North Atlantic Marine Weather Discussion
AGNT40 KWNM 201408
Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1008 AM EDT Mon May 20 2019
.FORECAST DISCUSSION...Major features/winds/seas/significant
.weather for the North Atlantic Ocean W of 50W from 30N to 50N.
A warm front has moved N into the Gulf of ME this morning and
passes NE of the area later this morning into the afternoon. A
cold front was inland and nears the N and central areas near the
coast by early tonight. At the moment winds were SW to about 25
kt over the New England into the northern mid Atlc waters. There
was a ship observation at around 11Z near 39N71W which showed SW
of 35 kt, but QC history with 33 wind observations shows a plus
14 kt bias. As the cold front enters the offshore waters later
tonight winds to 30 kt look likely. Elsewhere low pressure was
near 27N69W with model guidance tracking it NE. NHC currently has
about 60 percent chance of the system becoming a subtropical or
tropical cyclone later today or tonight. This system should
steer E of the offshore waters as the cold front moves SE into
the N and central mid Atlc waters later tonight and Tue. For the
morning update will be making few changes to previous forecast
grids which look reasonable.
The latest NCEP surface analysis indicates high pressure across
the W Atlc at 06Z with what was earlier a stationary front across
the central NT2 waters has become a warm front from the Delmarva
Peninsula SE. A complex area of low pressure extends from the
northern Great Lakes to northern New England with main center to
the W. The GOES infrared satellite imagery along with the
lightning density product shows a few remaining isolated
thunderstorms over the Gulf of Maine, New England and eastern NY
state and some scattered showers over the outer central NT2
waters near the front, mainly over the warmer SSTs in the Gulf
Stream. The previous weather grids had showers and tstms, so
planning on continuing with that in the next forecast while
making some adjustments based on current data. Otherwise, current
surface reports indicate winds 15 kt or less over the offshore
waters except 20 kt in the Gulf of Maine and S of New England in
the increasing southerly flow ahead of the approaching low
pressure, and also indicated in the evening ASCAT wind
retrievals. The 00Z models indicate that the winds will continue
increasing today as the low pressure passes just N of the area
alsong with its cold front approaching the coast. The 00Z
GFS/ECMWF indicate winds increasing to 25 kt over the Gulf of
Maine with a low level jet strengthening over NT1 just ahead of
the front. However, with the model 925 mb winds only indicating
roughly 45 kt ahead of the front, and with a strong low level
inversion expected to develop with warm advection over cold SSTs
the probability of anything higher than 25 kt seems low at this
point. As a result, am planning on starting out with the 00Z
W-ARW in the next forecast since it is representative of the 00Z
model guidance which is in good agreement for today through Tue.
The 00Z models then agree fairly well on the front passing
across the offshore waters Mon night and Tue as the associated
low center passes to the N of the region. The GFS is signaling a
chance of the SW winds reaching 30 kt over NT2 near the Gulf
Stream, though the rest of the 00Z guidance has remained
slightly weaker while the 00ZW-ARW also shows a chance of 30 kt.
Hence will stay with the W-ARW through Tue. The models then
indicate a better chance of stronger winds in the post- frontal
cold advection, as the stability becomes negative with unstable
lapse rates near the surface. This should allow for a deeper
mixed layer, though the 12Z ECMWF/UKMET have trended weaker. The
00Z GFS still indicates a marginal chance of gales in the outer
nrn NT2 water for Tue night, so confidence is just below average
on the previous gale headlines in this area. Since the gales are
in the forecast already, am planning on populating with a
blend of the previous grids with the 00Z GFS using the tool that
places the higher GFS first sigma layer winds in unstable areas
and 10 meter winds in stable areas.
Also of note, the latest NHC Outlook shows a 60 percent chance
of tropical cyclone development well S of Bermuda over the next
48 hours, and the guidance agrees fairly well on a low developing
and heading NE towards Bermuda. All of the 00Z guidance keeps
the system E of the offshore waters over the forecast period, and
indicates it will be absorbed into the aforementioned front Wed.
However, this system bears watching, so those with interests in
the region should monitor the latest NHC Outlooks and forecasts.
In the medium range, the 00Z models indicate a high pressure
ridge will build over the W Atlc behind the low pressure and
front Wed into Thu, before a developing surface low approaches
from the NW late Thu then moves SE off the northern mid-Atlantic
coast Fri. The 00Z models have all trended stronger with this
system as it moves SE pqast the Gulf Stream, with most to
varying degrees showing gales. The models disagree some on the
timing, track, and intensity of the low with 00Z ECMWF strongest
and farther N, actually extending gales into the southern NT1
waters. For this portion of the forecast will use the 00Z GFS
Wed and Wed night with support from other guidance then
transition to the 00Z UKMET which is closer to prior tracks of
the models and also the latest WPC medium range guidance and for
now limit gales to the northern and central NT2 outer waters.
Confidence is only average as there may need to be adjustments
to warnings in later forecasts based on new guidance.
.Seas...The 00Z Wavewatch and ECMWF WAM are initialized well
over the offshore waters with the latest observations showing
wave heights up to 5 or 6 ft over the region. Since the guidance
agrees fairly well over the forecast period, am planning on
using an even blend of the two solutions to especially later in
the forecast to account for some differences in the models.
.EXTRATROPICAL STORM SURGE GUIDANCE...No significant positive
surge events are expected during the next few days. Please
consult products issued by your local National Weather Service
forecast office for additional information.
.NT1 New England Waters...
.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
Gale Possible Friday.
.ANZ920...Baltimore Canyon to the Great South Channel...
Gale Possible Friday into Friday night.
.ANZ905...The Great South Channel to the Hague Line...
Gale Tuesday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
Gale Possible Friday into Friday night.
.Forecaster Rowland/Bancroft. Ocean Prediction Center.