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000
AGNT40 KWNM 180713
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
313 AM EDT Fri Aug 18 2017

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

Ascat wind retrievals from 0200 UTC indicate an anticyclonic wind
circulation over the nrn NT2 offshore waters with winds up to 15
kt. The latest NCEP surface analysis shows a high pres center in
the center over nrn NT2, and also a pair of weak stationary
fronts across the far nrn and far srn offshore waters. The
lightning density product shows that the srn boundary across srn
NT2 is acting as a focus for convection this morning, so will
adjust weather grids to match current conditions. As far as the
winds are concerned, the 00Z GFS/ECMWF are both initialized well
when compared with the current data, and indicate that the front
across srn NT2 will start moving slowly N later today as the nrn
stationary front over the Gulf of Maine also starts lifting N.
In addition, the analysis indicates an inland cold front moving
through the Great Lakes this morning. The 00Z GFS/ECMWF both
indicate the front will approach the coast today before moving
offshore tonight. The models agree well on the timing of the
front, though differ slightly on the intensity of the S to SW
over the cooler waters in NT1 by tonight. The GFS first sigma
winds show up to 30 kt, which is supported by the 00Z NAM/GEM
solutions. However, 30 kt seems a bit high considering the low
level inversion that is expected to set up over the cooler SSTs.
The 10m GFS winds seems a bit more reasonable, and indicate about
25 kt in NT1 ahead of the front. This is supported by the 00Z
ECMWF, while the 00Z UKMET is just slightly weaker. The previous
forecast went up to 25 kt, so planning on going up to that wind
intensity in the next forecast. Also, planning on starting out
with the 00Z GFS 10m winds, and continue with them into Tue.

In the medium range, the 00Z GFS indicates another cold front
will move into the offshore waters late Tue and Tue night in a
fashion similar to the one that is expected tonight. The models
once again indicate up to 25 kt in the S to SW over NT1, so will
again go with that max intensity as there is fair amount of
agreement. However, the GFS is a bit faster than the 00Z ECMWF,
which seems to be about the best compromise when compared with
the 00Z UKMET/GEM which are slightly slower with the front than
the 00Z ECMWF. As a result, planning on switching to the 00Z
ECMWF at 12Z Tue, and will continue with it through the remainder
of the forecast period. Otherwise, not planning on carrying any
warning headlines as winds are expected to remain below 34 kt
through the forecast period.

.SEAS...The 00Z Wavewatch and ECMWF WAM are initialized well and
agree into 12Z Tue. As a result, planning on using a 50/50 blend
of the two from today through Mon night. Will then transition
exclusively to the 00Z ECMWF WAM from 12Z Tue and beyond as it
matches the timing of the preferred 00Z ECMWF in the medium
range.

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

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Kells. Ocean Prediction Center.


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