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AGNT40 KWNM 201945
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
345 PM EDT Sun May 20 2018

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

Over the short term, the morning ASCAT overpass from around
1349Z missed a good portion of the offshore waters, but it did
catch the outer New England and northern mid-Atlantic zones
showing SW winds to 25 kt or so over these waters. The 12Z high
resolution Warw model winds are very close to these ASCAT winds.
Therefore, we will rely on something close to the 12Z Warw winds
for the afternoon package, tonight through Mon night. We will
cap winds at 30 kt over the offshore waters, as any gales will
likely occur in and near thunderstorm activity over the waters
during the next few days, or over areas east of the offshore
waters. The latest SREF thunderstorm guidance continues to show
the best chance for stronger storm off the SE U.S. coast into
tonight, and then near a cold front advancing SE over the New
England and mid-Atlantic offshore waters tonight and Mon. High
pressure will overspread waters in the wake of the cold front
Mon as it builds E and SE off the New England coast. The cold
front will stall over central NT2 waters later Mon and Mon night
as high pressure passes N of the front, and a weak high pressure
ridge dominates the southern waters, off the SE coast. Overall
confidence levels are above normal during the short term portion
of the afternoon package.

Over the medium range, we expect the stalled front to return N
as a warm front Tue into Tue night as low pressure organizes
over northern New England. The low will then pull a trailing
cold front SE over the region Wed into Thu as the low crosses
the New England waters early Wed, and reaches a position S of
Newfoundland by Wed night. High pressure will follow the front
and build off the E coast Thu and Thu night as the front stalls
and weakens over the southern NT2 waters, off the SE coast. The
12Z models are in good agreement over the area Tue through Thu
night. For the afternoon package we will use a 50/50 blend of
the 12Z GFS first sigma layer winds and 12Z ECMWF winds during
this time frame as we feel it will be representative of the
pressure gradient over the waters at that time. For Fri and Fri
night we will trend the forecast more toward the 12Z GFS as it
has been the most consistent over the past few days, has support
from the 12Z UKMET, and is rather close to the WPC medium range
guidance over the waters later in the week. The high will move
off to the E of the waters during Fri into Fri night, with the
front dissipating off the SE coast, and the next cold front
approaching the region from the NW. Confidence levels are close
to average over the medium range.

Seas...Both the 12Z Wavewatch and 12Z ECMWF WAM guidance appears
to be running a little on the low side mainly off the mid-
Atlantic and SE coast per the latest observations over the
waters, with maximum seas heights close to 9-10 feet. For the
afternoon package we will remain close to the previous grids and
adjust them manually to fit current conditions over the area
this evening into tonight. For Mon through Thu night we will use
a 50/50 blend of the Wavewatch and WAM models, and then as we
trend the forecast more toward the GFS winds as noted above,
also trend the forecast more toward the Wavewatch for Fri and
Fri night to match the wind forecast. Some manual editing will
be done in deference to nearby coastal WFO and TAFB grids over
the region during the next few days, as well.

.EXTRATROPICAL STORM SURGE GUIDANCE...n/a.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Mills. Ocean Prediction Center.


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