marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 353 am EDT sun Jul 21 2019
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Over the short term, we will populate grids with the 00z warw winds for today as they appear to be very representative of conditions noted over the offshore waters early this morning. An ascat pass from last evening around 0237z indicated a broad SW flow over the offshore waters with winds up to 20 kt mainly over central nt2 waters, off the Carolina coast. A coastal low pressure trough will persist today. A high pressure ridge will also persist and extend eastward along 30n or so, from Georgia and North Florida E toward and S of Bermuda, throughout today, and then slowly weaken during early and middle parts of the week. The latest GOES satellite IR, water vapor channels and lightning density data indicated very little in the way of thunderstorm activity over the offshore waters early this morning, with a few heavy thunderstorms E of the central nt2 waters, and well inland over western New England, the Great Lakes and Midwest. The latest sref thunderstorm guidance indicates that the thunderstorm activity will remain quite limited today, and then increase tonight through Mon as a cold front moves slowly se into western and northwestern portions of the region. Caution is advised for very gusty winds and very rough seas in and near any of the heavier thunderstorms that impact the waters, especially tonight through Mon. A cold front is still forecast to move se over the New England waters tonight, and then stall over the waters by Mon as low pressure organizes near New Jersey during Mon. The 00z guidance remains in very good agreement over the waters tonight and Mon. We will transition the wind grids more toward the 00z GFS for tonight into Mon, and use the smart tool which places the stronger first sigma layer winds over the warmer SSTs, mainly in and near the Gulf Stream. This will result in little change from the previous few opc forecasts for tonight and Mon, with winds increasing to 25 to 30 kt mainly over western portions of the nt2 waters by Mon afternoon. There is a slight chance for winds reaching gale force Mon afternoon. Chances remain too low to add gales to the forecast, at least at this time. Forecast confidence today into Mon is slightly above average.
Over the medium range, Mon night through Thu night, we will transition the grids more toward the 00z ECMWF guidance. We will also continue to boost the ECMWF winds by 10 percent or so mainly near the N and W walls of the Gulf Stream during the medium range portion of the forecast. The 00z ECMWF appears to be a decent middle of the Road solution regarding the slow movement of the above mentioned cold front across the waters along with a series of low pressure areas passing NE along the front. The first low will pass NE over western New England waters Mon night into early Tue, with the front then sliding slowly E and se over the New England and western mid-Atlantic waters later Tue and Tue night as a second low forms E of the Delmarva. This low and then a third low will move NE along the front which will extend from just off the south and North Carolina coasts NE over the waters toward Georges banks by Wed. The front and associated generally weak low pressure areas will drift E Wed night as the front is trapped between high pressure slowly building E over the eastern U.S. And more high pressure centered E of Bermuda. The 00z ECMWF has backed-off the idea for gales over the waters Mon night into Tue night, with it indicating only a modest threat for gales Tue night as low pressure tracks NE over the region. Widespread thunderstorm activity appears quite likely across the waters during the medium range, with the threat for at least gale force gusts or higher, and very rough seas in and near any of the heavier thunderstorms during most of the early and middle parts of the week. The chances for more sustained gale force winds appears to have diminished somewhat from the past few model cycles, and we will keep maximum winds at 30 kt for now. We will continue to evaluate future model guidance to see if gales might be needed at some point during the medium range part of the forecast. Please closely monitor the latest opc forecast over the next few days. Forecast confidence is above average Mon night, and then near average thereafter.
.Seas: the 00z wavewatch, 00z ECMWF wam guidance and the previous opc gridded forecast all appear to be initialized quite well early this morning with sea heights ranging from 3 to 6 ft over most of the offshore waters, with the higher seas off the North Carolina coast, and over the outer central and northern nt2 waters. For the early morning package we will use a 80 percent previous sea height, 20 percent 00z wavewatch blend for today into Mon, which will likely best fit to the wind forecast over the short term. For Mon night through Thu night we will transition the sea heights more toward the 00z wam and continue to adjust sea heights upward by 10-15 percent mainly near the N and W walls of the Gulf Stream where guidance typically under performs in this region. Sea heights up to around 12 ft or so still appear likely over and near this region Mon night into Thu. Also, throughout the forecast period we will make a few edits in deference to nearby coastal WFO and TAFB gridded sea height forecasts.
Extratropical storm surge guidance: no significant positive surge events appear likely during the next few days. For more information please refer to the latest coastal National Weather Service forecast.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Mills. Ocean prediction center.