marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 235 am EST Wed Jan 17 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Per 06z opc surface map, weak low pressure is now centered about 120 nm to the ENE of Wallops Island. Earlier ascat overpass before 0130 UTC provided crucial wind data, with widespread 30 kt retrievals along the east and southeast quadrant of the low. These strong winds continue to remain poorly initialized in the 00z guidance, including higher resolution guidance from the regional models NAM and 05z hrrr. Best fit continues to be first sigma layer winds from the latest GFS, and plan to populate with that data through about 21z this afternoon before the low clears northeast of the offshore zones.
Attention quickly turns to southern half of the offshore zones tonight through Thu eve. H5 low has already closed off over the Tennessee Valley, and is expected to slowly kick east through tonight beneath intensifying 150 kt jet streak. Continuing the trend first seen in the 15/12z UKMET, and then the 16/18z GFS, all models now suggest surface low formation se of Hatteras by 00z Thu, with rapid intensification of the surface low to a position east of the offshore zones by 00z Fri. With the ECMWF seemingly playing catchup to other guidance, will populate wind grids through 12z Fri with the 00z GFS. Will be very close if winds reach storm force over the opc offshore zones; the NAM and ECMWF do develop storm force winds over outer waters 925 and 930, while the UKMET and GFS (most consistent solutions) develop these storm force winds just east of the area. With little confidence either way, feels safest to Max winds to 45 kt and await further guidance from next round of models, but storm headlines remains a distinct possibility.
Fri 12z onwards, will trend forecast to previous grids for consistency which were based on the 12z ECMWF. High pressure ridge moves off the southeast US coast Thu night/Fri and remains quasi-stationary along 30n/31n through sun. The guidance is also relatively consistent that as a cold front drops south into New England, west to southwest winds will increase to 25 kt or 30 kt across all the offshore waters north of Baltimore Canyon late Fri night and Sat. The front is expected to then stall and weaken across the northern nt2 waters Sat night and sun.
Seas...both the ECMWF wam and ww3 seemed to initialize fine based on 00z observations. The wam tends to perform better in areas of low pressure rapid intensification, but since the ECMWF is pumping out storm force winds across the opc waters, its output is likely a little too high given the current 45 kt in the forecast. To account for this better guidance in rapid cyclogenesis, and the current wind grids which are based on the GFS, plan to use a blended 1:1 solution between the wave models. This will yield seas to about 18 feet in the 45 kt winds, compared to 21 ft from the wam and 15 ft from the ww3. Will then trend seas towards the 12z wam later Fri onwards towards the previous forecast.
Extratropical storm surge guidance...no major deviation from the latest surge guidance appears necessary through the forecast period..
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... .anz920...Baltimore Canyon to the great South Channel... gale Thursday. .Anz910...East of the great South Channel and south of 39n... gale Thursday. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale tonight into Thursday. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale tonight into Thursday. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale tonight into Thursday. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale tonight into Thursday. .Anz835...Inner waters from Cape Fear to 31n... gale tonight. .Anz935...Outer waters from Cape Fear to 31n... gale Thursday.
.Forecaster Collins. Ocean prediction center.