marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 345 am EDT Wed Aug 23 2017
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The late evening ascat overpass indicated S and SW winds around 25 kt occurring from the Gulf of Maine southwest to off the Jersey Shore. These winds matched up well with the ongoing opc forecast. The 06z preliminary opc-NCEP surface map shows a cold front extending from upstate New York south and southwestward to the Ohio Valley with a pre-frontal trough nearing the New England and mid-Atlantic coasts. The latest radar, satellite and lightning density data shows scattered thunderstorms near the trough extending from the Gulf of Maine southwest to near and east of Long Island toward the southern Jersey Shore. A few widely scattered thunderstorms are being detected further south off the north and South Carolina coasts. The latest sref thunderstorm guidance indicates that there will be a good chance for scattered thunderstorms mainly in advance of the trough and cold front over nt1 into nt2 waters today, with the thunderstorms shifting southeastward with the cold front tonight through Friday. Local wind gusts exceeding gale force and very rough seas can be found in and near any of the stronger thunderstorms over the region during the next few days.
Overall, the 00z models are quite similar from their previous cycles and with each other across the region into Friday. As a result, for the early morning package we will use a 70 percent previous forecast, 30 percent blend of the 00z GFS 10 meter winds, just to freshen up the grids and forecast slightly. Overall, there will be no major changes to the previous forecast for today into Friday, with confidence levels near to above average.
All eyes will then become focused on the potential for low pressure to form south of the offshore waters by Sat. The 00z ECMWF appears to be in good agreement with the 12z ECMWF for Fri night into Sun night over the offshore waters. We will use a 50-50 blend of the 12z ECMWF from yesterday and the 00z ECMWF to populate the wind grids from 00z Sat through Sun night as a result. This will also maintain continuity from the previous forecast. The low is forecast to slowly organize south of the waters during the day Sat into Sun night, with the low potentially lifting north or northeast into the waters during the early to middle parts of next week. The 00z ECMWF is now weaker and slower with the low as it moves into the waters next week, with the 00z GFS also weaker, but faster than the ECMWF. For the morning package we will use a 75 percent previous forecast, and 25 percent 00z ECMWF for the forecast for early next week beyond day 5, as we are not quite willing to jump all the way toward the slower and weaker 00z ECMWF solution at this time. There remains the threat for gales over portions of the nt2 waters early next week, beyond day 5. Confidence levels in the forecast are near average into the weekend, and then fall to below average for early next week as model differences increase in both timing and intensity of the previous mentioned low. The latest tropical outlook from the National Hurricane Center continues to forecast only a 30 percent chance for tropical cyclone formation of this system through day 5, with development expected to be slow to occur. Please closely monitor the latest opc forecasts and NHC outlooks over the next several days.
.Seas...For the early morning package it appears as if the previous grids are running close to the latest observations and altimeter data over the offshore waters. We will remain close to the previous grids and forecast for today into Fri, and then to match the winds use an even blend of the previous forecast and 00z ECMWF wam from Fri night into Sun night, and then a 75-25 blend of the previous forecast and 00z wam thereafter in an attempt to match the wind forecast.
.Extratropical storm surge guidance...N/A.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Mills. Ocean prediction center.