marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 402 am EDT Mon Apr 23 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The latest NCEP surface analysis shows a high pressure ridge across the offshore waters, and a developing low pressure system currently inland over the southern conus. The GOES-E infrared satellite imagery shows the system moving E towards the nt2 offshore waters, along with showers and some isolated tstms ahead of the associated cold front. The convective activity would indicate that the surface front is fairly strong, and that there is some moisture and instability ahead of the front as it moves toward the coast. The 00z GFS indicates that the front will approach the area today, before it moves offshore tonight. The models are all indicating an increase in the winds ahead of the front as it interacts with the slowly departing ridge, and the GFS is good agreement with the 00z ECMWF on the timing of the front over the next 48 hours as it moves into the region. However, the 00z GFS appears to be a bit stronger than the rest of the 00z models with the winds, and indicates a bullseye of higher winds up to 45 kt in the nt2 waters on Tue near the Gulf Stream. The 00z NAM/ECMWF/UKMET/Gem are all weaker with the winds, with all showing marginal gales. Currently thinking at this time is that the GFS is having some feedback issues, especially with the latest sref guidance showing a high probability that the current convective activity will follow along with the front as it moves offshore. As a result, planning on starting out with the 00z GFS 10m winds through the first 48 hours, opting for the first sigma winds in unstable areas to account for the deeper mixed layer but capping at 40 kt to account for the aforementioned feedback issues. It follows from this that confidence is above average on the short range gales, mainly due to the good agreement in the 00z guidance and also from the strong low level jet that is expected to set up ahead of the front and induce mixing from the vertical shear. As a result, planning on having gale headlines consistent with the 00z GFS, which is also close to continuity.
The 00z models start slowly diverging on the timing and track of the low center over the next several days as it turns N and stays along the coast Wed into Thu. The 00z GFS trended slightly W from its previous run, while the 00z ECMWF took a significant shift to the W and is now close to the 00z UKMET solution. However, there is still some uncertainty on the track of the low as the 00z ensemble guidance is showing a stronger signal for a track closer to the coast. This is near the 00z GFS track and also close to the track from the 22/12z ECMWF. Confidence is not very high on the 00z ECMWF as a result of the large swings in the track, and the changes from the previous run of the ECMWF continue with the next low to affect the offshore waters later in the period on Thu and Fri. Most of the differences emanate from differences on the handling of multiple shortwave troughs over the area, so preferring a solution at this time that does not stray too far from continuity as confidence is somewhat low in the guidance from the large model spread. For the wind grids, am planning on continuing with the 00z GFS to 12z Thu. Will then blend the 00z GFS 10m winds with the 22/12z ECMWF winds through the end of the forecast period.
Seas...the 06z sea state analysis shows generally light seas in the W Atlc under the high pressure ridge, and the 00z wavewatch and ECMWF wam are both initialized well with the wave heights. The wavewatch develops seas a little faster than the ECMWF wam over the short term as a result of the previously mentioned issues with the GFS winds being overdone. For the wave height grids, so am planning on using a 50/50 blend of the two models as a compromise solution which will also act to tone down the wave heights from the wavewatch. Will then switch to the 12z ECMWF wam in the blend with the wavewatch to account for the preferred solution of the 22/12z ECMWF winds blended with the 00z GFS.
Extratropical storm surge guidance...the 00z estofs is nearly 50 percent higher than the 00z etss with the surge along the se coast this afternoon into tonight in the increasing easterly fetch ahead of the developing low pressure system, and seems a little more reasonable with the strength of the winds as the estofs model indicates values close to 2 ft to the S of Cape Lookout. The models then show just over 1 ft in NY bight and Long Island Sound Wed as the system moves N, and then just under 1 ft along the New England coast on Thu. The guidance seems reasonable given the forcing from the GFS winds, though confidence decreases with time as a result of the increasing model spread in the 00z guidance.
.Nt1 New England waters... .anz815...south of New England... gale possible Wednesday.
.Nt2 mid-Atlantic waters... .anz915...Hudson Canyon to the great South Channel... gale possible Wednesday. .Anz920...Baltimore Canyon to the great South Channel... gale Tuesday night. Gale possible Wednesday. .Anz905...The great South Channel to the Hague line... gale possible Wednesday. .Anz910...East of the great South Channel and south of 39n... gale possible Wednesday into Wednesday night. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale Tuesday into Tuesday night. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale Tuesday into Tuesday night. Gale possible Wednesday. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale Tuesday into Tuesday night. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale tonight into Tuesday. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale tonight into Tuesday night. .Anz835...Inner waters from Cape Fear to 31n... gale today into Tuesday. .Anz935...Outer waters from Cape Fear to 31n... gale today into Tuesday.
.Forecaster Kells. Ocean prediction center.