marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 956 PM EDT Fri Jul 20 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
There appears to be no need for significant adjustments to the ongoing opc forecast for the evening update. We will adjust grids slightly to fit conditions noted just prior to forecast issuance time later this evening, and also in deference to nearby coastal WFO and TAFB grids through the weekend into early next week.
The preliminary opc-NCEP 00z surface analysis shows 1011 mb low pressure just S of Myrtle Beach, SC with an associated warm front extending E and NE over central nt2 offshore waters. The intensifying low and front will lift N over the region tonight into Sat night and sun. Gale force winds are still forecast to develop from S to N over the area Sat into Sat night. Based on the slightly deeper and slower trend shown by the 18z GFS when compared to the 12z cycle, we will expand the area of gales slightly from the afternoon package, but maintain gales over the same offshore zones for now. Additional forecast adjustments are possible later tonight into Sat, and the forecast may be altered once a full set of 00z guidance become available later tonight. Please closely monitor the latest opc and coastal WFO forecasts through the weekend.
The latest GOES-E IR satellite imagery and lightning density data indicates scattered thunderstorms occurring mainly from near the warm front southward over the offshore waters off the se coast. The latest sref guidance continues to show thunderstorms expanding N over the waters, especially from Long Island southward overnight into the weekend. Locally very strong winds, potentially exceeding gale force, and very rough seas can occur in and near any of the stronger thunderstorms tonight into the weekend. All mariners are urged to use caution over the offshore waters during the next few days.
Seas...sea heights appear to be building a little faster than previously forecast with seas near 6 to 7 feet already noted just E and S of Cape Hatteras according to the latest observations. For the evening update, we will adjust the initial grids and forecast upward by a couple feet over central and southern nt2 waters for tonight. Otherwise, no major changes will be made to the previous grids and forecast for the evening update. We will adjust grids slightly in deference to nearby coastal WFO and TAFB grids tonight, through the weekend into early next week.
----------------------------------------------------------------- Previous discussion...
GOES-E imagery along with surface observations indicate low pressure has formed along a stationary boundary just W of Hilton Head Island, SC. This coastal low will lift NNE along the eastern Seaboard tonight into Sat while intensifying, then track inland into southern NJ overnight Sat night. Southerly return flow then persists sun through Wed night as strong high pressure slowly drifts W toward the offshores, while a deep upper low becomes stationary while gradually weakening across the eastern half of the US.
Gales are forecast to develop by 15z Sat across the inner portions adjacent to the Outer Banks, then quickly expand and spread NNE through Baltimore Canyon (generally between 72w and 75w) Sat afternoon. Strong gales will develop adjacent to the southern Jersey Shore by 18z Sat, then spread N along the coast to western Long Island by 03z sun. Gales are expected to end around 09z sun. Given robust model consistency and general trend toward a stronger wind field, forecast confidence is high.
All available regional guidance suggests widespread showers and strong thunderstorms continuing off the se coast this evening and overnight associated with the developing coastal low, then spreading NNE as the low tracks along the mid Atlantic coast tonight through Sat night. Radar indicated waterspouts have already been noted in special marine warnings earlier in the day from the forecast office in Charleston, SC. Caution is advised for locally very strong winds and very rough seas in and near thunderstorms that impact the offshore waters through the weekend.
Details and model preferences...
Overall model guidance has converged to a relatively similar solution regarding the timing and track of the aforementioned coastal low. While the 12z GFS trended slower and more toward previous model consensus, the 12z ECMWF actually trended faster and farther W compared to its previous runs. Out of deference to ongoing opc forecasts and available ensemble means, and not ready to fully bite off on this faster ECMWF yet, will be using an even blend between a boosted version of the 12z ECMWF with the 12z GFS through 12z Sunday. All guidance develops an impressive 50-60 kt low level jet by late Sat atop relatively unstable areas, which should aid in mixing some of the stronger winds to the surface. Guidance has generally trended stronger with surface winds, and with this forecast see no reason to Buck the trend. Given the slight shift W in guidance, will continue expanding areal coverage of the gales west into a large portion of opc inner zones.
After 12z Sunday, strong high pressure over the W Atlantic gradually drifts W towards the offshores, delivering persistent southerly winds across most of the opc waters. Strongest S-se winds will be highest sun into Mon - generally in the 15 to 25 kt range - slowly easing NE to SW as the high noses farther west toward the offshores and the gradient weakens. Plan to use a 1:1 blend between the 12z and 00z ECMWF during this time period.
Seas: in similar fashion to the wind grids, plan on using a blend between the 12z ww3 and the 12z ECMWF through 12z sun. However out of deference to 12z gwes probabilistic output - which indicates a near 100 prob of seas greater than 12 ft - will bump the blended values about 10-15 percent higher which will yield Max seas to about 15 feet across Baltimore Canyon overnight Sat. Will then trend forecast to a 12z/00z wam blend for the duration of the forecast period.
Extratropical storm surge guidance: 12z estofs guidance indicates positive surge 1.0 to 1.5 ft (and possibly higher) across the Delmarva by 21z Sat, spreading north along the NJ coast bu 00z sun, then into and western Long Island by 03z sun. 12z etss values are seem to be about half of the estofs. Given the slight west trend + slight uptick in winds during the past few runs, would tend to give more credence to the higher possible surge values from the estofs. Please closely monitor the latest forecasts and advisories from the local coastal National Weather Service offices into the weekend.
.Nt1 New England waters... .anz815...south of Long Island... gale Saturday night.
.Nt2 mid-Atlantic waters... .anz820...Hudson Canyon to Baltimore Canyon... gale Saturday night. .Anz915...Hudson Canyon to the great South Channel... gale Saturday night. .Anz920...Baltimore Canyon to the great South Channel... gale Saturday night. .Anz825...Inner waters from Baltimore Canyon to Cape Charles Light... gale Saturday into Saturday night. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale Saturday. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale Saturday into Saturday night. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale Saturday. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale Saturday. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale Saturday.
.Forecaster Mills/Collins. Ocean prediction center.