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AGNT40 KWNM 201335
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
835 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 preliminary 12Z NCEP-OPC surface analysis shows a secondary
cold front moving further south and extending east from the
Jersey shore, roughly along 40-41N. High pressure is expanding
south and southeast from southeast Canada. Overall, the high
will become the dominate weather feature over the waters this
afternoon through Wednesday. For the morning update we will not
make major changes. We will fit the previous forecast to nearby
WFO and TAFB forecasts, and current conditions. Low pressure
potentially lifting northeast ahead of and along an approaching
cold front will likely impact conditions later in the week and
next weekend, with deteriorating conditions expected at that
time. For now we will pretty much leave the previous forecast
alone, and wait for a full set of 12Z models before making any
changes, if needed.

Seas...Again, little change will be made to the previous forecast
for the morning update. We will, however, trend grids a little
closer to nearby TAFB and WFO forecasts, and update grids to fit
current conditions noted just prior to forecast issuance.

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PREVIOUS DISCUSSION...

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 on Day 5. Low pressure is forecast to track east from
the Great Lakes Wednesday night, and reach northern Maine later
Thursday. An associated cold front will approach the New England
and northern mid-Atlantic coast by 00Z Friday, with an increase in
south and southwesterly winds likely over the waters ahead of the
front, which should remain below gale force. The front will slide
southeast over NT1 waters Thursday night and early Friday as the
low moves over the Canadian Maritimes. The front will stall along
40N or so Friday, before lifting north as a warm front by Friday
night as a strong low pressure system tracks from the Plains to
the Great Lakes. 00Z GFS is too fast and 00Z ECMWF diverges north
from its previous track of the low Wednesday night into Thursday
night. Also the 00Z ECMWF departs from its previous run in
pulling a low north into the NT2 waters from east of Florida ahead
of the strong system developing and moving toward the Great
Lakes on Day 5. 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 approaching the coast Friday night.

.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 Mills/Bancroft. Ocean Prediction Center.



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