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000
AGNT40 KWNM 221944
MIMATN

.Marine Weather Discussion for N Atlantic Ocean
.NWS Ocean Prediction Center Washington DC
345 PM EDT TUE 22 AUG 2017

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

This mornings Ascat offshore overpasses mostly missed the NT2
waters, but the 14Z pass has a few returns to 20 kt about 30 nm
east of Cape Cod indicating the southwest winds already started
to increase. The latest buoy observations across the coastal and
the inner offshore waters are only as high as 15 kt. The 12Z
models are consistent with the timing of the approaching cold
front, which should move offshore Wed/Wed night, and all
indicate winds should increase to about 25 kt this evening and
tonight across the New England waters and likely as far south as
Hudson Canyon. Versus its previous run the 12Z GFS was slightly
stronger with these winds, up to 30 kt across the Gulf of Maine
by 00Z. For the wind grids through Wed, will incorporate the
slightly higher 12Z GFS into the existing grids, and introduce
some limited 30 kt winds across the Gulf of Maine. The 12Z
GFS/UKMET/ ECMWF are in good agreement that the slow moving cold
front will reach the southern NT2 waters Thu night/Fri while the
prefrontal gradient gradually weakens. By late Friday and
through the weekend once the cold front stalls across the
southern NT2 waters, and as high pressure builds off the New
England and northern mid Atlantic coasts, the models are
consistent in forecasting an increasing east to northeast
gradient poleward of the stationary boundary.

To varying degrees, the models all indicate low pressure will
develop off the northeast Florida coast over the weekend,
potentially becoming a tropical cyclone. Versus their respective
previous runs, both the 12Z ECMWF and 12Z GFS trended slower in
moving the low northward toward the southern NT2 zones late
Sun/Sun night. The 12Z UKMET also supports this timing. The 12Z
GFS/UKMET/ECMWF are in well above average agreement with the
track/timing of the developing low. However, the 12Z GFS is
significantly deeper than the ECMWF/UKMET by days 6/7. The
latest NHC outlook indicates 30 percent or a low chance of
tropical or subtropical formation with this system over the next
5 days. With high pressure anchored across New England, we do
have somewhat higher confidence that gales across the
southern/central NT2 waters could develop early next week even
if low is ultimately baroclinic.

.Seas...The 9 to 10 second southeast swell will continue to
impact the mid Atlantic coast over the short term. The 12Z
Wavewatch III seems to have a slightly better handle than the
12Z ECMWF WAM with this swell. The models at least appear
equally well initialized with the west Atlantic wave heights
this afternoon. These models are in excellent agreement through
the end of the week. By the weekend, across the southern NT2
zones, OPC preference is then with the higher 12Z ECMWF WAM. For
the wave height grids through Fri night, used an even blend of
the 12Z ECMWF WAM and 12Z WW3 before transitioning to a 75/25
12Z ECMWF WAM/12Z WW3 blend thereafter.

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

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Clark. Ocean Prediction Center.


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