marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 844 PM EDT Sat Mar 24 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Current conditions...the 18z NCEP surface analysis shows a strong complex low E of the offshore waters, with associated troughing over the nt1 and NE nt2 waters. The analysis shows a weak 1014 mb low over Rhode Island, and a cold front approaching the New England coast from the NW. The analysis also indicates a high pressure ridge across the central and southern nt2 waters. Latest available ascat and ascat hi-res passes from this morning show 20 to 30 kt winds in NW flow over the outer nt2 waters, with 10 to 20 kt winds indicated elsewhere in the offshore waters.
Models/forecast...the 12z medium range models are in good agreement across the offshore waters for tonight through Tue, so have above average confidence in the storm warnings for the outer zones in the northern part of the nt2 area for Mon night, with all of the models showing strong N to NE flow across the north wall of the Gulf Stream. The representative 12z GFS 10m/30m solution with the stability smart tool will be used for the wind grids through Tue. The models are not in particularly good agreement with the timing and orientation of the overall flow pattern around the offshore waters for Tue night through Thu night, but the 12z ECMWF looks like a good median model solution and has good support from the latest wpc medium range forecaster guidance. So the 12z ECMWF will be used for Tue night through the rest of the forecast period.
Seas...the ECMWF wam and wavewatch both initialized reasonably well over the offshore waters, and are in decent agreement for tonight through Tue with some minor differences noted, so a 50/50 blend of the models will be used for the sea height grids for this timeframe. Will then use the 12z ECMWF wam exclusively for Tue night through the rest of the forecast period, in order to be consistent with the preferred 12z ECMWF winds over this period.
Extratropical storm surge guidance...the latest estofs and etss guidance indicates a positive surge in the 1.0 to 1.5 ft range along the mid Atlantic and se coasts for later sun through Mon, with looks reasonable given the fairly strong NE flow showing up in preferred GFS model for those regions.
----------------------------------------------------------------- Previous discussion...
The GOES visible satellite imagery indicates cold air cumulus in the cold air advection across the offshore waters this afternoon in the wake of a cold front E of the offshore waters. The lightning density product shows some showers and tstms just E of the nrn nt2 offshore waters which were over the area this morning, mainly in the unstable environment over the Gulf Stream. The convection is an indication of the low level mixing in the cold advection with stronger winds aloft mixing down towards the surface. The previous forecast had gales over nt2 for earlier today, but the imagery indicates that they have passed just E of the waters. In addition, the ascat wind retrievals from 15z indicate up to 30 kt over the offshore waters, and gales off to the E near the front. The 12z GFS winds are initialized well when compared with the data, and the model indicates that majority of the strongest cold advection will pass E of the area early tonight. In addition, the 12z GFS indicates that a developing low will move off the se coast tonight, then develop E of the nt2 waters over the next several days. The 12z GFS and ECMWF agree fairly well over the short range, so planning on starting out with with the 12z GFS in the update package.
The 12z models indicate the aforementioned low pres will move develop E of the area sun into Tue. There are differences on the track of the low, and the 12z ECMWF is just W of the 12z GFS in the short range before taking the low north of the 12z GFS/UKMET/Gem models. However, the models generally agree on the strong NE flow over the offshore waters, mainly over nt2. The previous forecast had storm force winds in far NE nt2 by late Mon night, with a large area of gales surrounding the storm force winds. Confidence is above average on the storm force as the strong NE flow over the Gulf Stream will more than likely be above the guidance when considering the low wind bias the models have had this season. As a result, planning on keeping previous headlines for the next forecast package, but will switch to the 12z ECMWF on 12z Tue.
On Thu, the 12z models all indicate another low pres system will approach the coast, though the 12z GFS is a bit faster than the rest of the 12z models and takes the low and associated cold front offshore. There is quite a bit of spread in the guidance, and the 12z ECMWF is a decent compromise when compared with the 12z UKMET and Gem. In addition, the 12z gefs members are showing a stronger signal for a slower solution, and are more similar to the 12z ECMWF. Also, the wpc medium range guidance is also favoring the slower solution, so will continue with the 12z ECMWF through the remainder of the forecast period.
Seas...the 00z ECMWF wam and wavewatch have both initialized reasonably well over the offshore waters, and remain in decent agreement through the short range period. As a result, will start out with a 50/50 blend of the two models. Will then transition to the 12z ECMWF wam at 12z Tue since will be using the 12z ECMWF winds at that point Onward.
Extratropical storm surge guidance...the 12z estofs and etss backed off slightly on the surge along the mid Atlantic and se coasts for later sun through Mon night, are now showing just over 1.0 ft as a maximum. The estofs is slightly higher than the etss, and seems slightly more reasonable in the NE flow expected along the coast.
.Nt1 New England waters... .anz900...Georges Bank east of 68w... gale Monday. Gale possible Monday night into Tuesday.
.Nt2 mid-Atlantic waters... .anz920...Baltimore Canyon to the great South Channel... gale Sunday night into Monday. .Anz905...The great South Channel to the Hague line... gale Sunday night into Monday. Storm possible Monday night. Gale possible Tuesday into Tuesday night. .Anz910...East of the great South Channel and south of 39n... gale Sunday night into Monday. Storm possible Monday night. Gale possible Tuesday into Tuesday night. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale tonight. .Anz835...Inner waters from Cape Fear to 31n... gale tonight into Sunday. .Anz935...Outer waters from Cape Fear to 31n... gale tonight into Sunday.
.Forecaster Scovil/Kells. Ocean prediction center.