North Atlantic Marine Weather Discussion

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AGNT40 KWNM 121349
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
849 AM EST Tue Dec 12 2017

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

We do not plan on making major changes to the ongoing OPC
forecast for the morning update. We will make minor adjustments 
mainly to fit the ongoing forecast to current conditions, and to
fit with nearby TAFB and coastal WFO grids and forecasts a 
little better. 

A strong cold front is still forecast to move E and SE over the 
waters today, with widespread gales developing within the next 
few hours into tonight, and storm force winds over the far NE 
NT2 waters by Thu. The 06Z GFS has remained quite consistent 
from the 00Z GFS which was used for the short term portion of 
the forecast into early Thu, which leads to more confidence in 
the ongoing forecast. 

A pre-frontal trough extending from N to S over the NT waters
early today, will move E as the front move offshore later this
morning and afternoon. The latest radar, satellite and lightning
density products indicate a few strong thunderstorms over 
central NT2 outer waters moving off to the E and NE. The latest 
SREF thunderstorm guidance indicates that the thunderstorms will
increase mainly over NT2 waters today into tonight as the front 
moves across the region. Caution for locally strong winds 
exceeding gale force and very rough seas in and near the stronger
thunderstorms over the NT2 and potentially southern NT1 waters 
today into tonight.

Seas are already building off the SE U.S. coast this morning with
seas heights around 7-8 feet being detected per the 13Z 
observations. Again, for the morning update we will only make a
few minor adjustments to the ongoing OPC forecast mainly to fit
our grids to current conditions and to fit with nearby coastal
WFO and TAFB grids and forecasts. Seas will continue to build
throughout today and tonight.

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PREVIOUS DISCUSSION... 

The 02Z Ascat overpass clearly delineated the stationary front 
across the Gulf of Maine and indicated north winds to 20 kt just 
poleward of the front. Elsewhere there were also some winds to 
20 kt off the Georgia and South Carolina coasts. The 00Z models 
continuted in excellent agreement across the west Atlantic for 
the next 48 hours or so. The potential for hazards becomes 
highly likely today ahead of strong low pressure that will move 
from the lower Great Lakes into the Canadian Maritimes today 
into Wednesday, accompanied by a strong cold front passing east 
across the NT1 and NT2 waters. Over the past few runs, the 
models have trended stronger both the prefrontal winds as well 
as with the widespread prolonged gales west of the front in the 
strong cold air advection tonight through Wed night. The 00Z GFS 
is the strongest of the models with the winds and now even shows 
some southerly winds reaching storm force across the nothern 
outer NT2 zones tonight. Although will likely see fairly large 
areas with storm force gusts across these outer waters, will cap 
the winds at 45 kt as the GFS only indicating the low level jet 
reaching 55 kt or perhaps 60 kt briefly. We continue to have 
well above average forecast confidence that widespread gales 
will develop across all of the offshore waters associated with 
the cold front today and extending through at least Wed night. 
As was noted above, storm force wind gusts are likely especially 
across the outer NT2 zones near the Gulf Stream where deep 
convection is expected to develop this afternoon into tonight. 
Used a stability smart tool for the wind grids through Wed 
night, which places the 00Z GFS 10m winds across the stable 
areas, and the higher GFS first sigma level winds across the 
unstable areas, which in this case is across all the offshore 
zones behind the cold front.

As the strong low lifts north, widespread gales will diminish 
gradually Wednesday into Wednesday night from southwest to 
northeast over the offshore waters. The 00Z models then all 
indicate a clipper type low pressure system will emerge off the 
Mid Atlantic coast early Thu. As was the case with the previous 
couple runs, the 00Z GFS continued to trend stronger with this 
feature and like the 18Z/11 run is again advertising storm force 
winds across ANZ905 and ANZ910 Thu. 00Z GFS/NAM low level lapse 
rates indicate that the low level environment will still be 
quite unstable Thu, so some there is some potential for the 
sustained winds to briefly reach storm force across these outer 
zones Thu. The 00Z ECMWF also trended toward the stronger GFS, 
and now is also further north with the track of the low, similar 
to the GFS. Based on this trend, am favoring the stronger GFS 
and will go ahead and upgrade the gales to storm force across 
those outer zones. This low should rapidly accelerate northeast 
passing south of the Canadian Maritimes late Thu and Thu night.  

Late in the period Fri/Sat, our forecast confidence wanes as the 
models continue to have difficulty coming to a consensus with 
how amplified the next upper level trough will be moving to the 
eastern seaboard. The 00Z GFS continued to be the strongest 
solution and also is less progressive than the ECMWF/UKMET. The 
latter models come into excellent agreement at 00Z. However, the 
latest GEFS and ECMWF EPS both suggest that the ECMWF/UKMET 
deterministic runs are too weak. Given the uncertainty, we are 
favoring the WPC medium range winds, which were based on a blend 
of the GEFS and ECMWF EPS means. However, out of deference for 
the consistently stronger GFS and with both the ECMWF and UKMET 
near or at gale with the post frontal winds Fri night into Sat 
night, will go ahead and boost the WPC winds slightly to get 
some marginal gales across the outer waters at that time.   

.Seas...Both the 00Z Wavewatch III and 00Z ECMWF WAM were well 
initialized with the west Atlantic wave heights at 06Z and are 
generally within 3 ft or so of each other across the offshore 
waters through Fri. Thereafter, the models diverge significantly 
with the GFS significantly stronger than the ECMWF with the Fri 
night/Sat night system. Will be using an even blend of the 
aforementioned guidance for the wave height grids through Fri 
before transitioning solely to the 00Z ECMWF WAM beginning Fri 
night. 

.EXTRATROPICAL STORM SURGE GUIDANCE...The consensus of the 
ESTOFS and ETSS guidance suggests any storm surge along the 
Maine coast to the Bay of Fundy will be around 1 ft this evening 
and tonight. The 00Z ETSS is slightly higher than the 00Z 
ESTOFS, but only a few tenths feet. 


.WARNINGS...Preliminary.

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

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

$$

.Forecaster Mills/Clark. Ocean Prediction Center.