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AGNT40 KWNM 210048
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
848 PM EDT Fri Oct 20 2017

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

Current Conditions...The 18z ncep surface analysis shows a cold
front extending SW into the far NE part of the nt2 area, with a
low pres trough extending NE to SW across the nt1 area.
Otherwise, the surface analysis shows a high pres ridge over the
remaining nt2 waters. Latest available ascat hi-res and ascat
passes from this morning show 15 to 20 kt winds over the nt1 and
northern nt2 waters, and 10 to 15 kt winds in the central and
southern nt2 waters.

Models/Forecast...The medium range models are in good overall
agreement across the offshore waters for tonight through Mon
night, so the representative gfs 10m/30m solution with the
stability smart tool will be used for the wind grids during this
timeframe. A strong cold front is expected to approach the east
coast later Tue, then move E across the offshore waters Tue night
through Wed night. A wave is expected to form along the front on
Wed or Wed night, either over the northern or central nt2
waters. The medium range models are not in good agreement
regarding the timing and strength of the cold front, and are
forming the frontal wave at different times and in different
locations. Even though some of the models indicate gales ahead
of the cold front for later Tue through Wed night, there is not
enough of a model consensus for us to put up gale warnings for
that timeframe and so we will be using the ecmwf starting Tue,
which keeps the winds sub-gale. The ecmwf will be time shifted 3
hours faster in deference to the faster gfs/ukmet solutions.

Seas...The wna wavewatch and ecmwf both initialized within a
foot or two of observations and recent altimeter passes in the
offshore waters. Both of the models are in good agreement for
tonight through Mon night, with only a few minor differences
noted in the nt2 waters on Mon and Mon night. A 50/50 blend of
the wavewatch/ecmwf wam will be used for the wave height grids
through Mon. Then will be using the ecmwf wam exclusively for Mon
night through the rest of the forecast period, in order to be
consistent with the preferred ecmwf wind grids. The ecmwf wam
will be time shifted 3 hours faster, as was done for the ecmwf.

Extratropical Storm Surge Guidance...N/A.

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

The latest NCEP surface analysis shows a high pres ridge across
the srn and central NT2 offshore waters, and a cold front across
far NE NT2. Ascat overpasses from 15Z this morning indicated
winds up to 20 kt over the offshore waters with the highest winds
over NT1 in NW flow behind the cold front. Current surface
reports in the W Atlantic show about the same wind intensities
over the W Atlantic, and 12Z GFS 10m winds are initialized well
when compared with the data. The GFS indicates the front will
move off to the E into Sun as the ridge expands over the entire
area encompassed by the offshore waters, and the 12Z
ECMWF/UKMET/GEM/NAM all agree well on the overall timing and
intensity of the synoptic-level features. The 12Z models all
indicate the winds will remain light in the short range, so not
expecting warnings. Planning on starting out with the 12Z GFS 10m
winds as a result of the good model agreement, but will use GFS
first sigma winds in unstable areas to account for the deeper
mixed boundary layer in those regions.

Otherwise, the 12Z GFS is in good overall agreement with the 12Z
through Mon before the ridge moves E of the region Tue. The
models then indicate a stronger cold front will move offshore
Tue, and move slowly E across the offshore waters through Wed
night. The 12Z ECMWF is a tad slower than the 12Z GFS solution,
but much slower than the 12Z UKMET. The 12Z GEM is stronger with
a wave that develops along the front. However, the front is
faster N of the low center. In addition, the 12Z ECMWF is weaker
with the winds, while the 12Z GFS is showing that gales will
develop ahead of the front on Tue and continue through Wed. The
12Z UKMET/GEM agree on the gales, but the timing and location
varies with each model solution. As a result of the model
differences and long range of the forecast, confidence is low on
the gales for Tue through Wed. Consequently, will cap winds at
30 kt. For the wind grids, am planning on switching to the 12Z
ECMWF on 12Z Mon and continue with it through the remainder of
the forecast period. However, will time shift it 3 hours faster
from 12Z Tue onward as a compromise to the timing differences
between the 12Z GFS and ECMWF.

Seas: The 12Z Wavewatch and ECMWF WAM are both initialized
within a foot or two of observations and recent altimeter passes
in the W Atlc. Both models agree reasonably well through the
short range, then start differing on the wave heights from 12Z
Mon onward as a result of the aforementioned intensity
differences between the 12Z GFS and ECMWF. For the short range,
am planning on using a 50/50 blend of the models. Will then
switch to the 12Z ECMWF WAM exclusively from 12Z Mon onward to
match the winds of the preferred 12Z ECMWF. Will also time shift
wave height grids 3 hours faster at 12Z Tue to match the time
shift used with the wind grids.

Extratropical Storm Surge Guidance: n/a.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Scovil/Kells. Ocean Prediction Center.


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