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000
AGNT40 KWNM 200157
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
857 PM EST Sun Feb 19 2017

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

The latest GOES infrared satellite imagery and lightning data
indicate thunderstorm activity concentrated over and just east of
the far east portion of the far southern ANZ935 early this
evening, just east of a weak 1013 mb low analyzed near 32N 73W
and an associated frontal boundary. From the satellite loop the
system is moving southeast so that by the time the evening update
goes out the activity will be east of the area except perhaps in
the far east central NT2 waters ahead of the cold front
approaching from the northwest where thunderstorms will be more
isolated as indiacted in the latest SREF thunderstorm guidance.

Otherwise, we will alter mainly wave grids slightly to fit
current conditions, but otherwise pretty much leave the previous
forecast based on 12Z GFS 10m/30m wind tool transitioning to 12Z
ECMWF Thursday alone for now, and wait for a full set of 00Z
guidance before making any changes, if needed.

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

Over the short term, a weak 1015 MB low pressure area located
over the far southeastern NT2 offshore waters, or near 32N 75W,
per the preliminary 18Z OPC-NCEP surface analysis will move east
and southeast away from the waters tonight. The latest GOES
visible satellite imagery and lightning density data indicates
scattered thunderstorms occurring this afternoon near the low,
with most of the activity located east of 76W, and south of 35N.
Per the latest SREF thunderstorm guidance most of the
thunderstorms will move east of the southern and central NT2
outer waters by midnight tonight. Any of the heavier thunder-
storms can produce winds exceeding gale force and very rough
seas into tonight. Overall, the 12Z global models remain in good
agreement with the previous model cycles over the short term.
For the afternoon package, as the 12Z GFS is very similar to the
00Z GFS from yesterday, we will rely on the previous grids for
tonight into Tuesday, with high pressure becoming the dominate
weather feature over the waters by Monday night and Tuesday as
it builds southeast across the region, and low pressure
organizes east of the waters. Confidence levels are above
average over the region into Tuesday.

Over the medium to long term part of the forecast, some model
differences are noted, but overall, the models appear to be in
somewhat better agreement than what was noted yesterday. The 12Z
models are in pretty good agreement over the waters from later
Tuesday into Thursday, with high pressure slowly passing east
and southeast of the area by Wednesday. A warm front will lift
northeast over NT1 waters Wednesday into Wednesday night, with a
high pressure ridge dominating most of the NT2 waters. Low
pressure is forecast to track east from the Great Lakes
Wednesday night, and reach northern Maine later Thursday. An
associated cold front will approach the New England and northern
mid-Atlantic coast by 00Z Friday, with an increase in south and
southwesterly winds likely over the waters ahead of the front.
These winds are expected to remain below gale force. The front
will slide southeast over NT1 waters Thursday night and early
Friday as the low moves over the Canadian Maritimes. The front
will stall along 40N or so Friday, before lifting north as a
warm front by Friday night as a strong low pressure system
tracks from the Plains to the Great Lakes. As the 12Z models
remain in pretty good agreement late Tuesday into Thursday we
will stay close to the 12Z GFS 10 meter winds. From later
Thursday into Friday night we will trend the forecast more
toward the 12Z ECMWF as it has remained the most consistent late
in the forecast period as has good support from the UKMET,
Canadian GEM and NAVGEM models, and is also quite similar to the
latest WPC medium range guidance. There will be an increasing
threat for gales over the offshore waters over next weekend as a
strong cold front moves east into the waters. At this time, no
gales appear likely over the waters into Friday night, with
confidence levels near average.

.Seas...Both the 12Z Wavewatch and ECMWF WAM appear to have
initialized fairly well over the offshore waters, with the
observations and altimeter data indicating sea heights within a
foot or so of the model guidance. For the afternoon package we
will continue to use a 50/50 blend of these two similar models
through Wednesday night, and then as we transition to using the
ECMWF for winds also transition toward the ECMWF WAM for forecast
sea heights.

.Extratropical storm surge guidance...N/A.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Bancroft/Mills. Ocean Prediction Center.


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