marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 1012 PM EDT Wed Aug 15 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The ongoing wind and significant wave height grids look to be in excellent shape this evening, and will be not making any impactful changes in the upcoming update.
See previous discussion below for details.
----------------------------------------------------------------- Previous discussion...
This mornings ascat overpasses missed most of the nt1 and nt2 offshore waters, and we had no data across any areas where the thunderstorms were occurring this morning. Thunderstorms across the mid-Atlantic waters south of Hatteras Canyon continue to slowly shift southeast as the mid/upper level winds are shifting to west to northest across the northern inner or western zones this afternoon. Also, at 18z, low pressure was centered just east of Cape Cod with a weakening cold front extending southwest from the low toward Hatteras Canyon. The latest GOES-16 water vapor channels indicate the weakening mid and upper level trough across the northern offshore waters continues to slowly move east this afternoon. We will continue to mention gale force winds in thunderstorms in the nt2 text forecast at least through today generally across the waters south of Hatteras Canyon. But will monitor the trend in the latest GOES-16 imagery and lightning data before making final decision. The aforementioned weak cold front should dissipate overnight.
Two stronger cold fronts are expected to move through at least the northern offshore waters over the forecast period. The latest models remained consistent with the first which is forecast to exit the New England coast late tonight into early Thu, stall south of New England and lift back north as a warm front late Thu through Fri night. Yesterdays 12z GFS and 12z ECMWF were in much better agreement with the weekend forecast, specifically with the next stronger cold front and potential developing surface low, late Sat through Sun night, than were the more recent runs. The 12z ECMWF, like yesterdays 12z run, was back to developing a fairly strong surface low off the mid-Atlantic coast late Sat night and sun. The 12z GFS is generally consistent with the 12z ECMWF on timing of the front and to a somewhat lesser degree the low development. We will be using a 75/25 12z ECMWF/12z GFS blend for the wind grids beginning Fri night, which will bump up the weekend winds slightly, but overall keeps continuity. Forecast confidence remains about average in the medium range. There is some potential that the northeast winds Sat night into early next week could be stronger than the 25 kt which is in the forecast at this time.
Seas: the 12z wavewatch and 12z ECMWF wam are well initialized with the west Atlantic wave heights this afternoon, and this guidance is in excellent agreement through the end of the week. The ECMWF wam is then slightly higher than the wavewatch with the wave heights within the increasing northeast winds over the weekend. We will be using an even blend of the aforementioned guidance through Fri, then relying on the ECMWF wam Fri night through the end of the period.
Extratropical storm surge guidance: no significant storm surge is expected through the remainder of the week. Late in the weekend, there could be minor surge that develops across the mid Atlantic coast poleward of the stalled front in east to northeast onshore flow. This, of course, depends on how strong the developing low and associated easterly gradient become, as well as how persistent any stronger winds are. At the moment, given the model variability, our confidence in any one given forecast scenario is low.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Collins/Clark. Ocean prediction center.