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000
AGNT40 KWNM 231422
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1022 AM EDT Mon Apr 23 2018

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

The latest available observations indicated conditions just below
gale-force over ANZ835, though there was a trend upward in
observations over the last few hours with gusts into the lower
30s knots in and just south of that zone. The coldest cloud tops
on infrared imagery were located just south of ANZ835, and there
should be some cloud-top cooling in that zone during the day as
the best isentropic lift on the 300K surface occurs there this
afternoon into this evening. 950mb winds on the GFS increase to
around 50kt this afternoon, and though sustained winds should be
in the gale category as they increase later this morning and into
this afternoon, there will likely be some convective gusts around
50 knots with the strongest storms. Plan to increase the maximum
sustained winds in the forecast, worded and digital, to 45kt this
afternoon and tonight in ANZ835. Will review later guidance today
to determine whether or not to increase the maximum similarly in
zones farther northeast in vicinity of the Gulf Stream off of the
mid-Atlantic and Carolina coasts into Tuesday and Wednesday.
Mariners should be prepared for gusty thunderstorms in ANZ835
this afternoon through tonight.

The gales will gradually spread northeast tonight through
Wednesday, and the current forecast handles this well. The
northeast progress will be slow as a modest amplitude mid-level
flow develops over the offshore waters, with a ridge axis aloft
over the waters by 12UTC Tuesday. Eventually, as the ridge axis
moves east, the upper low over the mid-Mississippi Valley will
fill and lift northeast as a wave over New England and offshore
by Thursday. Guidance suggests that between 09UTC and 12UTC
Thursday the higher 925mb and 950mb winds exit the offshore
waters. Will also review later forecasts to see if there could be
any need to spread low-end gales into the southern parts of
ANZ805 and ANZ900 by late Wednesday and Wednesday night.
Currently, not confident enough in the more stable areas to
include.

Beyond this, the forecast will currently remain hazard-free in
the OPC offshore waters for the end of the week through the
weekend. The UKMET is the most robust with winds reaching gale
force in the Gulf Stream waters off of the North Carolina and
Virginia coasts by Saturday afternoon. This is a result of the
UKMET being stronger and slower with the track of surface low
pressure moving over or just north of New England later in the
week. The ECMWF and GFS are similar in track and strength with
the low then, and neither of those models provides for sustained
winds greater than about 30 knots. Certainly will monitor later
forecasts, and the chance for gales then is non-zero, but there
is definitely not enough confidence now to include any gales
then. Unless there is a noticeable trend up in the ECMWF and GFS,
would anticipate the next forecast to remain sub-gale for the
weekend.

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

The latest NCEP surface analysis shows a high pressure ridge
across the offshore waters, and a developing low pressure system
currently inland over the southern CONUS. The GOES-E infrared
satellite imagery shows the system moving E towards the NT2
offshore waters, along with showers and some isolated tstms ahead
of the associated cold front. The convective activity would
indicate that the surface front is fairly strong, and that there
is some moisture and instability ahead of the front as it moves
toward the coast. The 00Z GFS indicates that the front will
approach the area today, before it moves offshore tonight. The
models are all indicating an increase in the winds ahead of the
front as it interacts with the slowly departing ridge, and the
GFS is good agreement with the 00Z ECMWF on the timing of the
front over the next 48 hours as it moves into the region.
However, the 00Z GFS appears to be a bit stronger than the rest
of the 00Z models with the winds, and indicates a bullseye of
higher winds up to 45 kt in the NT2 waters on Tue near the Gulf
Stream. The 00Z NAM/ECMWF/UKMET/GEM are all weaker with the
winds, with all showing marginal gales. Currently thinking at
this time is that the GFS is having some feedback issues,
especially with the latest SREF guidance showing a high
probability that the current convective activity will follow
along with the front as it moves offshore. As a result, planning
on starting out with the 00Z GFS 10m winds through the first 48
hours, opting for the first sigma winds in unstable areas to
account for the deeper mixed layer but capping at 40 kt to
account for the aforementioned feedback issues. It follows from
this that confidence is above average on the short range gales,
mainly due to the good agreement in the 00Z guidance and also
from the strong low level jet that is expected to set up ahead of
the front and induce mixing from the vertical shear. As a result,
planning on having gale headlines consistent with the 00Z GFS,
which is also close to continuity.

The 00Z models start slowly diverging on the timing and track of
the low center over the next several days as it turns N and
stays along the coast Wed into Thu. The 00Z GFS trended slightly
W from its previous run, while the 00Z ECMWF took a significant
shift to the W and is now close to the 00Z UKMET solution.
However, there is still some uncertainty on the track of the low
as the 00Z ensemble guidance is showing a stronger signal for a
track closer to the coast. This is near the 00Z GFS track and
also close to the track from the 22/12Z ECMWF. Confidence is not
very high on the 00Z ECMWF as a result of the large swings in
the track, and the changes from the previous run of the ECMWF
continue with the next low to affect the offshore waters later in
the period on Thu and Fri. Most of the differences emanate from
differences on the handling of multiple shortwave troughs over
the area, so preferring a solution at this time that does not
stray too far from continuity as confidence is somewhat low in
the guidance from the large model spread. For the wind grids, am
planning on continuing with the 00Z GFS to 12Z Thu. Will then
blend the 00Z GFS 10m winds with the 22/12Z ECMWF winds through
the end of the forecast period.

Seas...The 06Z sea state analysis shows generally light seas in
the W Atlc under the high pressure ridge, and the 00Z Wavewatch
and ECMWF WAM are both initialized well with the wave heights.
The Wavewatch develops seas a little faster than the ECMWF WAM
over the short term as a result of the previously mentioned
issues with the GFS winds being overdone. For the wave height
grids, so am planning on using a 50/50 blend of the two models
as a compromise solution which will also act to tone down the
wave heights from the Wavewatch. Will then switch to the 12Z
ECMWF WAM in the blend with the Wavewatch to account for the
preferred solution of the 22/12Z ECMWF winds blended with the
00Z GFS.

Extratropical Storm Surge Guidance...The 00Z ESTOFS is nearly 50
percent higher than the 00Z ETSS with the surge along the SE
coast this afternoon into tonight in the increasing easterly
fetch ahead of the developing low pressure system, and seems a
little more reasonable with the strength of the winds as the
ESTOFS model indicates values close to 2 ft to the S of Cape
Lookout. The models then show just over 1 ft in NY Bight and Long
Island Sound Wed as the system moves N, and then just under 1 ft
along the New England coast on Thu. The guidance seems
reasonable given the forcing from the GFS winds, though
confidence decreases with time as a result of the increasing
model spread in the 00Z guidance.

.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ810...South of New England...
     Gale Possible Wednesday.

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

$$

.Forecaster Figurskey/Kells. Ocean Prediction Center.



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