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104
AGNT40 KWNM 230745
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
345 AM EDT Wed Aug 23 2017

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

The late evening ASCAT overpass indicated S and SW winds around
25 kt occurring from the Gulf of Maine southwest to off the
Jersey shore. These winds matched up well with the ongoing OPC
forecast. The 06Z preliminary OPC-NCEP surface map shows a cold
front extending from upstate New York south and southwestward to
the Ohio valley with a pre-frontal trough nearing the New England
and mid-Atlantic coasts. The latest radar, satellite and
lightning density data shows scattered thunderstorms near the
trough extending from the Gulf of Maine southwest to near and
east of Long Island toward the southern Jersey shore. A few
widely scattered thunderstorms are being detected further south
off the north and south Carolina coasts. The latest SREF
thunderstorm guidance indicates that there will be a good chance
for scattered thunderstorms mainly in advance of the trough and
cold front over NT1 into NT2 waters today, with the thunderstorms
shifting southeastward with the cold front tonight through
Friday. Local wind gusts exceeding gale force and very rough seas
can be found in and near any of the stronger thunderstorms over
the region during the next few days.

Overall, the 00Z models are quite similar from their previous
cycles and with each other across the region into Friday. As a
result, for the early morning package we will use a 70 percent
previous forecast, 30 percent blend of the 00Z GFS 10 meter
winds, just to freshen up the grids and forecast slightly.
Overall, there will be no major changes to the previous forecast
for today into Friday, with confidence levels near to above
average.

All eyes will then become focused on the potential for low
pressure to form south of the offshore waters by Sat. The 00Z
ECMWF appears to be in good agreement with the 12Z ECMWF for Fri
night into Sun night over the offshore waters. We will use a
50-50 blend of the 12Z ECMWF from yesterday and the 00Z ECMWF to
populate the wind grids from 00Z Sat through Sun night as a
result. This will also maintain continuity from the previous
forecast. The low is forecast to slowly organize south of the
waters during the day Sat into Sun night, with the low
potentially lifting north or northeast into the waters during the
early to middle parts of next week. The 00Z ECMWF is now weaker
and slower with the low as it moves into the waters next week,
with the 00Z GFS also weaker, but faster than the ECMWF. For the
morning package we will use a 75 percent previous forecast, and
25 percent 00Z ECMWF for the forecast for early next week beyond
day 5, as we are not quite willing to jump all the way toward the
slower and weaker 00Z ECMWF solution at this time. There remains
the threat for gales over portions of the NT2 waters early next
week, beyond day 5. Confidence levels in the forecast are near
average into the weekend, and then fall to below average for
early next week as model differences increase in both timing and
intensity of the previous mentioned low. The latest tropical
outlook from the National Hurricane Center continues to forecast
only a 30 percent chance for tropical cyclone formation of this
system through day 5, with development expected to be slow to
occur. Please closely monitor the latest OPC forecasts and NHC
outlooks over the next several days.

.Seas...For the early morning package it appears as if the
previous grids are running close to the latest observations and
altimeter data over the offshore waters. We will remain close to
the previous grids and forecast for today into Fri, and then to
match the winds use an even blend of the previous forecast and
00Z ECMWF WAM from Fri night into Sun night, and then a 75-25
blend of the previous forecast and 00Z WAM thereafter in an
attempt to match the wind forecast.

.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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