marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 840 PM EDT sun Oct 22 2017
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Current conditions...the 18z NCEP surface analysis shows a strong high pres ridge over the offshore waters. Latest available ascat and ascat hi-res passes from this morning show 15 to 20 kt winds in E to se flow over the SW part of the nt2 area, with 5 to 15 kt winds indicated across the remainder of the offshore waters.
Models/forecast...the medium range models are in good agreement across the offshore waters for tonight and Mon, so the representative GFS 10m/30m solution with the stability smart tool will be used for the wind grids over this timeframe. The models are showing differences regarding the timing and strength of the cold front expected to pass E over the offshore waters for late Tue through Thu, with the ECMWF looking like the best median model solution. The GFS also looks different than the other models because it forms a low along the front in the nt2 area late Wed, then takes it NE while strengthening it Wed night through Fri night. Meanwhile, the ECMWF/Gem/UKMET do not even form a low along the front. The ECMWF/UKMET are in the best agreement for later Thu through Fri night, while the Gem and GFS are showing significantly different patterns and timing of the main weather features. In summary, the ECMWF will be used for the wind grids for Mon night through the rest of the forecast period.
Seas...the wna wavewatch and ECMWF wam both initialized reasonably well over the offshore waters, and are in good agreement with one another for tonight and Mon. A 50/50 blend of the wavewatch/ECMWF wam will be used for the wave height grids through Mon. The wavewatch is 3 to 6 ft higher than the ECMWF wam during the Mon night through Thu night timeframe, and the wavewatch looks overdone with the seas in this type of a regime where the prevailing winds are southerly. Thus will use the ECMWF wam exclusively for the wave height grids for Mon night through the rest of the forecast period.
Extratropical storm surge guidance...N/A.
----------------------------------------------------------------- Previous discussion...
This mornings offshore ascat overpasses returned winds to 20 kt across the zones south of Cape Fear. Also the 1520z pass indicated east to northeast winds as high as 20 to 25 kt just off the Maine coast. None of the 12z models were well initialized with these higher winds across coastal Maine. Recent buoy observations suggest that this gradient has relaxed somewhat the past few hours. Over the near term the 12z models are in good agreement that the high pressure ridge across the waters will continue to drift east through Mon. The weak surface low and warm front northeast of Cape Cod will further weaken while lifting north this evening. Will update the tonight/Mon wind grids with the 12z GFS which is well supported by the remaining guidance.
The 12z models are then consistent that the first mid/upper level trough will lift northeast just inland of the mid Atlantic and through New England while weakening Mon/Mon night before a second more amplified trough digs southeast toward the mid Atlantic coast Tue through Wed. This upper feature will support a strong cold front that will slowly approach the mid Atlantic and New England coasts Mon through Tue, and will be the weather feature which will have the most impact on the offshore waters through the forecast period. The models continue to diverge with the details of the strong frontal passage and potential low development(s) across the offshore waters Tue night through Thu. The 12z GFS/ECMWF/UKMET have come into much better agreement with the timing of the cold front at least through Wed. By later Wed night and Thu, the UKMET becomes more progressive than the GFS, with the ECMWF offering a compromise between the two. The models also differ with how widespread (or limited) any pre frontal gales may be, specifically determining how much of the 45 to 55 kt low level jet (as shown by the 12z GFS and 12z ecmwf) will mix to the surface. The 12z ECMWF also offers a compromise between the GFS, which is the strongest of the models with the southerly winds ahead of the front, and the weaker UKMET. The gales are likely to become more widespread ahead of the front across the northern outer nt2 and eastern nt1 zones as the upper trough becomes negatively tilted off the coast late Wed and Wed night. Again the 12z ECMWF seems to have a decent handle on this. The gales should shift east of the waters late Thu, and late in the week the models are consistent that another high pressure ridge will exit the mid Atlantic coast. Will populate the wind grids with the 12z ECMWF and make some minor edits to keep continuity with the previous gale hazards. We have much more confidence with the pre frontal southerly gales forecast across the warmer northern nt2 waters Wed/Wed night than those across the colder waters near Georges Bank at that time.
.Seas...Both the 12z wavewatch iii and 12z ECMWF wam are equally well initialized with the west Atlantic wave heights this afternoon, and also are in good agreement through Mon night. By Tue night through Thu night, with the GFS and ECMWF differing with the increasing southerly winds, the 12z ww3 is anywhere from 3 to 6 ft higher than the 12z ECMWF wam, and generally looks too high in this type of southerly flow regime. Will be populating with an even blend of the ww3/ECMWF wam through Mon before transitioning solely to the lower ECMWF wam Mon night through the remainder of the forecast period.
.Extratropical storm surge guidance...N/A.
.Nt1 New England waters... .anz800...Gulf of Maine... gale possible Wednesday night. .Anz805...Georges Bank west of 68w... gale possible Wednesday night. .Anz900...Georges Bank east of 68w... gale possible Wednesday night into Thursday.
.Nt2 mid-Atlantic waters... .anz920...Baltimore Canyon to the great South Channel... gale possible Wednesday into Wednesday night. .Anz905...The great South Channel to the Hague line... gale possible Wednesday night into Thursday. .Anz910...East of the great South Channel and south of 39n... gale possible Wednesday into Wednesday night.
.Forecaster Scovil/Clark. Ocean prediction center.