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AGNT40 KWNM 270104
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
904 PM EDT Mon Jun 26 2017

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

The NCEP surface analysis indicates weak low pres off the SE
coast along a weak stationary front across the NT2 waters, and
a high pres ridge to the the SE of the waters. The current
surface observations indicate up to 15 kt in the offshore waters,
though the infrared satellite imagery shows an area of weakening
convection over far srn NT2 ahead of the front which may have a
few gusts above 15 kt. The previous forecast indicated up to 20
kt over a majority of the offshore waters, and favored the 12Z
ECMWF into 18Z Tue. The 12Z/18Z models all agree on the front
remaining very while while the low off the SE coast moves NE
along it. Will only be making minor adjustments from the previous
forecast based on current data for tonight and Tue.

The models stay in generally good agreement over the forecast
period, except for the 12Z GEM which becomes an outlier solution
into the medium range period. The rest of the models indicate a
relatively weak pattern over the next 5 days, and the GFS/ECMWF
differ slightly on the track and timing of a few weak features
over the next few days. The previous forecast favored the 12Z GFS
which is supported marginally better by the 12Z UKMET. The 18Z
GFS has not changed much from the 12Z run, so will only be making
minor adjustments in the update package.

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

This mornings offshore Ascat overpasses returned winds to 30 kt
and even some retrievals to marginal gale south of 31N which
were all associated with the strong thunderstorms near 1000
fathoms from 31N to 32N occurring at that time. Elsewhere Ascat
showed some winds to 20 kt across the outer NT2 waters near the
front. Based on the recent lightning data and warming cloud
tops, this convection has weakened significantly since this
morning. The SREF based probability guidance did not seem to
have a good handle with these thunderstorms, instead showing
higher probabilities nearer the coast this morning. There will
likely be another diurnal maximum in this thunderstorm activity
around daybreak. Overall the 12Z models are in very good
agreement across the west Atlantic for the next several days.
The 12Z
GFS/ECMWF/CMC/UKMET all forecast a few weak lows to develop
across the southern/central NT2 waters tonight and move NE along
the weak stationary front Tuesday into Tuesday night. The 12Z
ECMWF shows the associated wind reaching 20 kt and this is well
supported by the higher resolution 4km NAM/HRRR. The 12Z models
then forecast a fairly amplified upper level trough will reach
the coast late Tuesday, and support a cold front passing east
across the waters Tuesday into early Wednesday. The guidance is
consistent with the timing of the cold front passing east of the
NT2 waters by early Wednesday, and then high pressure
subsequently building to the coast by Wednesday evening.

Then with the low forecast to track east across New England and
its associated front forecast to approach the New England coast
late Thursday and Friday, some timing differences remain between
the latest models. The 12Z ECMWF is faster  than the 12Z
GFS/UKMET/CMC in moving the cold front off the coast. The latest
GEFS and ECMWF EPS both support the slower GFS/UKMET. The 12Z
GFS looks reasonable with the return flow to 25 kt Thursday
night through Friday night, while the 12Z parallel GFS continued
to be overdone with 10m wind close to gale across the Gulf of
Maine. The 12Z GFS/UKMET are mostly consistent that the cold
front will stall across the Gulf of Maine Saturday and lift back
N as a warm front Saturday night. Meanwhile through the medium
range, a high pressure ridge will persist across the NT2 waters.


.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 are generally within a ft or so of each
other through the forecast period. To account for some minor
differences will populate wind grids with a 50/50 blend of the
two models through the forecast 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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