North Atlantic Marine Weather Discussion

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AGNT40 KWNM 151352
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
952 AM EDT Mon Oct 15 2018

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

High pressure is centered east of the northern NT2 waters this
morning with a ridge axis extending across the NT2 zones. As a
result, offshore wind speeds remain less than 25 kt across the
region. However, conditions will rapidly deteriorate later today
and tonight as a strong cold front moves off the New England and
northern Mid-Atlantic coast. The 06Z NAM Nest winds support our
current forecast with winds strengthening to gale force ahead of
the front overnight across portions of the NT1 and northern NT2 
waters. Post-frontal gales will likely persist into early Tue
before the system clears the region. With no changes to our 
forecast reasoning over the next couple days, grid edits were 
minimal with this update. This system is just one of a series of
cold fronts that will impact our area throughout the forecast
period, so further changes to the extended forecast may be 
necessary later today after evaluating the 12Z guidance.

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PREVIOUS DISCUSSION...

A series of strong cold fronts will pass SE over the waters
during the next several days. The first strong front will impact
conditions tonight into Tue, the second later Wed into Thu, with
a third strong front approaching the waters later Fri and Fri
night. The best chance for gales is expected with the first two
fronts, with a lower chance for gales later in the week with the
third front.

Over the short term, a late evening ASCAT overpass indicated
winds up to 15 kt or so mainly over the far southern offshore
and northeastern offshore waters as high pressure builds E over
the region. The first strong cold front will approach from the W
during the day today as the high moves off to the E of the area.
The front will cross the waters tonight into Tue, and then stall
off the North Carolina coast later Tue. A weak high pressure 
ridge will pass E over waters N of the stalled front later Tue,
with a weakening ridge over the far southern waters. There 
appears to be an increasing threat for widespread gales both in
advance of, and in the wake of the strong cold front. For the 
early morning package, we will rely on guidance from the high 
resolution 00Z Warw model output, except we will cap winds at 35
kt ahead of the front as the model attempts to place winds well
above 35 kt in patchy areas, and these higher winds are likely 
associated with thunderstorms that will likely form near and in 
advance of the strong cold front. For the early morning package 
we will go ahead and add gales to all of the New England waters,
and to the northern mid-Atlantic offshore waters as well, or all
of the northern NT2 waters, for tonight into Tue, as strong 925
mb winds of 50 to 65 kt develop and spread across these waters.
Caution is also advised for potentially very strong winds and 
very rough seas in and near any of the stronger thunderstorms 
that form near the cold front tonight into Tue. Forecast 
confidence is above average over the short term.

Over the medium range we will trend the forecast toward the
very consistent 00Z ECMWF winds for Tue night through Thu night 
over the region, and use the boost tool which increases the 
ECMWF winds by up to 15 percent on a sliding scale. We will also
make a few manual edits in deference to the previous few OPC 
forecasts, and nearby coastal WFO and TAFB grids. The next 
threat for gales will develop Wed night through Thu as the next
strong cold front crosses the waters from NW to SE. A low
pressure area may form on the stalled front, in advance of the 
next strong cold front, off the N. Carolina coast on Wed, with 
the 00Z ECMWF stronger with this low and associated winds verses
the GFS, UKMET and Canadian GEM guidance. Therefore, we will cap
winds at 30 kt associated with the low as it moves E over 
central NT2 waters on Wed, and continue to closely monitor 
future model guidance. Otherwise, we expect gales to develop as 
the second strong cold front crosses the waters Wed night and 
Thu, with the best chance for gales over eastern NT1 and NE NT2 
waters. There is some potential for gales occurring further W 
over the offshore waters Wed night and Thu, and we will continue
to make adjustments to the forecast over the next few days. 
Conditions will improve Thu night into Fri as high pressure 
builds E over the waters. The high will then shift E of the 
waters Fri night as yet another cold front approaches from the W
and NW. For Thu night through Fri night we will rely on 
something closer to the 00Z ECMWF winds and not boost the winds 
over this time frame, as forecast confidence remains near 
average, and no strong advection appears likely for most of this
time period. Some adjustments will be made in deference to 
nearby coastal WFO and TAFB wind forecasts over the medium 
range. It appears that any gales associated with this third cold
front will not occur until sometime over the upcoming weekend, 
and this threat will be addressed in future OPC forecasts. 

Seas...Both the 00Z Wavewatch and 00Z ECMWF WAM appear to have 
initialized quite well over the offshore waters early this 
morning with seas heights ranging from 2 to 5 feet over the 
region, with the higher seas over the outer offshore waters. 
These two wave models are in pretty good agreement over the 
waters over the next 5 days. We will use a 50/50 blend of these 
two wave models for the early morning package, with a few manual
edits in deference to nearby coastal WFO and TAFB grids, and in 
and near the gale force wind areas where we may add a foot or 
two especially near the north wall of the gulf stream where sea
heights are typically higher in strong cold air advection in the
wake of strong cold fronts.

.EXTRATROPICAL STORM SURGE GUIDANCE...The latest ESTOFS and ETSS
guidance show no significant positive surge over the region for 
most of this week. No major adjustments to the surge guidance 
appears necessary at this time.


.WARNINGS...Preliminary.

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

.NT2 Mid-Atlantic Waters...
.ANZ820...Hudson Canyon to Baltimore Canyon...
     Gale tonight.
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale tonight.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale tonight.
     Gale Possible Wednesday night.
.ANZ905...The Great South Channel to the Hague Line...
     Gale tonight into Tuesday. 
     Gale Possible Wednesday night into Thursday. 
.ANZ910...East of the Great South Channel and south of 39N...
     Gale tonight into Tuesday. 
     Gale Possible Wednesday night.

$$

.Forecaster Reinhart/Mills. Ocean Prediction Center.