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000
AGNT40 KWNM 310033
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
833 PM EDT Thu Mar 30 2017

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

Update...The 18z ncep surface analysis shows a high pres ridge
moving se over the offshore waters with a strong low centered e
of the nt1 area se of nova scotia. Latest available ascat and
ascat hi-res passes from this morning show 25 to 35 kt winds in
a strong pressure gradient between the low and the ridge over the
gulf of maine, georges bank and the ne nt2 waters. Ascat shows 15
to 20 kt winds over the sw nt1 and wrn nt2 waters.

Models...The medium range models are in decent overall agreement
across the offshore waters for tonight through sun night, so the
representative gfs 10m/30m solution with the stability smart
tool will be used for the wind grids during this timeframe. The
models are divided into two camps regarding the strength of the s
to se flow over the nt2 waters for mon and mon night, with the
gem/gfs stronger and the ecmwf/ukmet weaker, so as a comprimise
solution a 50/50 blend of the ecmwf/gfs will be used on mon and
mon night. The models are not in good agreement for tue and tue
night concerning the strength, trajectory and timing of a
developing low moving offshore, but the ecmwf with its winds
capped at 37 kt looks like a good median model solution at this
time. Am not planning to make any significant changes to the
current forecast trend.

Seas...The wna wavewatch will be used for the sea height grids
for tonight through sun night, in order to be consistent with the
preferred gfs model during this timeframe, then will go with a
50/50 blend of the wna wavewatch/ecmwf wam for mon and mon night
for the same reason. The ecmwf wam will be used exclusively for
tue and tue night in order to be consistent with the preferred
ecmwf model during this period.

Extratropical Storm Surge Guidance...N/A.

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

Over the short term, in the NNW gradient associated with the
intensifying low pressure center now SE of Nova Scotia, earlier
high resolution 1445Z ASCAT-A and 1359Z ASCAT-B scatterometer
passes confirmed that marginal gale force winds were present in E
portions of NT2 zones ANZ905 and ANZ910 as previously expected.
Per a consensus of the new 12Z models, expect this gradient to
weaken to below gale force by early tonight, then continue to
weaken later tonight into Fri as the low is forecast to continue
to move off to the E while a high pressure ridge drifts E across
the offshore waters.

Then with only minor timing differences, the 12Z models forecast
the next developing surface low to approach the Delmarva
Peninsula late Fri, track E across the offshore waters in the
vicinity of 40N Sat into Sat night with all of the models
forecast gale force associated boundary layer (BL) winds to
impact the Srn NT1 and Nrn/central NT2 waters, then pass off to
the E Sun/Sun night. In regards to the forecast track of this
low, the 12Z GEFS Mean and the other models indicate that the 12Z
GFS may be slightly too fast, but not significantly so. In
regards to the associated forecast gradients, would favor the
slightly stronger 12Z GFS/NAM/GEM solutions. Therefore plan on
populating our forecast wind grids with the 12Z GFS solution
using our smart tool that will place stronger GFS first sigma
level winds in unstable areas and weaker 10m winds in stable
areas for tonight through Sun night. Then in deference to the
other 12Z models, will time shift these winds 3 hours slower Fri
night through Sun night, and then make some additional minor
edits mainly in deference to the 12Z ECMWF.

In the long range, the 12Z models generally agree that a high
pressure ridge will build to the E coast Sun, then pass slowly E
across the waters Sun night into Mon night causing diminishing
conditions to develop in its vicinity. Then late Mon/Mon night to
varying degrees the 12Z models all forecast a strengthening Sly
gradient to develop in association with a warm fropa across the
NT2 waters. As a compromise, would favor a blended 12Z GFS/ECMWF
solution for this Sly gradient. Therefore will populate with a
50/50 blend of the 12Z GFS 10m and 12Z ECMWF BL winds on Mon/Mon
night.

Then Tue/Tue night, the disparity increases between the 12Z
global models in regards to a developing surface low and cold
front moving offshore. The 12Z GEM/UKMET both forecast their
surface low to track significantly further N than the 12Z
GFS/ECMWF. The 12Z GEFS Mean though indicates that the 12Z GFS
may be tracking its surface low too far S. Therefore in between
lies the 12Z ECMWF solution which perhaps looks like a reasonable
compromise. Therefore will transition to populating with 12Z
ECMWF BL winds on Tue/Tue night, time shifted 6 hours slower to
be more in line with the 12Z GEFS Mean and the WPC medium range
guidance. Then with this being Day 5 and due to the model
disparity, will cap these winds at 35 kt

.SEAS...The 12Z Wavewatch III has initialized the current seas
slightly better than the 12Z ECMWF WAM. With this in mind and
since the 12Z GFS solution will be favored, will populate our
forecast wave grids with the 12Z Wavewatch III for tonight
through Sun night. Then since a compromise 12Z GFS/ECMWF solution
will be favored, will transition to populating with a 50/50
blend of the two wave models for Mon/Mon night. Then Tue/Tue
night will populate with all 12Z ECMWF WAM seas (reduced 10
percent on average since the 12Z ECMWF BL winds will be capped)
since the 12Z ECMWF solution will be favored.

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

.WARNINGS...Preliminary.

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

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

$$

.Forecaster Scovil/Vukits. Ocean Prediction Center.



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