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AGNT40 KWNM 180210
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1010 PM EDT Tue Oct 17 2017

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

High pressure will persist to the west of the forecast waters and
a relaxed pressure gradient across the region will keep winds
below gale force threshold. Geocolor satellite images show
mostly clear skies inland west of the forecast waters but mostly
cloudy areas over the southern waters with some convection
confined to the far southern and southeastern waters. This is in
line with inland high pressure and a surface cold front across
the southeastern and far southern waters. Winds are mainly from
the northeast with weaker winds over the north areas and they
increase southward with maximum winds to 30 kt over the southern
waters in the vicinity of the front. NCEP map at 00Z has inland
high pressure over the southern states that extends its ridge
northeast across the mid Atlantic states into New England and
that has northeast winds to the east which is mainly the forecast
waters. Another high pressure 1030 MB to the east of the
southern waters generates southwest winds to the east of the
region. Low pressure 1003 MB northeast of the waters has its cold
front that stretches southwest into another low pressure 1012 MB
240 NM east of the Baltimore Canyon, front continues southwest
between the two highs, just east of the waters then into the
southern waters. Last scatterometer pass was this afternoon and
is explained in the previous discussion section below.

Seas are relatively smaller to the north of the Gulf stream with
6 ft along the gulf stream and just 2 ft over the far northwest
waters. Otherwise, seas mostly range between 6 and 9 ft except
the southeastern zones where seas are over 9 ft with a peak at 13
ft. Jason 3 pass about 1908Z returned peak significant wave
height of 13 ft over the southeastern waters. NWW3 generally fits
very well the observed seas pattern but just underdone by a foot
on the peak value. ECMWFWAVE matches the peak value but not so
much in line with observations to the north as it extends its 6
ft line farther north of the gulf stream. As such, an equal blend
of the two wave models is a better compromise and so will go
ahead with a blend.

Models GFS/CMC/UKMETHR/ECMWFHR/NOGAPS/NAM/GFSMEAN have
initialized the main synoptic features fairly well with just
minor differences on the actual position for the central pressure
value of the inland high and UKMETHR is the only model that does
not have a closed low just east of the Baltimore Canyon but a
deep trough. In the short term, models have a general agreement
on keeping high pressure just west of the region and that will
block strong frontal boundaries form reaching the waters and so
the pressure gradient will remain relaxed over the waters through
the forecast period. The models have also indicated instability
to persist over the southeastern waters as indicated by the
persistent negative lifted index and that will keep winds
relatively higher to the southeast of the Gulf stream with
maximum winds over the far southern waters. Will continue with
the previous model choice of GFS.

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

The latest NCEP surface analysis shows a cold front just SE of
the srn NT2 offshore waters this afternoon, and a high pres
ridge extending NE to SW along the New England through Mid Atlc
coast. The ridge is interacting with the frontal boundary, and
current wind observations off the SE coast indicate NE flow up
to 25 kt. However, the Ascat overpasses from 1530 UTC indicated
a large area up to 30 kt in the NE flow counter to the Gulf
Stream. The 12Z GFS indicates unstable lapse rates in the srn
offshore waters which is allowing stronger winds aloft to mix
down. The 12Z GFS also indicates that the front will move off to
the E tonight and Wed as the ridge continues to build over the
area. The previous forecast had gales to 18Z this afternoon, so
will allow the warning to expire as the 12Z models all indicate
the front will continue moving off to the SE and allowing the
pres gradient to weaken. The 12Z models all agree well on the
timing on the front and ridge, and also on the timing of another
weak cold front passing through NT1 Wed. As a result, planning on
starting out with the 12Z GFS 10m winds since they are
initialized well and are supported by the rest of the 12Z
guidance. However, will use first sigma winds in unstable areas
to reflect the deeper mixed layer expected in these regions.

In the medium range, the 12Z GFS/ECMWF agree fairly well on the
overall pattern, and indicate high pres will continue over the W
Atlc through the end of the forecast period. The models also
agree well on the timing of another cold front passing through
the NT1 waters Thu night into Fri. The 12Z GFS/ECMWF 10m winds
both indicate up to 25 kt in the SW flow ahead of the front over
the Gulf of Maine. This seems reasonable considering the cooler
SSTs in that region, so planning on staying with the 12Z GFS as a
result of the good model support. In addition, the models
indicate yet another cold front will approach from the NW on Sun,
though all 12Z solutions keep it to the W and NW of the offshore
waters by the end of the forecast period on Sun night. However,
the 12Z GFS is slightly stronger with the SW flow ahead of this
front on Sun night, and indicates a larger area with 25 kt than
the 12Z ECMWF. As a result, planning on continuing through the
remainder of the forecast period with the 12Z GFS, but will tone
done the winds slightly in deference to the weaker 12Z ECMWF
solution.

Seas...The 12Z Wavewatch and ECMWF WAM1 to 2 ft low off the SE
coast in the strong NE flow. However, the models are in good
agreement over the forecast period, so planning on using a 50/50
blend of the two solutions as a compromise to the minor
differences between them. Also, planning on adjusting seas higher
initially to account for the poor initialization off the SE
coast.

.Extratropical Storm Surge Guidance...The 12Z ESTOFS and ETSS
indicate a 1+ ft surge along the SE coast in the strong NE flow
offshore. The models both seem reasonable.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Musonda/Kells. Ocean Prediction Center.



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