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000
AGNT40 KWNM 120156
MIMATN

.Marine Weather Discussion for N Atlantic Ocean
.NWS Ocean Prediction Center Washington DC
855 PM EST MON 11 DEC 2017

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

The lastest models continued to be in excellent agreement with
the pair of amplifying mid and upper level shortwave troughs
dropping SE across the Mississippi Valley and Upper Great Lakes,
respectively, and forecast to approach the Mid Atlantic and New
England coasts Tue and Tue night. We continue to have well above
average forecast confidence that widespread gales will develop
across the offshore waters associated with the cold front Tue
extending through at least Wed night. The models then all
indicate a clipper type low pressure system will emerge off the
Mid Atlantic coast early Thu, with the 12Z UKMET and 12Z/18Z GFS
all trending stronger from their respective previous runs. The
18Z GFS is even showing some winds reaching storm force across
the northern outer ANZ905 AND ANZ910 zones around midday Thu.
Compared to the GFS/UKMET, the 12Z ECMWF is weaker and further
south with the low. The 18Z GFS and 12Z UKMET also suggest that
gale hazards may need to be adjusted slightly northward to
include ANZ905, but will monitor the trend in the 00Z models
before making any adjustments to the previous OPC grids. Late in
the period, today`s models are in no better agreement than was
the case yesterday. The GFS continued as the strong outlier with
another highly amplified upper level trough across the eastern
US late Fri/Sat. At this time given the widely varying model
solutions and forecast uncertainty, favoring a solution which
incorporates the ensemble means is probably the best course of
action. The 18Z Wavewatch III and 12Z ECMWF WAM are equally well
initialized with the west Atlantic wave heights this evening.

---------------------------------------------------------------
...Previous Discussion...

The latest observations and ASCAT data supports conditions well
below hazards, with winds as high as the mid 20s knots, and
waves in the high single digits feet well offshore over the mid-
Atlantic waters. The potential for hazards increases greatly
Tuesday ahead of strong low pressure that will move from the
lower Great Lakes into the Canadian Maritimes Tuesday into
Wednesday, accompanied by a strong cold front. The trends of the
previous forecast continue, and there is high confidence in
gales developing by Tuesday afternoon in the mid-Atlantic waters
as 925mb winds increases to 35 to 40 knots in an unstable
environment, with gales spreading across the waters from Tuesday
night into Wednesday morning.

The GFS and the ECMWF are very similar with the track and
strength of the low as it moves toward the Canadian Maritimes
and then farther north into Canada through Wednesday night. Raw
ECMWF 10-meter winds are running slightly under the GFS winds
during this period, and due to the degree of instability present
followed by the strong cold-air advection, really like the
output of the GFS winds adjusted for instability in the 10-meter
to 30-meter layer, which is closer to the adjusted 10-meter wind
values of the ECMWF. Winds should be below storm force in the
offshore waters, but could see sustained winds reach 45 knots in
the eastern part of ANZ905 and ANZ910 coincident with the front
and some likely deep convection late Tuesday night and early
Wednesday morning, with some gusts likely well into the 50s
knots there. Mariners should note there should likely develop a
fairly solid line of deep, vigorous convection Tuesday evening
from ANZ935 northeast through ANZ905.

As the strong low lifts north, widespread gales will diminish
gradually Wednesday into Wednesday night from southwest to
northeast over the offshore waters. Uncertainty then starts to
increase as the period progresses due to surface waves forecast
to move through the mid-Atlantic waters Thursday and again on
Friday. On Thursday, the GFS and ECMWF remain similar but with
the GFS just slightly deeper by a very few millibars as the
second low moves east Thursday. The GFS, ECMWF and the UKMET all
show a good potential for gales especially offshore of Virginia
and North Carolina Thursday and Thursday evening. Given that the
consensus of guidance is solidly in the gale category Thursday,
including the adjusted 10-meter ECMWF guidance, confidence in a
period of gales Thursday over the mid-Atlantic waters is
currently good.

Confidence is not so great Friday. The GFS, with a sharper 500mb
trough approaching the eastern seaboard late Friday, deepens
surface low pressure from the mid-Atlantic into the New England
offshore waters from Friday afternoon through Friday night. The
ECMWF also provides for a strengthening low, but just east of
the offshore waters Friday night into Saturday. The UKMET is
weaker and farther east as well. As a result, will lean more
toward the ECMWF and its weaker winds for Friday into Saturday,
keeping any gales, for now, just east of the offshore waters.

Late Saturday night, the ECMWF would provide for a weak
secondary low in the northwest flow with enough of a gradient to
provide for a period of marginal gales over the New England
offshore waters, but for this forecast, with the GFS showing
some surface ridging and growing uncertainty in a transient
feature, will cap winds just shy of gales then. Later, there
actually develops a little more consensus among the guidance for
the potential for gales in the southwest flow that should
develop late Sunday night into Monday ahead of yet another low
that will move toward the northern waters Monday. The gridded
forecast at the very end will show some marginal gales off the
coasts of North Carolina and Virginia by Monday morning.

As for seas, followed the Wavewatch closely through Thursday
night. Afterward, leaned toward the ECMWF WAM. Adjusted the WAM
lower slightly Saturday night into early Sunday near ANZ905 and
ANZ910 due to reducing the ECMWF winds slightly.

.EXTRATROPICAL STORM SURGE GUIDANCE...The consensus of the
ESTOFS and ETSS guidance suggests any storm surge to be under a
foot. The only exception to this is Tuesday night ahead of the
cold front. The ETSS would suggest values around a foot possible
then in the northern Gulf of Maine.

.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ800...Gulf of Maine...
     Gale Tuesday into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ805...Georges Bank west of 68W...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ900...Georges Bank east of 68W...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ810...South of New England...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night.
.ANZ815...South of Long Island...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night.

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night.
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale Tuesday into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Tuesday into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ905...The Great South Channel to the Hague Line...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Tuesday into Wednesday.
     Gale Possible Wednesday night into Thursday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles
Light...
     Gale Tuesday night into Wednesday.
     Gale Possible Thursday.
.ANZ828...Inner Waters from Cape Charles Light to Currituck
Beach Light...
     Gale Tuesday into Wednesday.
     Gale Possible Thursday.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras
Canyon...
     Gale Tuesday into Wednesday.
     Gale Possible Wednesday night into Thursday.
.ANZ830...Inner Waters from Currituck Beach Light to Cape
Hatteras...
     Gale Tuesday into Wednesday.
     Gale Possible Thursday.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale Tuesday into Tuesday night.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Tuesday into Wednesday.
     Gale Possible Thursday.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale Tuesday into Tuesday night.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale Tuesday into Tuesday night.

$$

.Forecaster Clark/Figurskey. Ocean Prediction Center.



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