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AGNT40 KWNM 280121
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
921 PM EDT Tue Jun 27 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 weak low pres over
the nrn NT2 waters pulling a weak cold front across the area.
Current surface observations indicate up to 15 kt across the W
Atlc, though the Ascat pass from this morning indicated up to 25
kt just east of the front. The GFS/ECMWF/UKMET/GEM all indicate
the strongest winds are now east of the offshore waters, so
planning on starting out with 15 kt in NT2. In NT1, the analysis
shows a low pres trough along the New England coast, and the
lightning density product along with the radar imagery shows an
area of showers and tstms approaching the coast. The previous
forecast had a mention of showers and tstms in NT1 for tonight,
so planning on maintaining it in the update package. As far as
the winds are concerned, the previous forecast had SW winds up to
15 kt ahead of the front, and the latest model guidance keeps
that intensity as the weak trough moves into the offshore waters
and weakens. Planning on staying with previous grids as a result.

The models remain in good overall agreement into Fri, and
indicate high pres will build over the NT2 waters Wed before
passing east of the offshore waters Thu into Fri. The GFS also
indicates a few weak surface boundaries will move through the
area in the short range. The 12Z GFS was preferred in the
previous forecast as a result of the good overall support, so
planning on staying along those lines as the 18Z GFS has not
changed much in the short range. However, the GFS becomes a slow
outlier with the the next few systems to move through the W Atlc
Fri into Sun, while the slightly faster 12Z ECMWF is in decent
agreement with the 12Z GEM/UKMET solutions on the overall track
and timing. The 18Z GFS trended slower from its 12Z run which
already lagged behind the rest of the guidance, so confidence is
somewhat low on the GFS solution. The previous forecast favored
the 12Z ECMWF from Fri onward, so planning on staying with it as
a result of the better support. Otherwise, not planning on making
any major changes in the update package from the previous
forecast.




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

A 14Z Ascat overpass across the outer NT2 zones south of
Hatteras Canyon west to southwest winds to 20 kt associated with
the low and stationary front. The 12Z GFS/ECMWF/CMC were well
initialized with these winds while the 12Z NAM/UKMET were
slightly weak. The 12Z GFS/ECMWF are in good agreement across
the west Atlantic over the next few days in forecasting the weak
frontal wave to move northeast through the NT2 waters tonight
with associated southerly 20 kt winds shifting east of the
waters by evening. The 12Z models are consistent that the cold
front will pass east of the offshore zones by early Wednesday.
As high pressure builds toward the coast tonight and Wednesday,
then moves offshore Wednesday evening, the 12Z GFS/UKMET both
advertise post frontal winds to reach 20 kt across the NT2 outer
waters after midnight tonight, and then transition southward
across the waters, becoming northeast to east across the
southern NT2 zones Wednesday through Thursday. Models then all
forecast the high pressure center to shift east of the waters
Thursday, while a series of sheared upper level shortwaves pass
east across New England through the end of the week. Over the
past few days, the models have generally trended weaker with
these upper shortwaves. At the surface, whereas the GFS/ECMWF
from a couple days ago was consistently moving an associated
cold front to near Georges Bank, more recent runs are only
moving the front as far south as the Maine coast and maintains
southerly winds across the all the offshore waters at least
through Sunday. Versus the GFS/ECMWF the 12Z UKMET moves the
front further offshore.

The 12Z models are mostly consistent that a moderately strong
southwest gradient will persist across the New England waters
Thursday night into Friday night as low pressure passes across
New England and a zonally oriented cold front drifts south
toward the Maine coast. Front should stall along the Maine coast
and then lift back north as a warm front Saturday night/Sunday.
The 12Z GFS/ECMWF/UKMET then have come into much better
agreement with the cold front forecast to approach the New
England and northern Mid Atlantic coasts and move just offshore
Sunday night. For wind grids through Friday, will be populating
with the 12Z GFS winds but using the tool which places the 10m
winds across stable areas (as defined by the GFS low level
static stability) and first sigma level winds across unstable
areas, which are really only across the NT2 waters behind the
cold front tonight and over the southern NT2 waters through
Thursday. Beginning Friday night, transitioned to the 12Z ECMWF
mainly to keep continuity.

.Seas...The 12Z Wavewatch III and 12Z ECMWF WAM are well
initialized with the offshore wave heights (4 ft or less) this
afternoon. These models continued to be within a ft or so of
each other across the offshore waters through the forecast
period. Will continue to populate the wave height grids with a
50/50 blend of the two models through the period.

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

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Kells/Clark. Ocean Prediction Center.


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