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000
AGNT40 KWNM 200911
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
411 AM EST Mon Feb 20 2017

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

The latest GOES infrared satellite imagery and lightning data
indicate thunderstorm activity associated with the weak low which
earlier was moving along 31N has moves southeast of the NT2
waters with another band of thunderstorms appearing well to the
north just east of the central NT2 waters. This band appears
associated with a frontal boundary on the 06Z NCEP surface
analysis. 0125Z high resolution ASCAT-B pass indicates winds
mostly 20 kt or less except a few 35 kt winds appear just ahead
of the weak surface low mentioned above and an associated trough
extending south. These higher winds are likely due to
thunderstorms in that area at that time. Farther north a
secondary low which formed near Nova Scotia at 00Z has moved off
to the southeast and the 06Z analysis shows a secondary cold
front extending west across the central NT1 area.

Over the short term, the weak 1011 MB low located just southeast
of the southernmost zone or near 30N 72W, per the preliminary
06Z OPC-NCEP surface analysis will move southeast away from the
waters today while taking its scattered thunderstorms with it.
The 00Z global models remain in good agreement with the previous
runs through midweek. For the early morning package, as the 00Z
models have come into better agreement with the development and
track of low pressure area(s) strengthening east of the offshore
waters and like in previous model cycles call for boundary later
winds staying below gale force before the circlation of the low
pressure system moves off to the northeast away from the waters
by Wednesday. We will use the representative 00Z GFS for the wind
grids through 12Z Thursday as high pressure becomes the dominant
feature over the waters by Monday night and Tuesday as it builds
southeast across the region, and low pressure organizes east of
the waters before lifting to the northeast. High pressure slowly
passes east and southeast of the area by Wednesday. A warm front
will lift northeast over NT1 waters Wednesday into Wednesday
night, with a high pressure ridge dominating most of the NT2
waters. Confidence levels are above average over the region
through Wednesday night.

For Thursday and beyond some model differences are noted, which
increase ofdshore on Day 5 as the faster sulution of the 00Z
ECMWF departs from its previous run in pulling a low north into
the NT2 waters from east of Florida. This is not supported by 00Z
ECMWF ensemble mean and GEFS ensemble members. For the remainder
of the forecast followed the continuity of the previous forecast
and used the old 19/12Z ECMWF. Otherwise model agreement is good
on timing and strength of cold front moving offshore Friday and
Friday night, with an increase in south and southwesterly winds
likely over the waters ahead of the front. These winds are
expected to remain below gale force. The Thursday to Friday night
forecast is similar to WPC medium range guidance.

.Seas...Both the 00Z Wavewatch and ECMWF WAM appear to have
initialized fairly well over the offshore waters, with the
observations and altimeter data indicating sea heights within a
foot or so of the model guidance. For the new package we will
continue to use a 50/50 blend of these two similar models through
Wednesday night, and then as we transition to using the 19/12Z ECMWF
for winds also transition toward the ECMWF WAM for forecast sea
heights.

.Extratropical storm surge guidance...N/A.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Bancroft. Ocean Prediction Center.


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