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AGNT40 KWNM 190104
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
904 PM 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.

The infrared satellite imagery and lightning density product
show a squall line with showers and tstms approaching the Mid
Atlc coast, with the nrn part of the line moving off the coast of
New Jersey at this hour. will modify previous weather grids to
match up with the current data. Otherwise, the latest NCEP
surface analysis indicates a cold front inland to the W of the
squall line, and the satellite imagery indicates it is moving E
toward the coast. Current surface observations in the offshore
waters already indicates winds up to 25 kt in a few isolated
locations in the SW flow ahead of the approaching boundary. The
128Z GFS 10m winds are well initialized when compared with the
current data, and the model indicates that the front will move
offshore later tonight while pushing the moderately strong SW
flow to the E over the Gulf of Maine and Georges Bank. The
12Z/18Z GFS and 12Z ECMWF/UKMET agree fairly well on the wind
intensity, though the 12Z GEM/NAM are much stronger and seem
overdone. However, the models are agreeing well on the overall
timing of the boundary, so planning on staying with previously
populated grids over the short range as the 18Z GFS supports the
previous forecast.

In the medium range, the models indicate another cold front will
move offshore at some point late Tue into Wed, and again increase
the winds over the NT1 waters in southerly flow. The 12Z GFS
trended slower from the 06Z run, and matched the timing of the
front when compared with the 12Z ECMWF/UKMET/GEM. However, it was
well to the W of the guidance with the associated low center over
eastern Canada. The 18Z GFS then caught up with the rest of the
12Z guidance with the timing of the low, but trended faster with
the front and became more progressive than the rest of the 12Z
guidance. As a result of the poor run to run consistency,
confidence in the GFS solution is low. The previous forecast
favored the 12Z ECMWF solution, and it is supported by the the
12Z UKMET/GEM solutions. As a result, planning on staying with
the 12Z ECMWF through the medium range period.

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

Over the short term, the new 12Z models share similar forecast
timing for a cold front to push off the N mid Atlantic and
southern New England coasts late tonight, pass SE across the NT1
and northern NT2 waters Sat/Sat night, then drift slowly S across
the central NT2 waters Sun/Sun night. In regards to the forecast
gradients associated with this front, the 12Z models agree that
the southerly prefrontal gradient will be stronger than the
westerly postfrontal gradient. Overall, believe the 12Z GFS
solution for the associated gradients (with max southerly
prefrontal winds up to 20-25 kt) looks representative. So similar
to the previous offshore forecast package, plan on populating
our forecast wind grids with the 12Z GFS 10m winds for tonight
through Sun night, with some additional minor edits mainly in
deference to the 12Z ECMWF.

In the long range, the 12Z models generally agree that the cold
front will stall across the S central NT2 waters Mon into Tue
and wash out while high pressure builds offshore causing weak
associated gradients with light winds. Since it still looks
representative, will continue to populate on Mon through Tue with
12Z GFS 10m winds for these light winds.

Then the 12Z global models share similar forecast timing for the
next cold front to approach the New England and mid Atlantic
coasts Wed, then push offshore into the NT1 and NW NT2 waters Wed
night with a moderately strong (generally 20-25 kt) SSW
prefrontal gradient developing across the northern waters. With
the 12Z GEFS Mean indicating that the 12Z GFS attendant surface
low associated with this front is likely tracking too far NW,
would favor a solution closer to the 12Z ECMWF for this fropa.
Therefore, will transition to populating with 12Z ECMWF boundary
layer winds on Tue night through Wed night.

.SEAS...The 12Z Wavewatch III and 12Z ECMWF WAM have both
initialized the current seas well. With this in mind and with
their associated 12Z GFS/ECMWF solutions being similar, will
populate our forecast wave grids with a compromise 50/50 blend of
the two wave models for tonight through Tue. Then since the 12Z
ECMWF solution will become favored, will transition to populating
with all 12Z ECMWF WAM seas on Tue night through Wed night.

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

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Kells/Vukits. Ocean Prediction Center.



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