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AGNT40 KWNM 270232 CCA
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Marine Weather Discussion for N Atlantic Ocean...CORRECTED
NWS Ocean Prediction Center Washington DC
932 PM EST Sun Feb 26 2017

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

The infrared satellite imagery indicates moderately strong cold
air advection across the nrn offshore waters in the wake of a
cold front well east of the area. Ascat wind retrievals from 15Z
late this morning indicated winds up to 30 kt in the Gulf of
Maine. However, current surface observations indicate up to 25 kt
in the nrn zones, and then variable 5 to 10 kt in NT2 where high
pres is building over the area. The 18Z GFS winds are well
initialized when compared with the data, and indicates the winds
will remain light over NT2 as high pres passes east across the
waters. However, the GFS indicates that another cold front will
pass to the N of the waters Mon night, and increase the winds
over NT1 in the SW flow ahead of the front. The GFS indicates 30
kt in the first sigma winds with a marginally unstable
environment. The NAM/GEM agree with the GFS on the intensity,
though the 12Z ECMWF/UKMET both only show 25 kt. The previous
forecast went up to 30 kt, and this still seems reasonable given
the good model agreement and taking into account the low wind
biases of the ECMWF and UKMET. As a result, planning on staying
with the previous intensity in the update package.

Otherwise, the models indicate an active pattern through the
remainder of the forecast period with a few more systems moving
through the area in the longer range. The GFS indicates the first
system will start impacting the offshore waters on Tue night as
it approaches from the west, then pass through the area late Wed
and Wed night. The models have all been indicating gales in the
SW prefrontal flow, and seems reasonable with a strong low level
jet setting up along the front in the unstable environment over
the Gulf of Stream. Confidence is above average with the gales. The
GFS also indicates a second round of gales in the Gulf of Maine
Thu in the postfrontal cold advection. The rest of the 12Z
models are marginally supportive of the gales, and confidence is
about average. The previous forecast had gales consistent with
the GFS, though time-shifted them slower to account for the fast
timing. This still seems reasonable, so planning on maintaining
previous headlines and grids with this system.

On Fri, the models all indicate a low pres system will pass
through the offshore waters. The GFS has been the strongest
solution, and is not well-supported by the rest of the 12Z
models. The ECMWF indicates a low, but is much weaker and tracks
it north of the GFS solution. The UKMET/GEM both indicate a
trough with no low, though the 18Z GEFS members do indicate a low
with most members clustered fairly close to the GFS. However,
confidence with gales is just below average at this time as a
result of the poor support, so planning on keeping winds at
marginal gale force as was done in the previous forecast. The
previous forecast also favored the 12Z ECMWF from Friday onward as
it is supported better by the rest of the guidance. This still
seems reasonable, so will continue along these lines for the
update package.

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PREVIOUS DISCUSSION...

Over the short term, the new 12Z models remain consistent in
forecasting high pressure to build E off the mid Atlantic coast
tonight, then pass E across the NT2 waters Mon with light winds
in its vicinity. N of the high the models forecast a weak cold
front to approach from the NW tonight into Mon with a
strengthening WSW gradient developing in its advance across the
NT1 waters. Similar to their previous respective runs, the 12Z
NAM/GFS remain consistent in forecasting some gale force first
sigma level winds to develop in this gradient in the Nrn Gulf of
Maine. But, at the same time, the 12Z GFS remains consistent in
forecasting the lower levels to become stable which would make
the weaker 12Z GFS 10m winds more likely to verify. So its going
to be close as to whether gales develop (the latest WFO Caribou
CWF with gale force gusts looks fine) in the NE portion of NT1
zone ANZ800, but for now with the 12Z ECMWF/UKMET forecasting
weaker gradients will maintain the previous forecast continuity
and will hold off on issuing a gale warning.

Then with slight timing differences, the 12Z models agree that
the next significant synoptic feature will be a warm front
developing off the Nrn Mid Atlantic coast Tue, then lifting NE
into the NT1 waters Tue night with a strengthening SSW gradient
developing to its S. Overall would favor the similar 12Z
GFS/ECMWF solutions for this fropa, which will result in gale
warnings being issued for the outer central/NE NT2 zones in the
vicinity of the gulf stream Tue night. Therefore plan on
populating 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 Tue
night.

In the long range, the 12Z models forecast an active weather
pattern to develop. The first significant long range feature is
expected to be a strong cold front passage on Wed night into Thu
night. The 12Z GFS/ECMWF/GEM have converged towards a similar
timing for this fropa, while the 12Z UKMET now looks too slow.
Therefore overall would favor a compromise 12Z GFS/ECMWF solution
for this fropa. So plan on continuing to populate with 12Z GFS
first sigma/10m winds on Wed through Thu night (time shifted 3
hours slower to better match the 12Z ECMWF timing) and then will
blend these winds 50/50 with the 12Z ECMWF BL winds.

Then further out in the long range, the 12Z GFS has trended
stronger than its previous respective runs in regards to a
clipper-type developing surface low racing E across the Srn NT1
waters Fri, then intensifying NE of the area Fri night causing a
strengthening W to NW gradient to develop in its immediate
vicinity and wake with gale force associated boundary layer (BL)
winds forecast to develop throughout the NT1 and Nrn/central NT2
waters. If anything, the 12Z GEFS Mean indicates that the 12Z GFS
is likely both too fast and too strong with this system. The 12Z
UKMET forecasts a significantly weaker system, but by late Fri
night does forecast a moderately strong NNW gradient to develop
with gales hinted at across the Nrn/central NT2 waters. The 12Z
GEM forecasts no low at all, but forecasts a surface trough
passage with an associated strengthening Nly gradient developing
with gale force BL over an area similar to the 12Z GFS. The 12Z
ECMWF looks like it perhaps offers a compromise solution for this
system since its similar, but slower and somewhat weaker than
the 12Z GFS. Therefore, will transition to populating with the
12Z ECMWF BL winds on Fri and Fri night, with these winds then
boosted up 10-15 percent late Fri night to better account for the
cold air advection in the Nly gradient.

.SEAS...The 12Z Wavewatch and 12Z ECMWF WAM have both initialized
the current seas equally well. With this in mind and with
their associated 12Z GFS/ECMWF solutions being similar, will
populate our forecast wave grids with a 50/50 blend of the 12Z
Wavewatch III (time shifted 3 hours slower Wed through Thu
night) and the 12Z ECMWF WAM for tonight through Thu night. Then
since the 12Z ECMWF will become favored, will transition to
populating with all 12Z ECMWF WAM seas on Fri/Fri night.

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


.WARNINGS...Preliminary.

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

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

$$

.Forecaster Kells/Vukits. Ocean Prediction Center.



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