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000
AGNT40 KWNM 181338
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
938 AM EDT Tue Sep 18 2018

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

No significant changes are planned for the morning update. The
06Z GFS remains consistent with the previous 00Z run throughout
the forecast period, and also seems to be trending closer to the
other global models. The current warning headlines also still
look reasonable based on the track of the low forecast to move
east of the NT2 waters and high pressure building into western
portion of the waters. The 12Z sea state analysis indicated that
observed seas continue to match up well with the WNA and ECMWF
WAM forecast values, with a max of 7 FT just east of the central
New Jersey coast. Will also not be making any changes to the wave
grids at this time.

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

The western edge of the 0130Z Ascat overpass across the
Mid-Atlantic waters returned south winds to 20 kt from east of
Delware Bay to southeast of Cape Hatteras, but only within about
40 nm of the coast. The 00Z models generally agree that a series
of mid/upper level disturbances embedded within the broad trough
across the eastern US will move east off the coast and across
the New England and Mid-Atlantic offshore waters today into Wed
night. However, the models differ significantly at the surface
with multiple low developments associated with these upper level
shortwaves, including the evolution of post-tropical cyclone
Florence, which at 06Z was centered inland over central
Pennsylvania. Given the number of surface lows which the models
are developing along the front in responce to these upper level
vorticity maximas, it is difficult to pinpoint the progression
of post-tropical Florence at the mid levels or at the surface.
Our forecast confidence with the details of the timing and
development of these surface low is below average. With the 00Z
GFS trending toward the ECMWF/UKMET/CMC, at least through Wed
night into Thu, the guidance has come into better agreement with
the amplifying upper shortwave at the base of the upper trough
Wed and Wed night. This is the feature which the past several
runs of the UKMET and ECMWF have been focusing with gales
appearing more likely within the strong northeast flow across
the outer Mid-Atlantic zones late Wed into Thu. In addition, the
00Z HiResW/NMM and ARW and 00Z NAMNest are all advertising gales
as well. The wind grids were mostly based on the 00Z ECMWF
through the forecast period. We did blend some of the 00Z GFS
first sigma level winds with the ECMWF Wed through Thu. With the
high pressure ridge building southeast to the New England and
Mid- Atlantic coasts, it will not take a very strong surface low
to result in the northeast gales, especially given the slightly
unstable low levels across the warmer Gulf Stream waters.

In the medium range, the non-GFS models all move this southern
surface low back westward toward the southeast US coast late in
the week into the weekend. We maintained forecast continuity and
continued to populate the grids with the ECMWF winds through the
weekend. Will reiterate that forecast confidence is below
average through the forecast period.

OPC continues to provide guidance to WPC for the post-tropical
phase of Florence as it passes E over the OPC offshore and high
seas areas. We expect Florence to become absorbed into the
strong baroclinic zone just east of the offshore waters Thu.

Post-Tropical Cyclone Florence Advisory Number 75:

24H  19/0600Z 39.5N 71.5W 25 KT 30 MPH...POST-TROPICAL
36H  19/1800Z 38.5N 67.5W 30 KT 35 MPH...POST-TROPICAL
48H  20/0600Z 38.0N 64.5W 30 KT 35 MPH...POST-TROPICAL
72H  21/0600Z ABSORBED ALONG FRONT

Seas: Both the 00Z Wavewatch and 00Z ECMWF WAM were well
initialized with west Atlantic wave heights this morning. We
used a 50/50 blend of the aforementioned models through Thu, and
then boosted these wave heights by 10 to 20 percent across the
areas with the strong to gale force northeast winds Wed into
Thu. Beginning Thu night, we transitioned to the 00Z ECMWF WAM.

Extatropical Storm Surge Guidance: No significant positive
extratropical surge events appear likely during the next few
days.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Wednesday into Wednesday night.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Wednesday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Wednesday night.
     Gale Possible Thursday.

$$

.Forecaster Kosier/Clark. Ocean Prediction Center.


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