Marine Weather for HS 100


marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 901 PM 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.

The preliminary 00z opc-wpc surface analysis shows a cold front extending SW from a low pressure center over Nova Scotia to the southern Gulf of Maine to southern New England and then further W toward PA and OH. A high pressure ridge prevails just S of the waters along 30n or so, from North Florida eastward to S of Bermuda. The latest GOES-IR satellite imagery and lightning density data show very little in the way of thunderstorm activity over the offshore waters, with the most concentrated thunderstorms inland from southern New England W and SW toward northern Maryland and points to the W. Additional thunderstorms are occurring over eastern Georgia and southward into Florida. The latest sref guidance does indicate that the chances for thunderstorms will increase over the offshore waters, especially off the New England and mid-Atlantic coasts, later tonight through at least early parts of the week as a series of low pressure areas track NE along a slow-moving cold front crossing the region. Caution is advised for locally very strong winds, potentially near or exceeding gale force, and very rough seas in or near any of the heavier thunderstorms. Otherwise, based on the latest guidance, including the 18z GFS and NAM models, we do not plan on making any major changes to the ongoing opc forecast for the evening update, with just a few minor edits in deference to nearby coastal WFO and TAFB grids over the next few days.

.Seas...Sea heights, according to the 00z ra1 opc sea state analysis, range from 3 to 6 ft over most of the area, except 2 to 3 ft over the far southern portions of the offshore waters. For the evening update, we will not make major changes to the ongoing opc forecast, with just a few edits in deference to nearby coastal WFO and TAFB grids over the next few days.

----------------------------------------------------------------- Previous discussion...

This mornings ascat overpasses over the offshore waters returned 15 to 20 kt across most of the nt2 zones north of Cape Fear. The previous opc wind grids which were largely based on the 00z hiresw-arw agreed well with the ascat winds. At 18z a cold front extended just offshore across the northern Gulf of Maine. As the front sinks south tonight, thunderstorms are forecast to develop over the New England and northern mid-Atlantic offshore zones. The 12z href guidance shows a chance, or about a 30 to 40 percent probability, that some of these thunderstorms could produce gale force winds mainly over the south of New England zones this evening through about 04z. Thunderstorms are expected to be much more limited over the central and southern nt2 waters at least through early Mon. The 12z models are consistent that the front will stall near 40n early Mon. By late Mon and Mon night, the guidance all indicates that the southwest winds will abruptly increase from south to north as low pressure consolidates and tracks northeast from near Long Island. The 12z GFS and 12z ECMWF have come into excellent agreement with this first frontal wave, and both show winds up to 30 kt expanding northward from the central nt2 zones Mon evening, and the northern nt2 and southern nt1 waters Mon night into Tue evening. There continues to be a slight chance for winds to increase to gale force Mon night and Tue. With the guidance depicting the stronger winds coincident with the zones where sea surface temperatures are cooler, there should be very little vertical mixing of the 40 to 50 kt low level jet. Therefore, chances remain too low to add gales to the forecast, at least at this time. Forecast confidence for tonight through Tue is slightly above average.

During the medium range, we will trend the forecast more toward the ECMWF which will keep close continuity with the previous opc forecasts/grids. Versus previous runs, the 00z ECMWF trended deeper and stronger with the winds associated with the next set of developing surface lows expected to track northeast across the central/northern nt2 waters Tue night into early Thu. The 12z ECMWF was generally consistent with this stronger solution. We will use a blend of the 12z ECMWF and the previous grids, which will maintain some winds to 30 kt over the northern outer nt2 zones Wed into Wed night. As high pressure builds to the New England coast toward the end of the week, the front is forecast to stall and weaken over the central and southern nt2 waters. As more frontal waves track northeast over these areas, winds are expected to increase to 20 or 25 kt. Forecast confidence during the medium range is about average.

.Seas: wave heights across the Gulf of Maine are about 1 ft or so lower than the wavewatch and ECMWF wam guidance this afternoon. We will use an even blend of previous wave height grids and the 12z ECMWF wam through the period. However, the 12z wam and 12z wavewatch are in very good agreement across the offshore waters.

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/Clark. Ocean prediction center.

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