North Atlantic Marine Weather Discussion

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AGNT40 KWNM 231326
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
826 AM EST Thu Nov 23 2017

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

GOES visible satellite imagery indicates a strong low pres system
centered NE of the area over the Gulf of St Lawrence, and cold
air cumulus streaming across the offshore waters in the cold
advection behind the associated cold front now E of the area. 
The Ascat winds from last night indicated gales in the cold
advection over the offshore waters, but the strongest cold
advection has moved off to the NE and current surface reports 
show up to 20 kt in the NE zones. 06Z GFS 10m winds show a few 
remaining 25 kt winds over the eastern Gulf of Maine at 12Z, but 
the models all indicate the winds will decrease early today as a 
high pres ridge builds over the W Atlc today into tonight. The 
previous forecast started out with the 00Z GFS for today which is
supported by the 06Z run, so planning on staying with it in the 
update package.

The satellite also indicates low pres over the NE Gulf of Mexico,
and the 00Z GFS/ECMWF indicate the low will move NE into the srn
NT2 offshore waters late this afternoon into tonight along the 
tail end of the aforementioned frontal boundary. The models all
indicate the winds will increase N of the low as it moves over
the Gulf Stream, with the GFS showing few gales tonight in the
enhanced NE flow. The 00Z ECMWF is a little slower with the track
of the low, but overall agrees well with the GFS on taking it
over the Gulf Stream. It is slightly weaker with the winds, and
indicates up to 30 kt. However, confidence with the gales is low
as the models all indicate only about 30 kt at 925 mb, so the
likelihood of gales mixing down is low. Also, there is still a
fair amount of disagreement on the low center itself, with the
GFS a bit faster than the 00Z ECMWF/UKMET/GEM. There is also a
strong signal in the 06Z GEFS members for a slower solution. As a
result, planning on staying with the previous grids which were
based on the 00Z ECMWF solution. Will also cap winds at 30 kt in
the NE flow.

In the medium range, the GFS/ECMWF come into better agreement on
the low as it moves well NE of the area. The models indicate
strong cold advection behind the front by Sun into Sun night over
NT1 and nrn NT2. The GFS has been a bit stronger with the winds
in the cold advection, and shows a few gales in the first sigma
winds. However, it is not supported well by the rest of the 00Z
guidance with only the 00Z GEM indicating a brief period of
marginal gale force winds. The previous forecast stayed with the
00Z ECMWF through the medium range period, so planning on staying
with it as it shows a brief period of 30 kt in the cold
advection. Otherwise, not planning on any major changes through
the remainder of the forecast period.

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PREVIOUS DISCUSSION...

The latest GOES satellite imagery, observations and preliminary
06Z OPC-NCEP surface analysis shows a deepening 990 mb low over
central Nova Scotia moving NE away from the offshore waters with
high pressure centered over the Ohio Valley moving E. An ASCAT
overpass from last evening indicated gale force winds moving E
from the far NE NT2 and eastern NT1 waters associated with the
deepening low. The threat for gales over the offshore waters is
expected to remain rather low for the next 3-5 days.

Over the short term, the 00Z models are in very good agreement
over the region today with a high pressure area building E into 
the offshore waters as the low mentioned above tracks NE away 
from the region. A low currently over the NE Gulf of Mexico will
move E and NE over the next few days, and allow conditions to
deteriorate over southern NT2 waters by tonight. For today, we
will stay close to the 00Z GFS guidance, and use the smart tool
which places the higher first sigma layer winds over the 
unstable areas, and slightly lower 10 meter winds over the 
stable locations. For tonight through Sun, we will then trend 
the forecast more toward the 00Z ECMWF guidance as it is quite 
similar to the 12Z Wed ECMWF that was used for the previous 
package. This will also maintain continuity as the GFS remains 
rather unstable at this time, beyond today. Therefore, we still
expect low pressure to move into the far southern NT2 waters
later tonight into Fri, continue organizing over the SE NT2
waters Fri night, and then pass just E of the central and 
northern NT2 waters Sat. A moderate to strong cold front will
approach the waters from the W and NW Sat. Any gales that 
develop with the low are expected to remain E of the offshore 
waters at this time, with chances for gales over the offshore
waters not high enough to add to the forecast at this time. 
There will be an increasing threat for thunderstorms mainly over
southern NT2 waters tonight into Fri night, with local wind
gusts near or exceeding gale force and rough seas possible in
and near of the heavier thunderstorms. The low will then lift NE
away from the waters Sat night into Sun as the cold front 
crosses the waters from NW to SE, with cold air advection 
beginning over the waters at that time. High pressure will then
begin to slowly build E toward the area during Sun.

Over the medium range, we see no need to deviate too much from
the 00Z ECMWF guidance for Sun night through Mon night over the
region, with the 00Z ECMWF remaining quite consistent. The high
will slowly overspread the waters from W to E Sun night through
Mon night with improving conditions likely over the region. Any
gales associated with the rather strong cold air advection will
likely remain E of the NT1, and northern NT2, waters Sun night
into Mon evening.

Seas...Once again the 00Z ECMWF WAM appears to have initialized
slightly better when compared to the current data over the 
offshore waters. In order to fit the sea heights forecast with 
the wind forecast will we stay fairly close to the 00Z ECMWF WAM 
through Mon night over the region, which will tend to result in 
little change from the past few forecasts. We will make only a 
few manual adjustments to fit sea height grids to initial 
conditions, and to fit with nearby coastal WFO and TAFB grids.

.EXTRATROPICAL STORM SURGE GUIDANCE...n/a. 


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Kells/Mills. Ocean Prediction Center.