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AGNT40 KWNM 250723

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
323 AM EDT Sun Jun 25 2017

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

The latest NCEP surface analysis indicates a pair of frontal
boundaries across the offshore waters. The infrared satellite
imagery along with the lightning density product indicates an
area of showers and tstms along the srn boundary which is current
over the nrn and central NT2 waters in the vicinity of the Gulf
Stream. The previous weather grids indicated the convective
activity will continue along the boundary over the next 48 hours
as the 00Z GFS indicates the front will stall over the offshore
while weakening. As a result, planning on maintaining mention of
showers and tstms in the next forecast package as the front is
expected to continue triggering the convective activity in the
marginally unstable environment over NT2 and the rest of the 00Z
models agree somewhat well on the timing and intensity of the
weak front. As far as the winds are concerned, current surface
observations indicate up to 20 kt along the SE coast, though data
is fairly sparse. Ascat wind data from the 02Z overpass from late
last night indicated a large area of 20 kt in NT2 with a few
isolated areas with up to 25 kt. The 00Z GFS first sigma winds
are initialized well over the unstable NT2 waters near the front,
and indicates a small increase in wind intensity as a weak low
develops along the front and moves E through the area. The 00Z
ECMWF/UKMET/GEM/NAM agree somewhat well with the GFS, though the
NAM/GEM are slightly stronger. The 00Z models also indicate the
weak front over srn NT1 will pass E of the area today into
tonight while weakening, before another weak front moving into
the nrn waters Sun into Sun night. With the 00Z models in good
agreement over the first 48 hours on all of the aforementioned
features, planning on starting out with the 00Z GFS 10m winds in
stable areas, and using the first sigma GFS winds in unstable
areas to account for the deeper mixed boundary layer.

The 00Z GFS/ECMWF remain in fairly good agreement throughout the
remainder of the forecast period on the overall pattern. The
models all indicate a ridge will build over the W Atlc on Wed
into Thu, before a stronger cold front approaches from the NW on
Fri. The GFS has been a little inconsistent on the timing of the
front, and slowed from the previous few runs. Earlier runs of the
GFS had been a little faster than the ECMWF solution, but the
slower trend of the GFS has caused it to now lag behind the ECMWF
solution. In addition, the GFS 10m winds seem overdone over the
NT1 waters in the SW flow ahead of the front. The model has been
indicating up to 30 kt, while the ECMWF has been showing 25 kt in
the Gulf of Maine. This seems more reasonable considering sea
surface temperatures in the 50 degree Fahrenheit range over NT1
should lower the boundary layer air temperature and create a low
level inversion while limiting vertical mixing. As a result,
planning on switching to the 00Z ECMWF front 00Z Thu onward.

.Seas...Both the 00Z Wavewatch III and ECMWF WAM are initialized
a foot or so low when compared with the latest observations and
altimeter passes which indicated up to 12 ft just SE of the nrn
NT2 waters. Will need to adjust the guidance to account for the
poor initialization. However, the models agree somewhat well
throughout the forecast period, so planning on using a 50/50
blend of the two.

.Extratropical Storm Surge Guidance...N/A.


.NT1 New England Waters...

.NT2 Mid-Atlantic Waters...


.Forecaster Kells. Ocean Prediction Center. is the U.S. government's official web portal to all federal, state and local government web resources and services.