marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 949 PM EDT Tue Aug 22 2017
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
For the evening update we will adjust grids to fit current conditions and to fit with nearby coastal waters and TAFB forecast a little better. The preliminary 00z opc/NCEP surface analysis indicates high pressure centered east of the waters with a warm front lifting northeastward from Nova Scotia and the Canadian Maritimes, and a cold front extending from the eastern Great Lakes southwestward to the mid-Mississippi River valley. The front is still forecast to cross nt1 and northern nt2 waters Wed into Thu, before stalling over southern nt2 waters by Fri night. The latest radar, satellite, and lightning density imagery shows a line of strong to severe thunderstorms extending over southwestward over eastern New England to near Baltimore, with another section of the line further west over the Ohio River valley. All of the thunderstorms are moving toward the east and northeast, and will likely impact the offshore waters later tonight through Wednesday into Thursday, with the threat for thunderstorms diminishing with frontal passage from northwest to southeast across the region beginning tomorrow. Caution for local wind gusts exceeding gale force and very rough seas in or near any of the stronger thunderstorms over the next 24 to 72 hours.
Seas...sea heights have risen a little faster off the New England and mid-Atlantic coast this evening, per the latest observations, with seas near 7 feet noted off the Jersey Shore and over the Gulf of Maine. We will adjust the grids and forecast upward by a foot or so initially for the evening forecast update. Otherwise, we will not make major changes to the ongoing opc forecast at this time. Caution for very rough seas in or near any of the stronger thunderstorms that will impact the offshore waters later tonight through Wed.
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This mornings ascat offshore overpasses mostly missed the nt2 waters, but the 14z pass has a few returns to 20 kt about 30 nm east of Cape Cod indicating the southwest winds already started to increase. The latest buoy observations across the coastal and the inner offshore waters are only as high as 15 kt. The 12z models are consistent with the timing of the approaching cold front, which should move offshore Wed/Wed night, and all indicate winds should increase to about 25 kt this evening and tonight across the New England waters and likely as far south as Hudson Canyon. Versus its previous run the 12z GFS was slightly stronger with these winds, up to 30 kt across the Gulf of Maine by 00z. For the wind grids through Wed, will incorporate the slightly higher 12z GFS into the existing grids, and introduce some limited 30 kt winds across the Gulf of Maine. The 12z GFS/UKMET/ ECMWF are in good agreement that the slow moving cold front will reach the southern nt2 waters Thu night/Fri while the prefrontal gradient gradually weakens. By late Friday and through the weekend once the cold front stalls across the southern nt2 waters, and as high pressure builds off the New England and northern mid Atlantic coasts, the models are consistent in forecasting an increasing east to northeast gradient poleward of the stationary boundary.
To varying degrees, the models all indicate low pressure will develop off the northeast Florida coast over the weekend, potentially becoming a tropical cyclone. Versus their respective previous runs, both the 12z ECMWF and 12z GFS trended slower in moving the low northward toward the southern nt2 zones late sun/Sun night. The 12z UKMET also supports this timing. The 12z GFS/UKMET/ECMWF are in well above average agreement with the track/timing of the developing low. However, the 12z GFS is significantly deeper than the ECMWF/UKMET by days 6/7. The latest NHC outlook indicates 30 percent or a low chance of tropical or subtropical formation with this system over the next 5 days. With high pressure anchored across New England, we do have somewhat higher confidence that gales across the southern/central nt2 waters could develop early next week even if low is ultimately baroclinic.
.Seas...The 9 to 10 second southeast swell will continue to impact the mid Atlantic coast over the short term. The 12z wavewatch iii seems to have a slightly better handle than the 12z ECMWF wam with this swell. The models at least appear equally well initialized with the west Atlantic wave heights this afternoon. These models are in excellent agreement through the end of the week. By the weekend, across the southern nt2 zones, opc preference is then with the higher 12z ECMWF wam. For the wave height grids through Fri night, used an even blend of the 12z ECMWF wam and 12z ww3 before transitioning to a 75/25 12z ECMWF wam/12z ww3 blend thereafter.
.Extratropical storm surge guidance...N/A.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Mills/Clark. Ocean prediction center.