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000
AGNT40 KWNM 131315
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
815 AM EST Wed Dec 13 2017

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

For the morning update we will mainly update the ongoing forecast
and grids for initial conditions noted prior to issuance, and
also adjust grids and the forecast closer to the nearby coastal
WFO and TAFB grids. The latest observations indicate that
widespread gales are still occurring over most of the offshore
waters with strong cold air advection in the wake of the strong
cold front. GOES satellite imagery and lightning data clearly
indicate that the front has passed E of the waters and extends
from Nova Scotia S to just W of Bermuda per the 12Z preliminary
OPC-NCEP surface analysis. The next low that will impact the
waters is currently located over SE Minnesota. It will move
quickly E and SE and cross the offshore waters, roughly along
40N or so, during Thu. Another low will impact the waters Fri
into Sat. The 06Z GFS has remained consistent from the 00Z
cycle, with the 06Z NAM very similar regarding timing and
intensity of these low pressure areas as they cross the region
over the next few days. Therefore, no major changes appear
needed for the morning update.

As far as sea heights are concerned we will likely not make many
changes to the ongoing forecast and grids as they appear to fit
fairly well with the latest observations over the area. Sea
heights range from around 20 feet over the outer NT2 waters to
near 6 feet or so closer to the coastal waters per the latest
observations.

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

Last evenings Ascat overpasses across the offshore waters
confirmed the widespread post frontal gales extending from
Delaware Bay south to about 30N. The 01Z and 0145Z passes
indicated the cold front was located just southeast of the outer
ANZ905 and ANZ910 zones with gales extending about 150 nm
southeast of the front. Overall, the Ascat data suggests the
higher 00Z GFS first sigma level winds were the best initialized
with the offshore winds, and even these winds appeared somewhat
weaker just off the North and South Carolina coasts. Based on
this will continue to populate the short term wind grids with
the stability smart tool which places the GFS first sigma level
winds across areas with negative low level lapse rates, and the
lower GFS 10m winds elsewhere. With the strong cold air
advection occurring offshore, this will result in the all
offshore zones being populated with the higher first sigma level
winds. The 00Z models are then consistent in forecasting a
clipper low to move offshore near Delaware Bay/Southern New
Jersey early Thu and rapidly track east near 40N later Thu
morning into early Thu afternoon. Although the past few runs of
the GFS/ECMWF have generally backed off with the strength of the
low, plan to retain the storm warnings across ANZ910 and ANZ905
Thu, and gale warnings elsewhere across the NT2 zones north of
Cape Fear. These hazards will also maintain OPC forecast
continuity. Forecast confidence with these hazards is well above
average.

Versus their runs 24 hours ago, the 00Z models have come into
much better agreement with the next upper level trough which
will be moving to the eastern seaboard late Fri/Sat. That is,
the GFS has trended flatter and toward the more consistent
ECMWF/UKMET. However, the 00Z GFS is still initially stronger
than the ECMWF/UKMET with the associated developing surface low
moving east across the central NT2 waters late Fri/Fri night.
Then with the 00Z models sharing a similar timing with gales
again developing in the cold air advection Sat into Sat night,
we have more forecast confidence with the Sat/Sat night gales
than with the Fri/Fri night event. To account for some of these
model differences used a blend of the 00Z GFS and 00Z ECMWF for
the wind grids Fri through Sat night, but used the higher GFS
first sigma level winds in the blend to retain the previously
forecast gales.

The 00Z models share a similar timing in forecasting a high
pressure ridge passing east off the coast Sat night and moving
east across the offshore waters Sun. Models also agree that the
upper pattern across the eastern seaboard will briefly become
more zonal late in the weekend. This is where the model
similarities end, with significant timing differences popping up
with progression of the next long wave trough into the eastern
US. Although, versus its previous run, the 00Z ECMWF trended
slower, it still remains the most progressive outlier, and also
is the most amplified of the deterministic solutions. The 00Z
UKMET also trended faster compared to its previous run. To keep
some continuity with the previous OPC forecst, and also to
account for some of the above average forecast uncertainty, will
be using an even blend of the past two runs of the ECMWF for the
wind grids from Sun night onnward.

.Seas...The slightly higher 00Z ECMWF WAM may be a touch better
initialized than the 00Z Wavewatch III. There have not been any
recent altimeter overpasses across the offshore waters, and the
only buoy across the outer NT2 waters is 41002, which reported
17 ft at 06Z. Will populate the near term wave height grids with
the 00Z ECMWF, then use an even blend of the aforementioned
models tonight through Sun. To be consistent with the blended
wind grids, will then use an even blend of the past two runs of
the ECMWF WAM beginning Sun night.

.EXTRATROPICAL STORM SURGE GUIDANCE...No significant issues with
the latest surge guidance is noted at this time.


.WARNINGS...Preliminary.

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

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

$$

.Forecaster Mills/Clark. Ocean Prediction Center.


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