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North Atlantic Marine Weather Discussion

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AGNT40 KWNM 120203
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1003 PM EDT Tue Aug 11 2020

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

The preliminary OPC-WPC surface analysis indicated a cold front
over New England, from northern Vermont to eastern and southern
New York state, and an inland low pressure trough extending from
Maryland S and SW to northern Georgia. The main concern over the
next day or two will likely continue to be the threat for 
locally strong winds to gale force and rough seas in and near 
thunderstorms, as the front moves offshore Wed night, and 
progresses southward into the northern Mid-Atlantic waters by Thu
night into Fri. The best chance for thunderstorms will be near 
and ahead of the front tonight into later parts of the week. The 
latest lightning data and GOES-IR satellite imagery indicates 
scattered to isolated thunderstorms over many portions of the 
central and southern NT2 waters, with a line of thunderstorms 
over New England, along the cold front as noted previously. The 
forecast becomes more uncertain by the weekend as model solutions
diverge regarding the placement, movement and intensity of low 
pressure forecast to develop and move E along a stalled front off
the Mid- Atlantic coast. For the evening update, no major 
changes will be made to the ongoing forecast, with minor edits in
deference to nearby coastal WFO and TAFB grids.

.Seas: Sea heights range from 1 to 5 ft over the offshore waters
this evening with the highest seas over the Gulf of Maine waters
based on the latest observations. Overall, no major changes will
be made for the evening update from the late afternoon forecast
package.

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PREVIOUS DISCUSSION...

Per the 18z NCEP surface analysis a weak high pressure ridge was
located over the OPC offshore waters. A low pressure trough was
located over the far SE parts of the SE U.S. waters. GOES16 
satellite data and the lightning density product revealed 
scattered to numerous showers and thunderstorms off the SE U.S. 
waters. Hires ASCT overpasses from roughly between 14-16z,
revealed 5-15 kt winds over most of the OPC offshore zones, with
an area of 20-25 kt winds noted off the coast of Georgia near
31N79W.

Through Thu I will use a 1:1 blend of the 12z GFS/12z ECMWF bias
corrected winds through Thu 18z, given the 12z global models were
in fairly good agreement through that time. Models are in
agreement that the weak high pressure ridging over the waters,
slides E through Wed, as a cold front nears the New England and
Mid-Atlantic coast. The front then slowly moves offshore Wed
night into Thu, with the front then becoming nearly stationary
over the N. Mid-Atlantic waters. At this point, the 12z GFS
becomes a slower outlier in regards to low pressure developing
and moving E/NE off the Mid-Atlantic coast Thu night into the
weekend. With the 12z ECMWF/UKMET/CMC and 12z NAM12km in
agreement on moving low pressure E along the front late Fri into
Sat, while the GFS keeps the low inland over the Mid-Atlantic.
It's for this reason I will switch to the 12z ECMWF from Thu 21z
through Sat 12z. Beyond that time frame, the 12z ECMWF actually
becomes faster, moving the low E, then NE of area late Sat into
Sun, while the 12z ECMWF/UKMET/CMC/NAVGEM moves the low E of the
Mid-Atlantic waters early Sun, but then holds the low just E of
the Mid-Atlantic waters through late Sun. Based on that, I felt
it was reasonable to transition to the 12z CMC from Sat 15z and
use that through the end of the forecast period. I will reduce
the 12z CMC winds in the N semicircle of the low, over the New 
England and N. Mid-Atlantic waters, by about 10-15 percent and 
further cap winds at 25 kt or so Sat night through Sun night.
Despite the model differences over the weekend, there are some 
signals in the 12z GFS/UKMET/CMC regarding the possibility of
gales in the N semicircle of the low, but with uncertainty in the
guidance regarding the low track, I felt it was reasonable to not
bite on gales at this time. None the less with uncertainty still
present among the guidance in regards to the weekend regarding 
this low, we will continue to monitor the situation.

.SEAS...I will favor a 1:1 blend of the 12z WaveWatch III/ECMWF
WAM through Thu 18z, transition to the 12z WAM through Sat 12z,
and finally the 12z CMCwave for the rest of the period.


.EXTRATROPICAL STORM SURGE GUIDANCE...No significant positive 
surge events are expected during the next several days. Please 
monitor products from coastal National Weather Service offices 
for detailed water level information.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Mills/Holley. Ocean Prediction Center.