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AGNT40 KWNM 292342
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
742 PM EDT Mon May 29 2017

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

The moderately strong surface low near 39N66W continues to track
E pretty much as previously expected. Earlier ASCAT scatterometer
passes showed max winds in the vicinity of this low were in the
20-30 kt range and per the latest model runs suspect winds remain
just below gale force over the NE NT2 waters. Max associated seas
are likely in the 7-10 ft range immediately S of the low, which
are being handled slightly better by the 12Z ECMWF WAM versus the
18Z Wavewatch III.

Over the short term, the latest models present no significant
forecast problems. The models all forecast the surface low to
pass E of the NT2 waters tonight into Tue with a trailing cold
front becoming stationary across the central NT2 while the
associated conditions gradually diminish. Then late Tue into Wed
night, would favor the similar 18Z/12Z GFS and 12Z ECMWF
solutions which look like a reasonable compromise between the
more progressive 12Z GEM and slower 12Z NAVGEM/UKMET solutions of
the front lifting slowly NEwd as a warm front with a weak low
developing on the front with max associated winds generally in
the 15-25 kt range, or 30 kt at most. So with the 18Z GFS
remaining consistent, will continue to use the previously
populated 12Z GFS winds for this fropa with just some minor
additional edits in deference to the 12Z ECMWF.

In the long range, would continue to favor the similar 18Z/12Z
GFS and 12Z ECMWF forecast timing for a weak cold front to push
SE across the NT1 and Nrn/central NT2 waters Wed night into Thu
night with weak associated gradients (generally no higher than
15-20 kt). So again the previously populated 12Z GFS winds still
look representative.

Then Fri night into Sat night, the 12Z ECMWF looks like a
reasonable compromise between the slower 18Z/12Z GFS and 12Z
UKMET and more progressive 12Z GEM solutions for the next
moderately strong cold front to push SE across the NT1 and Nrn
NT2 waters and its also in better agreement with the latest WPC
Medium Range guidance. So will continue to use the previously
populated 12Z ECMWF boundary layer winds and associated WAM seas
then.



-----------------------------------------------------------------
PREVIOUS DISCUSSION...

Over the short term no major changes to the ongoing OPC forecast
appear needed. The 18Z OPC-NCEP surface analysis indicated 1007
mb low pressure near 38N 68W moving off to the east and
southeast. A late morning ASCAT overpass indicated a few gales
developing over the southeast quadrant over the low, just east
of the offshore waters.

The 12Z models all appear to handle the movement and intensity
of this low rather well when compared to the ASCAT overpass, and
observations. Therefore, we will stay close to the previous
grids into tonight, and keep any gales associated with the low
east of the northern NT2 offshore waters. For Tuesday into
Wednesday night we will rely on the 12Z GFS as it has good
support from the 12Z ECMWF and 12Z NAM guidance over the region.
We will also continue to use the smart tool which places the
stronger first sigma layer winds over the more unstable areas
and warmer SSTs near and south of the gulf stream; and somewhat
lighter 10 meter winds over the cooler waters mainly north and
west of the stream. A cold front associated with the low noted
above will become stationary over the central NT2 waters
tonight. The front will drift north as a warm front Tuesday and
Tuesday night with weak low pressure developing along the front
over northern NT2 waters. The low will track northeast along the
front later Tuesday into Wednesday, reaching a position near or
just south of Georges Bank by Wednesday evening. The low will
then likely dissipate east of the waters Wednesday night as a
weak cold front slides east and southeast over the waters.
Scattered thunderstorms are likely near the low described above
into tonight, with the warm front and weak low Tuesday into
Wednesday, and near the weak cold front Wednesday night. Locally
very gusty winds and rough seas can be expected in or near any
thunderstorm activity. Confidence levels are near to above
average for the short term part of the forecast tonight through
Wednesday night.

Over the long term, the 12Z model guidance remains in generally
good agreement over the waters Thursday and Thursday night. The
cold front will continue to move east and southeast Thursday,
then stall over southern NT2 waters Thursday night as weak high
pressure builds east over waters north of the front. The 12Z
models appear to be coming into somewhat better agreement over
the region Friday into the weekend. For now, we will continue to
follow the 12Z GFS winds for Thursday and Thursday night, and
then trend the forecast more toward the 12Z ECMWF model Friday
into the weekend as it appears to be a bit more consistent and
in good agreement with the latest WPC medium range forecast.
Therefore, we still expect a moderate to strong cold front to
approach the region from the west and northwest Friday into
Friday night, with increasing southwest winds ahead of the front
over most of the offshore waters. The front will slide southeast
over NT1 waters Saturday, then stall over northern NT2 waters
Saturday night as low pressure organizes along the front over
the midwest. There is some chance for gales developing in the
southwest flow ahead of the front Friday into the weekend. For
now, we will keep winds below gale force and continue to monitor
future model cycles to see if any gales might need to be added
to the offshore forecast. The confidence in the forecast is near
average from Thursday into Saturday night.

.Seas...Both the 12Z Wavewatch III and 12Z ECMWF WAM appear to
have initialized the current seas quite well again this
afternoon. We will rely on the 50/50 blend of these two wave
models through Thursday night, and then as we switch to using
more of the 12Z ECMWF for winds for Friday into Saturday night
also rely on the 12Z WAM for seas heights during this time
frame. Also, it appears that the WAM may be building seas a
little too high later in the forecast period. We will,
therefore, cap sea heights at 11-12 feet or so by Saturday
night, and continue to adjust sea heights as needed in future
OPC forecasts.

.Extatropical Storm Surge Guidance...N/A.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Vukits/Mills. Ocean Prediction Center.



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