marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 1001 PM EDT Sat Oct 20 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Storm force winds ahead of a cold front and now just over the eastern edge of the central waters will be confined to the east of the region in the short term. GOES 16 rgb geocolor satellite images show cold clouds with a band of lightning strikes within the vicinity of the cold front. The latest NCEP weather map has to the west a ridge that extends from high pressure to the northeast of the waters. A strong cold front stretches from Georges Bank across the region to the southwestern waters. Pressure gradient is fairly tight over the waters and maximum winds are in the storm force threshold with gale force winds over the northeastern waters. Stability index map show instability to continue over the region and that will spread gale force winds to central waters in the short term.
Seas are initially relatively large over the eastern waters with a peak at 20 ft but can be higher in storm force winds. Nww3 and ecmwfwave have initialized well the observed seas. Both wave models still agree well in the short term on keeping larger seas to the east of the waters but will subside as the front passes over the waters. Seas will then build again over the central waters as gale force winds spread into the central waters. Will use nww3 for the seas grids.
The latest model run for GFS/ukmethr/ecmwfhr/CMC have initialized fairly well the synoptic surface observations though there are just minor differences on the actual position of the front over the waters. Models have continued to show enhanced winds near the Gulf Stream. For consistency, will retain most of the official forecast winds and will stay with GFS at first then switch to ECMWF in the extended period.
----------------------------------------------------------------- Previous discussion...
The GOES visible satellite imagery shows a cold front across the N portion of the offshore waters, and a low pressure trough over the S portion with scattered showers along both features. The lightning density product indicates some isolated tstms embedded within the convective activity, mainly in the unstable environment in nt2 over the warmer waters in the Gulf Stream. Will adjust weather grids accordingly to match the current convection, and mariners should be cautious of locally higher wind gusts and seas associated with tstms. Otherwise, ascat wind retrievals from 15z this morning showed a large area of 30 kt winds in the nt2 waters mainly over the Gulf Stream with a couple of 35 kt barbs to the se of Cape Lookout. Current surface reports over the offshore waters also show winds up to 35 kt, and the 12z ECMWF/GFS winds are initialized well when compared with the data. However, the higher first sigma GFS winds seem more representative of areas with unstable lapse rates, and the model indicates a low will develop along the front tonight as it moves NE across the area. The GFS indicates a brief period of storm force winds over the far outer nrn nt2 offshore waters tonight near the developing low, and this has been consistent for several runs now. The 12z ECMWF has also shown development, but has not been as strong as the GFS and normally has a low wind bias in this type of situation. The previous forecast had a large area of gales tonight into sun, and also a second round off the mid Atlc coast sun into Sun night with a secondary cold surge. This is supported by the bulk of the 12z guidance, and the models have been consistent on this scenario for several days. As a result confidence is above average on the gales and small area of storm force in nt2 tonight, so will be continuing previous headlines while expanding them slightly as the models are slightly stronger with the secondary surge. Will also be populating with the 12z GFS winds, but will use the first sigma winds in unstable areas to account for the deeper mixing.
The 12z models continue to differ on the next system on Tue, and the GFS has been consistently faster than the rest of the guidance as it also keeps the associated surface low further N as it passes just to the N of the offshore waters. The rest of the 12z guidance is slower, while the 12z ECMWF has been consistent with taking the low through the nrn Gulf of Maine. The ECMWF also shows a strong level jet setting up ahead of the front, and indicates higher winds in the outer nrn nt2 waters for Tue night. The 12z UKMET/Gem are slightly faster than the 12z ECMWF, but overall show a stronger signal for the slower solution. The 12z gefs members also have a stronger signal for a slower solution from the previous cycle, and the wpc medium range guidance is closest to the ECMWF solution. As a result, am planning on transitioning to the 12z ECMWF Mon night, and continue with it through the rest of the forecast period since it is also near the previous forecast. Will also maintain previous gale headlines on Tue night with average confidence in the outer nrn nt2 waters as is continues to be supported by the 12z ECMWF.
.Seas...Both the 12z ECMWF wam and wavewatch are initialized a ft or two low in the offshore waters, so planning on boosting the guidance to start out with. The models agree over the short range, but diverge some in the medium range as a result of the aforementioned differences between the associated atmospheric models. As a result of the previously stated preferences for the winds, am planning on using an even blend over the short range period before transitioning to the 12z ECMWF wam exclusively in the medium range. However, will also be making further adjustments as needed, mainly where the guidance seems underdone with the expected hazard conditions.
.Extratropical storm surge guidance...There is no indication of any significant positive surge events over the region for the next few days.
.Nt1 New England waters... .anz800...Gulf of Maine... gale Sunday into Sunday night. .Anz805...Georges Bank west of 68w... gale Sunday. .Anz900...Georges Bank east of 68w... gale Sunday into Sunday night. .Anz810...South of New England... gale Sunday. .Anz815...South of Long Island... gale Sunday.
.Nt2 mid-Atlantic waters... .anz820...Hudson Canyon to Baltimore Canyon... gale tonight into Sunday. .Anz915...Hudson Canyon to the great South Channel... gale Sunday. Gale possible Thursday. .Anz920...Baltimore Canyon to the great South Channel... gale tonight into Sunday. Gale possible Thursday into Thursday night. .Anz905...The great South Channel to the Hague line... storm tonight. Gale Sunday into Sunday night. Gale possible Tuesday night. .Anz910...East of the great South Channel and south of 39n... storm tonight. Gale Sunday. Gale possible Tuesday night. .Anz825...Inner waters from Baltimore Canyon to Cape Charles Light... gale tonight into Sunday. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale tonight into Sunday. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale tonight into Sunday. Gale possible Thursday. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale tonight into Sunday. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale tonight into Sunday. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale tonight into Sunday.
.Forecaster musonda/Kells. Ocean prediction center.