marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 934 PM EST sun Dec 16 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The GOES infrared satellite imagery indicates an area of low pressure just S of Long Island, and the associated cold and occluded fronts moving across the offshore waters. Coastal buoys just S of Long Island have dropped to just under gale to 30 kt sustained gusting to 35 kt with seas near 12 ft. The earlier ascat wind retrievals showed gales along the coast this morning, but the 18z GFS and 12z ECMWF show that the strongest winds have moved off to the E over Georges Bank. The models indicate that the low and upper trough will move E across nt1 tonight as cold advection starts building in behind the front. The GFS shows gales across the nrn zones in the cold advection, and the rest of the 12z guidance agrees fairly well on the higher winds and strong cold air advection. The previous forecast kept gales over the offshore waters into Tue night, so not planning on making any major changes since it is well supported in the guidance.
The models then show high pressure building across the W Atlc Wed before it passed E of the offshore waters Thu. The GFS then indicates another developing low will move N to the W Fri as its associated cold front passes E over the area. The GFS/ECMWF have been consistent on gales in the southerly flow over the warmer waters in nt2 as a strong low level jet sets up ahead of the front and increases the mixing with the enhancement from the vertical shear. The previous forecast had gales for mainly the nt2 zones with the negative static stability, and also over Georges Bank where SSTs are up to 4 degrees celsius anomalously warm. Confidence remains about average on the previous headlines as a result of the good agreement in the guidance, so planning on staying near continuity through the medium range.
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This mornings ascat overpasses from 1340z to 1520z confirmed gale force winds north of the surface low and occluded front across the northern outer nt2 as well as within about 60 nautical miles south of western Long Island. The 12z models were generally initialized too weak with the strongest winds seen in this mornings ascat. The previous opc wind grids matched up fairly well with these higher winds, and were based on a ECMWF/ hiresw-arw blend, and then adjusted slightly higher. Based on the 12z models and the latest observations, we will probably not be making any significant changes to the previous wind grids through Mon. The models are consistent that these east to northeast gale force winds will continue to shift eastward and northward across Georges Bank this afternoon and into the southern Gulf of Maine this evening, and the guidance all indicates the surface low will continue to deepen as it moves just south of Nantucket tonight, and northeast over Georges Bank late tonight into early Mon. And as the low strengthens southeast and east of Cape Cod tonight, models all indicate that the strengthening gradient southwest of the will support gales developing across the northern nt2 and southern nt1 zones tonight and Mon. We have above average confidence with these hazards through Tue evening.
The 12z models then all move a reinforcing cold front offshore in the wake of this low pressure system late Mon and Tue, resulting in wider spread northwest gales developing in the strong cold air advection over the New England and northern mid-Atlantic offshore waters Mon night and Tue. Winds and seas should diminish Wed as a high pressure ridge builds to the coast and moves east over the waters. The 12z models then continue to show a strong possibility for widespread gales to again develop toward the end of the week as another amplified upper low/trough is expected to move east and northeast across the eastern US. Overall, the models are in very good agreement with the timing of the associated cold front, with the GFS only slightly more progressive than the ECMWF/UKMET. We prefer the slightly slower ECMWF/UKMET, and for the winds we will blend the 12z ECMWF with the previous opc wind grids. We will retain the gales ahead of the front Thu night and then expand them northward through the remainder of the nt2 zones Fri and Fri night. With the low levels of the atmosphere remaining quite stable in this southerly flow and limiting any vertical mixing, we will limit the gales across the colder New England waters to the southern portions of Georges Bank where sea surface temperatures are approaching 60 f, about 5 to 12 degrees warmer than those across the Gulf of Maine and the south of New England zones. Given the model consistency the the strength of this southerly flow late Thu and Fri, we again have above average confidence with these gale hazards at least over the nt2 waters.
.Seas: buoys south of Long Island continue with significant wave heights in the 12 to 14 ft range over the last few hours, which is about 2 to 5 ft higher than the 12z wavewatch, and up to about 2 ft higher than the 12z ECMWF wam. The previous wave height grids were boosted higher by about 15 to 20 percent and appear representative at the moment. We will not make much adjustment to the previous wave height grids through the near term. We then preferred the higher 12z ECMWF wam Mon night through Wed, mainly across the areas with the northwest gales. From Wed night Onward we used a 50/50 blend of the 12z ww3 and 12z ECMWF wam. However even these blended wave heights were over 20 ft late Thu night and Fri across the outer nt2 zones. We adjusted these wave heights lower by about 15 percent.
.Extratropical storm surge guidance...The higher 12z estofs model appears to have a better handle than the 12z etss with the storm surge along the coast from Delaware Bay to near Marthas Vineyard. Based on the latest water level observations, even the 12z estofs could be slightly low. Please monitor coastal flood/tidal products from your local National Weather Service forecast office for more information.
.Nt1 New England waters... .anz800...Gulf of Maine... gale tonight. Gale Monday night into Tuesday. .Anz805...Georges Bank west of 68w... gale tonight into Tuesday. Gale possible Friday night. .Anz900...Georges Bank east of 68w... gale tonight into Tuesday. Gale possible Friday night. .Anz810...South of New England... gale Monday into Tuesday. .Anz815...South of Long Island... gale Monday night into Tuesday.
.Nt2 mid-Atlantic waters... .anz820...Hudson Canyon to Baltimore Canyon... gale Monday night into Tuesday. Gale possible Thursday night into Friday. .Anz915...Hudson Canyon to the great South Channel... gale tonight into Tuesday. Gale possible Friday into Friday night. .Anz920...Baltimore Canyon to the great South Channel... gale tonight into Tuesday. Gale possible Thursday night into Friday night. .Anz905...The great South Channel to the Hague line... gale tonight into Tuesday. Gale possible Tuesday night. Gale possible Friday night. .Anz910...East of the great South Channel and south of 39n... gale tonight into Tuesday. Gale possible Friday into Friday night. .Anz825...Inner waters from Baltimore Canyon to Cape Charles Light... gale possible Thursday night into Friday. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale possible Thursday night into Friday. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale tonight. Gale Monday night into Tuesday. Gale possible Thursday night into Friday night. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale possible Thursday night into Friday night. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale possible Thursday night into Friday night. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale possible Thursday night into Friday night. .Anz835...Inner waters from Cape Fear to 31n... gale possible Thursday night into Friday night. .Anz935...Outer waters from Cape Fear to 31n... gale possible Thursday night into Friday night.
.Forecaster Kells/Clark. Ocean prediction center.