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AGNT40 KWNM 192044
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
444 PM EDT Mon Aug 19 2019

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

Per the 18Z NCEP surface analysis a stationary front extended
off the SE coast of Maine, and SE across Georges Bank over the
NT1 waters. A 1015 mb low was located over the NE NT2 waters
near 39N68W with a trough extending SE from the low center.
Another low pressure trough was located over the central NT2
waters in zone ANZ925. The latest GOES-East satellite imagery
and lightning data revealed showers and thunderstorms, mainly
over the NT2 waters. A majority of the thunderstorm activity was
located over the central NT2 waters S of the low, and along the
zones just off the SE coast. The latest ASCT high resolution data
revealed 5-15 kt winds over the NT1 waters. 5-15 kt winds were
also noted over the NT2 waters, except of the NE parts, in the
vicinity of the low, where 20-25 kt winds were noted. A small
area of 30 kt winds were noted over ANZ910, however analyzing
satellite data, those winds likely correspond to shower and
thunderstorm activity in that area.

The 12Z guidance was in good agreement through Friday 12Z, with
some differences noted afterward through about Saturday 12Z in
regards to the speed of a cold front anticipated to push E across
the offshore waters. For this forecast package I will populate
the grids through Tuesday 12Z with the 12Z Warw, then a 50/50
blend of the 12Z GFS/ECMWF through Friday 00Z, then allow the old
grids to remain in afterward. The reasoning for going with a
persistence forecast after Friday 12Z is due to differences in
the speed of the front in the guidance, namely the 12Z
GFS/ECMWF/UKMET, and the belief it`s reasonable to not make
changes and await future model runs to see if the guidance comes
into better agreement concerning the speed of the front.

Stationary front over the NT1 waters lifts N tonight, while the
low over the NE NT2 waters moves NE, then E of the NT2 waters.
Again per the latest ASCT data winds over the NE NT2 waters in
the vicinity of the low are in the 20-25 kt range, so feel
starting off the forecast in those zones with winds in the 20-30
kt range makes sense. A weak cold front slides E across the NT1
waters overnight into Tuesday, with a high pressure ridge moving
overhead behind it. This solution has been consistent in the
models over the last few runs. The next major feature will be the
aformentioned cold front expected to near the E coast late
Wednesday into early Thursday, before pushing E through the NT1
waters later Thursday through Friday night, and the northern and
central NT2 waters later Thursday night into Saturday, with the
front over the southern waters by Saturday night. Again guidance
differs some beginning Thursday night in regards to the speed of
the front as it moves through the waters. The 12Z GFS is the
fastest of the guidance with the 12Z ECMWF/UKMET/GEM/NAVGEM all
slower and with a similar position of the front come Friday. By
Friday night into Saturday however, guidance comes into better
agreement excluding the 12Z ECMWF with then becomes the slower,
outlier model. Again these differences are the reasoning for
going with a persistence forecast after Friday 12Z. That said,
despite the timing differences on Friday into Saturday, guidance
is in agreement in southerly winds increased ahead of the front
Wednesday night, continuing into Friday night into early Saturday
morning. The 12Z GFS, like its 00Z run, remains bullish with
pre-frontal winds Thursday night into Friday and continues to
suggest a gales in the vicinity of the gulf stream over the NE
NT2 waters. That said with no other model support in regards to
this, feel its reasonable to keep winds below gale force.

.SEAS...Will favor the 12Z WaveWatch III through Tuesday 12Z,
then a 50/50 blend of the 12Z WaveWatch III/ECMWF WAM through
Friday 00Z, then allow the old grids to remain in afterwards.

.EXTRATROPICAL STORM SURGE GUIDANCE...The 00Z ESTOFS and ETSS
indicate no significant positive surge events will occur over the
next few days. This seems reasonable considering the atmospheric
models indicating the higher winds well offshore. For the latest
information, refer to the latest forecasts and discussions


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Holley. Ocean Prediction Center.


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