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AGNT40 KWNM 241931
MIMATN

.Marine Weather Discussion for N Atlantic Ocean
.NWS Ocean Prediction Center Washington DC
330 PM EDT SAT 24 JUN 2017

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

This mornings 1420Z and 15Z Ascat overpasses indicated
widespread 20 to 30 kt winds east of the offshore cold front.
The 30 kt winds were limited to the NT2 waters. These Ascat
winds were slightly higher than the GFS/ECMWF/UKMET 10m or
surface winds, but were in line with the previous OPC wind
grids, which were based on a blend of the GFS first sigma level
and 10m winds. At 18Z the cold front extended southwest off the
Maine coast, near Cape Cod and across the northwest portion of
the Mid Atlantic offshore waters. The 12Z models are consistent
that the front will move southeast across Georges Bank this
evening and continue pressing south across the NT2 waters
tonight before stalling early Sunday. The 12Z guidance is also
in good agreement that the moderately strong southwest gradient
will continue to shift southeast ahead of the front tonight and
then persist across the central NT2 waters Sunday into Sunday
night. To varying degrees the models all forecast a developing
low to move northeast along the front across the northern NT2
waters Sunday into Sunday night. The 12Z ECMWF/GFS/CMC are
slightly faster than the 12Z UKMET/NAM. But the models are
generally consistent that the associated winds across the south
quadrant of the low should reach 25 kt within the northern outer
NT2 zones. The 12Z GFS/ECMWF/UKMET/NAM then share similar timing
with another, albeit a weaker cold front moving off the New
England coast Sunday night. This weak feature should wash out
Monday/Monday night as models suggest it merges with the more
southern stationary boundary. Through Sunday, will populate wind
grids with the same blend which matched up so well with this
mornings Ascat data. That is, a 67/33 12Z GFS 10m/12Z GFS first
sigma level wind blend.

By late Tuesday and Tuesday night, the 12Z models are consistent
in forecasting a sharper upper level trough will approach the
New England and northern Mid Atlantic coast and then move east
through the offshore waters. And there is decent model consensus
with the associated development of a weak surface low, which as
it tracks northeast across the central NT2 waters and south/
southeast of Georges Bank, will ultimately pull the persistent
front east of the offshore waters Wednesday. We prefer the
slightly faster GFS which is well supported by the 12Z UKMET.
The guidance is then still consistent through midweek in
forecasting a high pressure ridge to reach the coast Wednesday,
move offshore Wednesday night, then shift east of the offshore
waters Thursday. By Thursday night, the 12Z GFS appears overdone
with the return flow across the New England waters, with winds
up to 30 kt. For wind grids Sunday night through Thursday,
populated with the tool which places the 12Z GFS 10m winds
across stable areas, and the 12Z GFS first sigma level winds
over unstable areas as defined by the GFS low level lapse rates.
Given the predominant warm air advection through the period, the
grids are mostly 12Z GFS 10m winds. To tone down the GFS winds
Thursday night, used a 50/50 blend of the 12Z GFS and 12Z ECMWF
at that time.

.Seas...Both the 12Z Wavewatch III and 12Z ECMWF WAM are within
a foot or so of the latest west Atlantic ship/buoy observations.
Coincident with the areas of the strongest southerly winds
tonight, the 12Z ECMWF WAM is up to 3 ft higher than the 12Z
Wavewatch III. Thereafter, these models are generally within a
foot of each other through the forecast period. Populated the
wave height grids with an even blend of the models through
Thursday night.

.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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