marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 859 PM EDT Fri Mar 23 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 strengthening 1011 mb low centered just E of zone 910 in the NE nt2 waters, with an associated trough extending SW into the southern nt2 area. The analysis also indicates another low pressure trough extending SW into the northern Gulf of Maine. Latest available ascat and ascat hi-res passes from this morning show 15 to 20 kt winds in NW flow across the far eastern part of the nt1 area, and also in W to NW flow over the central and southern nt2 waters. Otherwise, 5 to 15 kt winds are indicated across the offshore waters. Lightning density product data at 23z shows isolated showers and tstms along the eastern edge of zone 925 in the outer central nt2 waters.
Models/forecast...the low just E of the northern nt2 waters will move E and strengthen tonight and Sat, which will enhance the NW flow over the northern and central nt2 waters. A developing low will move off the se coast late Sat night, pass E over the southern nt2 waters sun while strengthening, move off to the NE Sun night and Mon while continuing to strengthen, retrograde NW Mon night through Tue night towards the offshore waters, then track se while weakening Wed and Wed night. Will be using the 12z hi-res warw for the wind grids for tonight and Sat, in order to be consistent and maintain the gales in the outer zones of the nt2 area for that timeframe. Will then go with a representative 75/25 blend of the 12z namnest/ECMWF for Sat night and sun, which yields gales over the nt2 waters for sun. The 12z/00z ECMWF has good support from both the UKMET and the wpc medium range forecaster guidance while the GFS still looks too fast and weak with the strengthening low, so a boosted 50/50 blend of the 12z/00z ECMWF will be used for Sun night through the rest of the forecast period.
Seas...the 12z ECMWF wam and 12z wavewatch both initialized reasonably well over the offshore waters. Will use the 12z ECMWF wam as a basis for the wave height grids through the forecast period, but while making adjustments to the guidance. Will adjust the wave heights up by 15 to 20 percent over areas with the gales for tonight into Sat. Then will use a 50/50 blend of the 12z/00z ECMWF wam for the wave height grids for Sat night through the rest of the forecast 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 night, which looks reasonable since there is a good chance that strengthening NE flow could impact those regions.
----------------------------------------------------------------- Previous discussion...
This mornings ascat overpasses missed the offshore areas with the highest winds, making it difficult to Guage how well the 12z models are initialized with the marine winds. The latest models remained in good agreement that northwest winds will increase across the mid-Atlantic waters as a mid/upper level shortwave trough digs southeast from the lower Great Lakes to the mid- Atlantic coast and offshore this evening and tonight. Versus its previous few runs, the 12z GFS is slightly stronger with the winds across the outer nt2 zones, and now shows some marginal gales late tonight into Sat. We have above average confidence with the gales forecast to develop across some of the outer nt2 zones tonight per the latest hiresw arw/nmm and namnest. Through Sat we will be populating the winds grids with the 12z hiresw arw.
The 12z GFS was consistent with its previous run, and remained faster than the other deterministic models and nearly all the 00z gefs/ECMWF eps members with the developing surface low expected to depart the southeast US coast late Sat night. The 06z/12z GFS were slightly faster than previous runs, so these most recent runs did narrow the timing differences. The 12z models also appear quicker to develop the system across the southern nt2 waters late Sat night into sun, and now nearly unanimously are indicating that the associated winds will increase to gale force before the low passes east of the offshore waters. With using a blend of the slower 12z ECMWF and 12z namnest for the wind grids, Sat night and sun, we will be introducing marginal gales across the outer mid-Atlantic zones south of Hatteras Canyon by 15z sun. Then with the high pressure ridge, greater than 1040 mb, reaching the mid-Atlantic and New England coasts late sun/Sun night and persisting through midweek, and the aformentioned surface low deepening just east of the waters Mon into Tue, and eventually retrograding back westward toward the northern nt2 waters, widespread northeast gales are expected to develop and persist across most of the offshore waters. Although the 12z models showed good run to run continuity with their respective previous runs, there remains a good deal of disparity between the GFS/gefs and ECMWF/UKMET/ECMWF eps. Compared to the latter, the former remained weaker and further east with the surface low. Opc model preference continues with the stronger and more westward ECMWF/UKMET, which is also supported by the latest wpc medium range guidance. Given the trend in the 12z ECMWF/UKMET, we will expand the gales significantly late sun into midweek and also introduce storm force winds across the outer anz910 and anz905 Mon night into Tue night. The wind grids were based on a blend of the past two runs of the ECMWF Sun night through the remainder of the forecast period. We have above average confidence with the gales and only slightly less so with the storm force winds expected. By Wed/Wed night, the guidance is consistent that the low will finally weaken and drift east and southeast away from the waters.
.Seas...Both the 12z ECMWF wam and 12z wavewatch are reasonably well initialized with the offshore wave heights this afternoon. We will be using the 12z ECMWF wam as a basis for the wave height grids through the forecast period, but making some adjustments to the guidance. First, will be adjusting the wave heights up by about 15 to 20 percent over areas with the gales tonight into Sat. During the medium range, we populated the wave height grids with an even blend of the past two runs of the ECMWF wam.
Extratropical storm surge guidance...there is some potential for a moderate surge event with early next week, but should be somewhat limited as the strongest northeast winds are expected to be further offshore. By early Mon, the 12z estofs is close to 1 ft higher than the 12z etss along the Georgia/South Carolina coast.
.Nt1 New England waters... .anz800...Gulf of Maine... gale possible Tuesday into Tuesday night. .Anz805...Georges Bank west of 68w... gale possible Sunday night into Tuesday night. .Anz900...Georges Bank east of 68w... gale possible Sunday night into Tuesday night. .Anz810...South of New England... gale possible Sunday night into Wednesday. .Anz815...South of Long Island... gale possible Tuesday into Tuesday night.
.Nt2 mid-Atlantic waters... .anz915...Hudson Canyon to the great South Channel... gale possible Sunday night into Wednesday. .Anz920...Baltimore Canyon to the great South Channel... gale tonight into Saturday. Gale Sunday. Gale possible Sunday night into Wednesday. .Anz905...The great South Channel to the Hague line... gale possible Sunday night into Monday. Storm possible Monday night into Tuesday night. Gale possible Wednesday. .Anz910...East of the great South Channel and south of 39n... gale possible Sunday night into Monday. Storm possible Monday night into Tuesday night. Gale possible Wednesday. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale Sunday. Gale possible Sunday night. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale tonight into Saturday. Gale Sunday. Gale possible Sunday night into Wednesday. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale Sunday. Gale possible Sunday night. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale Sunday. Gale possible Sunday night. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale tonight into Saturday. Gale Sunday. Gale possible Sunday night. .Anz835...Inner waters from Cape Fear to 31n... gale Sunday. Gale possible Sunday night. .Anz935...Outer waters from Cape Fear to 31n... gale Sunday.
.Forecaster Scovil/Clark. Ocean prediction center.