution will become favored, will transition
to populating with all 12Z ECMWF WAM seas on Wed through Fri
night.

.EXTRATROPICAL STORM SURGE GUIDANCE...N/A.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Scovil/Vukits. Ocean Prediction Center.



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