marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 1020 PM EDT Sat Oct 19 2019
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The latest GOES-IR satellite imagery, regional radar imagery and lightning density data all indicate widespread rain, showers and locally strong thunderstorms increasing along and off the se U.S. Coast this evening, from around Cape Hatteras southward. The stronger thunderstorms are currently about 70-100 nm off the Georgia and southern S. Carolina coasts, as well as, inland over NE Georgia and much of S. Carolina, with all activity moving generally off the NE. Widespread rain, showers and thunderstorms will cover most of the nt2 waters by sun and Sun night before moving E of most of the waters Mon or Mon night. The stronger thunderstorms have the potential to produce winds exceeding gale force and very rough seas across the region tonight through Sun night into Mon. The preliminary 00z opc surface analysis shows a low pressure center, or Post-tropical cyclone Nestor, over southern Georgia moving off to the NE, with a warm front developing E and NE from the low, and a cold front extending S into Florida and the eastern Gulf of Mexico. Further N, high pressure covers most of the nt1 and NE nt2 waters. The latest observations already indicate at least gale force gusts occurring off the Georgia and southern S. Carolina coasts. For the evening update, we will not alter the previous hazards with gale force winds expected to expand N and NE over the nt2 waters later tonight into Sun night, and potentially reach the far SW nt1 waters as well Sun night.
The latest model guidance still shows some differences regarding the future track and strength of Post-tropical Nestor as it passes NE and E over the offshore waters through Mon into Tue, with the GFS, NAM and Gem keeping the system stronger and further N, while the ECMWF, UKMET and navgem are a little further S and not as strong. For the evening update, as we did not want to alter the previous hazards too much, but we did want to produce a more representative wind field, we will populate grids with the first sigma layer 18z GFS winds for tonight into Sun night, and then transition the grids more towards the 12z ECMWF thereafter, with edits to maintain the previous hazards and to match up with nearby coastal WFO and TAFB grids. Hopefully most of the yet to arrive 00z guidance will begin to show better agreement, which will allow US to have more confidence in the forecast and associated hazards. Please closely monitor the latest opc forecast, with additional adjustments quite possible as additional guidance arrives later tonight and sun.
.Seas...Sea heights will be adjusted to fit the winds as noted above, with manual edits to fit best with initial conditions and then in deference to nearby coastal WFO and TAFB grids over the next few days. Sea heights currently range from 11-12 ft off the Georgia and far southern S. Carolina coasts to 2 to 4 ft or so further N off the mid-Atlantic and New England coasts.
----------------------------------------------------------------- Previous discussion...
Per the 18z NCEP surface analysis a high pressure ridge was located across the opc offshore nt1 and nt2 waters. Post-tropical nestory was located at 29.8n 85.0w. The latest GOES-east satellite imagery revealed showers and thunderstorms, associated with Post-tropical cyclone Nestor, were present over the southern nt2 waters, with the heaviest concentration of activity off the South Carolina and Georgia coasts. The latest available asct high resolution data revealed 10-20 kt winds over the eastern nt1 waters, extending down into the central nt2 waters. Over the SW nt2 waters 20-30 kt winds were noted, which is to be expected due to the influence of Post-tropical Nestor in the NE Gulf of Mexico.
For this forecast package, the track of Post-tropical Nestor will be based upon the 21z advisory from the National Hurricane Center, through Wed 18z, which per the NHC will be their last advisory on this system. Concerning the background winds, I will populate using the 12z GFS through Mon 12z as that best matches nhc's track through that time, then transition to using the 12z ECMWF through Tue 18z as that becomes the better model in regards to handling Nestor. Then for the remainder of the forecast I will populate the grids using a 1:1 ratio of the 12z GFS/ECMWF as that best fits the synoptic pattern for the rest of the forecast period.
Overall the same general thinking remains as gale force winds, associated with Post-tropical cyclone Nestor, will develop over the SW waters tonight, then spread NE as Nestor moves NE across northern Florida the rest of this evening, then across eastern sections of south and North Carolina sun and later sun, before emerging off the coast in the vicinity of the Outer Banks, NC Sun night. Post-tropical Nestor will then move E across the central nt2 waters through Tue morning. Regarding Nestor, all marine interests, especially those near and off the mid-Atlantic and se U.S. Coasts should closely monitor the latest information from wpc, opc and coastal WFO forecasts. From Tue night on the main weather concern will be a moderately strong cold front anticipated to move E across the waters Tue night through Wed night. Right now we aren't carrying any wind warnings with this cold front, though the 12z Gem remains bullish in suggesting gales over parts of the northern nt2 waters both ahead and behind of the front. None the less we will continue to monitor the trends in the guidance through the weekend into early next week.
.Seas...Based on the winds used, I will populate the grids with the 12z wavewatch iii through Mon 12z, then the 12z ECMWF wam through Tue 18z, then a 1:1 ratio of the 12z wavewatch iii/ECMWF wam through the rest of the period.
.Extratropical storm surge guidance...The increasing east to northeast winds along the coast from the southeast coast to near Delaware Bay could produce a minor to more moderate storm surge here this evening through Sun night. The latest etss and estofs guidance still indicate surge values around 1 ft, and only over a relatively brief time period. Coastal interests should monitor the latest information from coastal National Weather Service offices.
.Nt1 New England waters... .anz815...south of Long Island... gale Sunday night.
.Nt2 mid-Atlantic waters... .anz820...Hudson Canyon to Baltimore Canyon... gale Sunday night. .Anz915...Hudson Canyon to the great South Channel... gale Sunday night into Monday. .Anz920...Baltimore Canyon to the great South Channel... gale Sunday night into Monday. Gale possible Monday night. .Anz910...East of the great South Channel and south of 39n... gale Sunday night into Monday. Gale possible Monday night into Tuesday. .Anz825...Inner waters from Baltimore Canyon to Cape Charles Light... gale Sunday into Sunday night. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale Sunday into Sunday night. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale Sunday into Monday. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale Sunday into Sunday night. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale tonight into Sunday. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale Sunday into Sunday night. .Anz835...Inner waters from Cape Fear to 31n... gale tonight into Sunday. .Anz935...Outer waters from Cape Fear to 31n... gale Sunday.
.Forecaster Mills/Holley. Ocean prediction center.