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000
AGNT40 KWNM 260111
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
811 PM EST Sat Feb 25 2017

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

The latest satellite and radar imagery, and lightning density
data shows locally heavy thunderstorms moving east just ahead of
a strong cold front moving east and southeast off the mid-
Atlantic coast. The most recent trends show that the most of the
thunderstorms are slowly weakening as they move over the coastal
and into the western portions of the offshore waters from
eastern Long Island south and southeast to near Virginia Beach.

The latest SREF thunderstorm guidance maintains at least scattered
thunderstorms moving east and southeast over most of the waters
through the night, just ahead of the cold front. Winds may
exceed gale force along with very rough seas in and near any of
the heavier thunderstorms through tonight, with the thunderstorm
threat ending with the passage of the strong cold front from the
west and northwest to east and southeast through early Sunday
morning. The best chance for heavier thunderstorms will be over
northern and central NT2 waters tonight.

Overall, no major changes appear needed to the ongoing OPC
forecast for the evening update. The 18Z GFS is quite similar to
the 12Z cycle. Low pressure moving north from near Georges Bank
will pass over Nova Scotia later tonight, with the strong cold
front as noted above passing east and southeast over the
offshore waters. For the evening update, we will adjust the
forecast to fit current conditions, and fit nearby coastal WFO
and TAFB forecast.

As for the seas, we will again make only minor adjustments to the
previous forecast, primarily to fit the ongoing forecast to the
conditions noted just prior to forecast issuance and to fit with
nearby TAFB and coastal WFO forecasts. Otherwise, seas heights
will be adjusted slightly higher over the Gulf Stream tonight as
strong cold air advection develops in the wake of the cold front
crossing the warmer Gulf Stream waters. Per the 00Z RA1 OPC sea
state analysis seas heights have risen to near 9 feet east of
Cape Hatteras, and to around 12 feet over the far northeast NT2
waters associated with low pressure passing north over Georges
Bank. Locally very rough seas can occur in or near any heavy
thunderstorms through tonight.

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

Over the short term, the 12Z models share similar forecast timing
for a moderately strong cold front to race SE across the
coastal/offshore waters tonight, followed by the attendant
surface low intensifying well NE of the area Sun. For the
associated gradients with this system, the 12Z GFS solution,
which is well supported by the other 12Z models, especially the
12Z NAM/GEM, looks representative for this fropa. So with the 12Z
GFS remaining very consistent, similar to the previous offshore
forecast package, will populate our forecast winds with our smart
tool that will place stronger 12Z GFS first sigma level winds in
unstable areas and weaker 12Z GFS 10m winds in stable areas for
tonight through Sun night.

Then late Sun night/Mon while all of the models forecast high
pressure to pass E across the NT2 waters with diminishing
conditions, further N the models all forecast a cold front to
approach from the NNW, then move offshore into the NT1 waters and
weaken Mon night. As the front approachs to varying degrees the
12Z models all forecast a strengthening WSW gradient to develop
across the NT1 waters, peaking early Mon. With the 12Z GFS
forecasting the lower levels to transition from marginally
unstable to stable early Mon it looks close as to whether
marginal WSW gales will be able to mix down to the surface across
the Nrn Gulf of Maine. So with the 12Z UKMET/ECMWF forecasting
slightly weaker gradients then, while continuing to populate with
our smart tool with 12Z GFS first sigma/10m winds for Mon/Mon
night, for now will continue to hold off on forecasting any
associated gale warnings, but this will have to be monitored
closely in the future model runs.

In the long range, to only slightly varying degrees the 12Z
models all forecast a warm front to develop off the Nrn mid
Atlantic coast Tue and then lift NNE into the NT1 waters Tue
night. Overall would favor a solution closer to the 12Z
GFS/UKMET/ECMWF for this warm fropa and would keep its associated
gradually strengthening gradients subgale. Therefore, will
continue to populate with representative 12Z GFS first sigma/10m
winds on Tue/Tue night.

Then, the disparity between the 12Z global models increases in
regards to the next strong cold frontal passage. The 12Z GEM
looks like a fast outlier solution, while the 12Z UKMET looks a
little too slow. Therefore would favor a compromise 12Z GFS/ECMWF
solution which brings the strong cold front towards the E coast
late Wed night, offshore Thu, then E of the offshore waters Thu
night with widespread associated gales developing throughout much
of the waters. Therefore as a compromise, will populate with a
50/50 blend of the 12Z ECMWF boundary layer winds and the 12Z GFS
first sigma/10m winds (time shifted 3 hours faster) for Wed
through Thu night.

.SEAS...The 12Z Wavewatch and 12Z ECMWF WAM have both initialized
the current seas reasonably well. With this in mind and with
their differences generally remaining the 1-2 ft range, will
populate our forecast wave grids with a 50/50 blend of the two
wave models for today through Tue night. Then since a compromise
12Z GFS/ECMWF solution will be used, will continue with a 50/50
blend of the two wave models (with the 12Z Wavewatch III time
shifted 3 hours faster as was done with the 12Z GFS winds) as a
compromise for Wed through Thu night.

.EXTRATROPICAL STORM SURGE GUIDANCE...N/A.


.WARNINGS...Preliminary.

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

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

$$

.Forecaster Mills/Vukits. Ocean Prediction Center.


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