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AGNT40 KWNM 201245
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
845 AM EDT Fri Apr 20 2018

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

Note: AWIPS data will not be available at opc until 17z on Apr
21, due to planned maintenance.

Current Conditions...The 06z ncep surface analysis shows a 996 mb
low centered just SE of Nova Scotia, with an associated cold
front extending SW from the low across the central and southern
nt2 waters and into the Jacksonville region. The analysis also
shows a low pressure trough extending SE to NW across New
Brunswick, Maine and Quebec.

Models/Forecast...The cold front will pass SE of the offshore
waters later today. A series of low pressure troughs will pass SE
over the nt1 area today into Sun night. A strong high pressure
ridge will move SE over the offshore waters this evening into
Mon, then shift NE Mon night through Tue night. Low pressure will
move E along the Gulf Coast Sun night through Mon night, then
track NE along the SE coast Tue and Tue night. A strengthening
pressure gradient between the low and the ridge Sun night through
Tue night will result in winds increasing into the 30 to 40 kt
range over the nt2 waters for the Mon through Tue night period.
The medium range models are in good overall agreement across the
offshore waters for today into early Mon, so the representative
00z gfs 10m/30 solution with the stability smart tool will be
used for the wind grids into early Mon. There are some timing
differences with the low for later Mon through Tue night, so will
use a 50/50 blend of the 00z ecmwf/gfs to resolve the
differences starting later Mon. Confidence in the gale warnings
for Mon through Tue night is average, since most of the models
are in decent agreement during that timeframe.

Seas...Both the wna wavewatch and ecmwf wam initialized well
versus the latest buoy observations this morning. The models are
in good agreement into Mon, then differ somewhat regarding the
sea heights associated with the gales in the nt2 area. So will
use a 50/50 blend of the wavewatch and ecmwf wam over the entire
forecast period to smooth out the model differences.

Extratropical Storm Surge Guidance...Estofs and etss indicate a
positive sure in the 1 to 2 ft range along the SE coast for late
Sun into Tue, then expanding along the mid Atlantic coast for
late Tue and Tue night. Estofs and etss look reasonable with the
surge, given the strong E to SE flow forecast for the nt2 waters
for Sun night through Tue night.

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

The latest NCEP surface analysis indicates strong low pressure
SE of Nova Scotia, with a cold front extending back to the SW
across the central and srn NT2 offshore waters. The GOES infrared
satellite imagery indicates cold cumulus across the offshore
waters, indicative of the strong cold advection in the wake of
the cold front. The Ascat wind retrievals from 02Z last night
indicated a few gales ahead of the front as it was passing to the
E of the offshore waters, and a second area of gales in the cold
advection behind the front, mainly over the Gulf Stream. The 00Z
GFS indicates the system will move quickly off to the E today,
taking the strongest cold advection with it. The rest of the 00Z
models are in good agreement with the GFS on the timing of the
front, and most indicate winds will decrease after 12Z today. The
previous forecast had gale headlines up into 12Z today, mainly
near the Gulf Stream. Confidence with gales continuing into 12Z
is about average as the bulk of the guidance is near or just
below gale. However, would prefer not to drop headlines yet as
moisture and heat flux from the Gulf Stream to the cold air above
will deepen the boundary layer and potentially mix higher winds
aloft down to the surface. As a result, planning on allowing
previous gales to continue into 12Z today, and then letting
warnings expire as previously forecast.

The 00Z GFS and 00Z ECMWF remain in good overall agreement
throughout the remainder of the forecast period, and the models
all indicate a developing low will move to the SE coast late Mon
night into Tue. The GFS had been slightly faster than the rest
of the models with this low in previous runs, but the 00Z GFS
trended slower with the track of the low and is now in decent
agreement with the 00Z ECMWF/UKMET/GEM despite some minor timing
differences. In addition, the 00Z models are all showing another
round of gales will develop to the N of the warm front over the
NT2 waters, though the GFS seems a bit overdone as it indicates a
small area of storm force winds over the Gulf Stream by Tue
night and seems to be a feed back issue. However, confidence is
about average on the gales as a result of the good agreement, so
planning on staying close to previous gale headlines in the next
forecast while making small adjustments based on the trends from
the 00Z guidance.

For the wind grids, am planning on using the 00Z GFS 10m winds
starting 15Z today through 12Z Mon, opting for the first sigma
winds in unstable areas to account for the deeper mixed layer
expected in these areas. Will then use a 50/50 blend of the 00Z
GFS 10m winds with the 00Z ECMWF winds from 12Z Mon through the
end of the forecast period as a compromise to the minor
differences between the two models.

Seas...Both the 00Z Wavewatch and ECMWF WAM have been initialized
well when compared with conditions at the 06Z this morning. The
models agree well into Mon, then differ slightly on building seas
associated with the aforementioned gales in NT2. As a result, am
planning on using a 50/50 blend of the two models throughout the
forecast period while making only minor adjustments.

Extratropical Storm Surge Guidance: no significant positive
surge events are expected during the next few days. However, the
00Z ESTOFS indicates a positive surge near 2.0 ft along the SE
coast will develop as a result of the strong easterly flow
expected with the developing low along the coast. The model has
backed off some since yesterday, but confidence is higher as the
models are starting to come to a consensus with the associated
low pressure system.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

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

$$

.Forecaster Scovil/Kells. Ocean Prediction Center.



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