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000
AGNT40 KWNM 151427
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
927 AM EST Fri Dec 15 2017

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

Surface low formation clearly evident in observations along the
Outer Banks this morning; buoy 41063 reporting winds out of the SW,
while just 40 nm to the west BFTN7 reporting winds out of the
NNE. Supported by 170+ kt jet streak and digging mid to upper
level trough, and atop the warm waters of the Gulf Stream, rapid
intensification / bombogenesis expected through today and tonight
as the low quickly traverses the central portions of the OPC
offshore zones. Very brief, marginal storm force wind warnings
remain headlined for zones 910 and 905 at 06z tonight associated
with the strengthening of the 55-70 kt low level jet before the
surface low quickly clears east of the waters. Departing low will
pull another widespread cold air mass across the northern
waters, and another round of gales remains in the forecast
through late Sat night. Confidence during this time frame is
average.

Weak gradients expected Sun into Mon night as high pressure
settles over the W Atlantic, with a weak stationary front along
36-37n Highest winds to only 20 kt forecast to form Mon afternoon
as a weak wave forms along the stalled boundary, then moves E of
the area Mon night while pulling the front through the S waters.

Tue and Tue night: southwest flow increases ahead of the next
forming cold front, highest to about 30 kt over the northern
waters. Front pushes through the area Tue night, as another cold
air mass spills over the W Atlantic. Models have been hinting
next round of possible gales in the CAA behind this next front,
beginning late Tue night, however overnight and early morning
guidance - along with ensemble means - did trend significantly
weaker and east with axis of strongest winds. Will re-evaluate
with upcoming 12z guidance for later afternoon package when extra
day (Wed and Wed night) are tacked onto the forecast.


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

The latest surface observations show low pressure likely around
KLBT and low pressure will emerge off of the North Carolina coast
and move northeast during the day today. The GFS, UKMET and ECMWF
guidance all portray the surface low beginning to intensify
quickly as it approaches the warmer waters of the Gulf Stream in
the eastern mid-Atlantic zones late this afternoon and this
evening. While subtle, probably the most significant change as it
affects the forecast in the near term is the slightly farther
north track as the low moves northeast and deepens. As such,
925mb winds at or above storm force that were confined to the
southern part of ANZ910 now move into ANZ905. The ECMWF would
indicate winds at that level near 70kt at 06UTC in the eastern
part of ANZ905, while the GFS suggests lesser but still robust
values around 55kt at the same time, continuing south into
ANZ910. With the low intensifying near the Gulf Stream, and with
the strong southwest flow, will extend the storm warnings north
one zone into ANZ905. It is possible that, with deep convection,
eastern parts of even ANZ915 and ANZ920 could see brief storm-
force conditions. Confidence is higher just to the east, though.

Most of the gale-force conditions should be begin this evening in
the mid-Atlantic zones, but the NAM12 would suggest a period of
gales in ANZ925 as early as late this afternoon as the southwest
flow strengthens. In deference to the that model, will note some
gales in that zone then. Using guidance taking into account
instability, gale-force winds could reach as far west as the
eastern parts of ANZ825 and ANZ828 this evening. Most of the
10-meter winds keep gales just east of there, but factoring in
instability the GFS would certainly suggest gales along and east
of the vicinity of the Gulf Stream there.

As the low moves northeast, hurricane-force conditions should
develop well to the northeast of the offshore waters. It should
be noted that the ECMWF, near 43N57W by 16/18UTC, would provide
for winds around or above 70kt. Back to the west over the
offshore waters, the tight surface gradient in the northwest flow
should result in a good potential for gales developing Saturday
afternoon southeast of Long Island and then spreading east late
Saturday and Saturday night over the cluster of zones mainly
southeast of Massachusetts.

The surface gradient relaxes during Sunday and into Monday. On
Tuesday, the consensus of guidance indicates that the surface
gradient should begin to tighten again ahead of a cold front that
will move through the waters from late Tuesday afternoon through
Tuesday night, possibly moving through ANZ935 as late as
Wednesday morning. The consensus of the 00UTC guidance is for
less potential for any marginal gale-force conditions in the
southwest flow, with the GFS 10-meter/30-meter fields more stable
and the UKMET and ECMWF lighter with their 10-meter winds as
well. Potential is still non-zero, but for now highest winds from
Tuesday into Tuesday night will be capped from 20 to just above
30 kt. If there was a higher alternative to this, it would
probably be in the Gulf of Maine.

By Wednesday behind the front, as thicknesses fall, the potential
for gales increases to at least a moderate level over a large
part of the northern waters. The guidance has been fairly
consistent with at least marginal gales then north of ANZ825 and
ANZ925 developing Wednesday and diminishing Thursday, when high
pressure should build in once again. In this period currently,
the potential for anything greater than gales over northern
waters seems remote.

.SEAS...Leaned toward higher seas throughout the seven-day
forecast period. As such, leaned toward the ECMWF WAM through
Tuesday, which results in seas that should build to at or just
above 20 feet late Saturday and Saturday night in the eastern
parts of ANZ905 and ANZ910. From Tuesday night through the end of
the seven-day forecast, leaned more toward the higher Wavewatch.
In the northwest flow with that model, seas could reach the upper
teens feet in the aforemention locations for a period on
Thursday.

.EXTRATROPICAL STORM SURGE GUIDANCE...In the vast majority of the
forecast period, both the ETSS and the ESTOFS have any surge
values less than a half-foot. The ESTOFS continues to note
slightly more positive surge Tuesday night in the Gulf of Maine,
though less than its run of 24 hours ago. Tuesday night values
there are around a half-foot, compared to a half-foot to a foot
in its run 24 hours ago.


.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ805...Georges Bank west of 68W...
     Gale Saturday into Saturday night.
.ANZ900...Georges Bank east of 68W...
     Gale Saturday into Saturday night.
.ANZ810...South of New England...
     Gale Saturday into Saturday night.
.ANZ815...South of Long Island...
     Gale Saturday.

.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale tonight into Saturday night.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale tonight.
     Gale Saturday night.
.ANZ905...The Great South Channel to the Hague Line...
     Storm tonight.
     Gale Saturday into Saturday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Storm tonight.
     Gale Saturday into Saturday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale tonight.
.ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light...
     Gale tonight.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale today into tonight.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale tonight.

$$

.Forecaster Collins/Figurskey. Ocean Prediction Center.


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