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AGNT40 KWNM 191355
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
955 AM EDT Thu Oct 19 2017

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

A weak stationary front remains draped just south and southeast
of the southern waters this morning, with persistent east to
northeast flow generally south of about 34n resultant from the
combination between this front and high pressure ridging draped
over the central waters. Previous overnight grids contained
forecast winds to about 25 kt, which was based on good overnight
model agreement, but only seeing winds to 20 kt with latest ship
and buoy obs from 12-13z. Will make the initial downward
adjustment accordingly, but still expect winds to increase to
about 25 kt this afternoon as a frontal wave develops along the
boundary well east of the area, but strong enough to sufficiently
increase the pressure gradient over OPC southern zones. This
remains well supported from 06z GFS and NAM.

Farther north, strong surface low over far northern Quebec will
move east and into the Labrador Sea this afternoon and overnight,
pulling a cold front across the northern areas. Models remain in
excellent agreement with timing and strength of boundary, and
expect winds to increase to 25 kt by 21z for areas north of about
41n.

Friday onwards: as far as mid-October is concerned, relatively
calm winds and flat seas expected beneath strong high pressure
ridge over the W Atlantic. Southerly return flow forecast late
Monday night as the high moves farther east, and deep upper
trough begins to approach the East Coast. Overnight shift
preferred the slower non-NCEP solution with respect to timing of
the surface cold front, which was also supported by the overnight
00z WPC guidance. At this time see no reason to make any changes
in the medium range. Will re-evaluate timing and strength of
boundary with upcoming 12z suite of guidance.

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

The 06Z NCEP surface analysis indicates a high pres ridge across
offshore waters, and a stationary front to SE of the southern
NT2 waters. ASCAT from 0224 in the evening indicated up to 25 kt
in the outer waters off the SE coast in the moderately strong
pressure gradient between the front and the ridge, but current
observations from 41002 which is west of the 25-kt ASCAT winds
show around 20 kt. The ASCAT-B pass which was a bit earlier
showed 25 kt farther w, and in fact 41002 at 01Z showed winds 23
G27 kt. The 00Z GFS first sigma winds are initialized well when
compared with the data, and indicates that the winds over the
southern NT2 area will continue in the 20 to 25 kt range as a
wave develops along the front early today. The 00Z ECMWF agrees
well with the GFS on the overall development, and both show the
low developing into an intense system to the E of Newfoundland
Fri night. As far as the NT2 waters are concerned, winds are
expected to slowly decrease below 25 kt by Thu night as the low
and front move E away from the area. In addition, the analysis
shows a ridge extending NE over the NT1 waters and the next cold
front is well inland over the Great Lakes. The 00Z GFS indicates
the ridge will weaken over the waters today with a southerly flow
developing in the NT1 waters ahead of the next cold front. The
00Z GFS first sigma winds look reasonable over unstable areas in
the short range, while the 10m winds seem more representative in
the more stable environment N of the Gulf Stream, so planning on
starting out with the 00Z GFS in the next forecast.

Otherwise, the 00Z models all indicate high pres will dominate
the pattern over the W Atlc through the forecast period, though
they also indicate another weak cold front will pass through the
Gulf of Maine Thu night into Fri. The 00Z GFS indicates SW flow
with 10m winds ahead of the front up to 25 kt. This seems
reasonable as the 00Z ECMWF/UKMET agrees well on the intensity,
and also makes sense since the mixed layer should be somewhat
shallow with the cooler SSTs N of the Gulf Stream helping to
create a stable boundary layer. As a result, planning on staying
close to the 00Z GFS for this system.

In the medium range, the models all indicate another cold front
will approach from the W Mon and Mon night. The GFS again
indicates strong southerly flow ahead of the front, and the 10m
winds seem reasonable and well supported by the 00Z ECMWF.
However, the 00Z GFS and UKMET are a bit faster than the 12Z/00Z
ECMWF and the 00Z GEM is a fast outlier. Also WPC medium range
guidance favors the timing of the 12Z ECMWF, so planning on
switching to 12Z ECMWF after 12Z Mon.

.SEAS...Both the 00Z ECMWF WAM and Wavewatch are initialized
within a ft or two of the current data in the offshore waters.
The models are in reasonably good agreement into early Mon, so
planning on using a 50/50 blend of the two models. Beyond that,
am planning on transitioning to the 12Z ECMWF WAM exclusively to
match the preferred timing of the 12Z ECMWF winds rising ahead of
the front on Mon and Tue.

.EXTRATROPICAL STORM SURGE GUIDANCE...NA.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Collins/Bancroft. Ocean Prediction Center.



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