marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 341 PM EDT Mon may 29 2017
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Over the short term no major changes to the ongoing opc forecast appear needed. The 18z opc-NCEP surface analysis indicated 1007 mb low pressure near 38n 68w moving off to the east and southeast. A late morning ascat overpass indicated a few gales developing over the southeast quadrant over the low, just east of the offshore waters.
The 12z models all appear to handle the movement and intensity of this low rather well when compared to the ascat overpass, and observations. Therefore, we will stay close to the previous grids into tonight, and keep any gales associated with the low east of the northern nt2 offshore waters. For Tuesday into Wednesday night we will rely on the 12z GFS as it has good support from the 12z ECMWF and 12z NAM guidance over the region. We will also continue to use the smart tool which places the stronger first sigma layer winds over the more unstable areas and warmer SSTs near and south of the Gulf stream; and somewhat lighter 10 meter winds over the cooler waters mainly north and west of the stream. A cold front associated with the low noted above will become stationary over the central nt2 waters tonight. The front will drift north as a warm front Tuesday and Tuesday night with weak low pressure developing along the front over northern nt2 waters. The low will track northeast along the front later Tuesday into Wednesday, reaching a position near or just south of Georges Bank by Wednesday evening. The low will then likely dissipate east of the waters Wednesday night as a weak cold front slides east and southeast over the waters. Scattered thunderstorms are likely near the low described above into tonight, with the warm front and weak low Tuesday into Wednesday, and near the weak cold front Wednesday night. Locally very gusty winds and rough seas can be expected in or near any thunderstorm activity. Confidence levels are near to above average for the short term part of the forecast tonight through Wednesday night.
Over the long term, the 12z model guidance remains in generally good agreement over the waters Thursday and Thursday night. The cold front will continue to move east and southeast Thursday, then stall over southern nt2 waters Thursday night as weak high pressure builds east over waters north of the front. The 12z models appear to be coming into somewhat better agreement over the region Friday into the weekend. For now, we will continue to follow the 12z GFS winds for Thursday and Thursday night, and then trend the forecast more toward the 12z ECMWF model Friday into the weekend as it appears to be a bit more consistent and in good agreement with the latest wpc medium range forecast. Therefore, we still expect a moderate to strong cold front to approach the region from the west and northwest Friday into Friday night, with increasing southwest winds ahead of the front over most of the offshore waters. The front will slide southeast over nt1 waters Saturday, then stall over northern nt2 waters Saturday night as low pressure organizes along the front over the Midwest. There is some chance for gales developing in the southwest flow ahead of the front Friday into the weekend. For now, we will keep winds below gale force and continue to monitor future model cycles to see if any gales might need to be added to the offshore forecast. The confidence in the forecast is near average from Thursday into Saturday night.
.Seas...Both the 12z wavewatch iii and 12z ECMWF wam appear to have initialized the current seas quite well again this afternoon. We will rely on the 50/50 blend of these two wave models through Thursday night, and then as we switch to using more of the 12z ECMWF for winds for Friday into Saturday night also rely on the 12z wam for seas heights during this time frame. Also, it appears that the wam may be building seas a little too high later in the forecast period. We will, therefore, cap sea heights at 11-12 feet or so by Saturday night, and continue to adjust sea heights as needed in future opc forecasts.
.Extatropical storm surge guidance...N/A.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Mills. Ocean prediction center.