marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 930 PM EDT Thu Mar 22 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 992 mb low centered about 200 nm se of Cape Cod, with a 989 mb low centered about 100 nm E of Cape Sable. The analysis shows an occluded front wrapping around the southern low and extending NE into the northern low. The surface analysis indicates a low pressure trough just E of the nt2 area, with another trough inland but approaching the se coast. Latest available ascat and ascat hi-res passes from this morning show 25 to 35 kt winds over most of the offshore waters, except for the SW part of the area off the se coast where winds are in the 20 to 30 kt range.
Models/forecast...the deep lows will move off to the NE tonight and Fri. Another low will form just E of the northern nt2 waters late Fri, then move E Fri night and Sat while strengthening. A developing low will move off the se coast later Sat night, pass E over the far southern nt2 waters sun, move off to the E Sun night and Mon, then retrograde NW towards the northern nt2 waters Mon night through Tue night. A strong high pressure ridge will build se over the offshore waters Sun night into Tue night. The representative 12z GFS 30m solution will be used for the wind grids for tonight. There is some support from the 12z hires warw/nmm and 12z namnest for marginal gales over the outer zones of the nt2 waters for late Fri and Fri night with NW flow expected across the Gulf Stream, so will maintain the gale warnings here but with average confidence at best. The 12z GFS is slower than the other medium range models with the developing low for later sun into Mon, so the slower 12z ECMWF will be used for Sat through sun, before switching to a 50/50 blend of the 12z/00z ECMWF for Sun night through the rest of the forecast period.
Seas...the 12z ECMWF wam initialized well over the offshore waters, and since the 12z ECMWF is the preferred model for the wind grids over most of the forecast period, the 12z ECMWF wam will be used for the sea height grids across the entire forecast period. The 12z ECMWF wam will be adjusted upwards by 10 percent for Fri and Fri night, since we have gale warnings up for that timeframe.
Extratropical storm surge guidance...N/A.
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This mornings ascat overpasses indicated winds only as high as 45 kt south of the low pressure center and across the outer anz910 waters. The storm warnings across these outer waters were already scheduled to expire this afternoon. At 18z, the strong low was centered near 41n 65.5w just southeast Georges Bank. Buoy 44037 over the eastern Gulf of Maine was still reporting gale force gusts last hour. The 12z models also continued to show some gale force winds across the offshore waters east of 69w, and are consistent that the gales should shift east of the waters by 00z. The 12z models have then trended somewhat sharper with the upper level shortwave trough which is expected to dig southeast off the northern mid Atlantic coast Fri and amplify further across the nt2 waters late Fri and Fri night. Based on this trend and the 12z hiresw arw/nmm and 12z namnest, we will introduce some marginal gales across anz920, anz925, and anz930 zones in the cold air advection. Although gales will not likely be widespread, the steep low level lapse rates across the Gulf Stream suggest that there will be periods where gales can mix to the surface. We have average forecast confidence with these gales. We populated the winds grids with the 12z hiresw arw 12z Fri through 12z Sat. A high pressure ridge should then reach the mid Atlantic coast Sat and shift southeast of the mid Atlantic waters Sat night.
The 12z models continued to offer large differences with the west Atlantic upper low expected to close off across or just east of the offshore waters late in the weekend into early next week. Versus its previous two runs, the 12z ECMWF was less amplified and did not retrograde the associated surface as far west, but was still stronger than and west of the 12z GFS. The 12z UKMET and 12z gefs mean also continued to support the stronger and more western ECMWF. Also, a lead upper level shortwave is expected to move off the southeast US coast sun south of the amplified system to the north. We prefer the slower ECMWF/UKMET with this feature versus the more progressive GFS. There is some potential for winds associated with the low across the southern nt2 waters to reach gale sun. However, given the model differences, we do not yet have sufficient confidence to add any hazards to these southern waters at this time. On the other hand, with the 1040+ mb high expected to move into New England, the surface low east of the waters will not require much deepening to support gales at least across the outer offshore waters late sun through Tue. The Hazards Sun night through Tue night will remain mostly consistent with the previous offshore forecasts. With uncertainty as to how strong and how far west the developing surface low will become, we chose to populate with an even blend of the past two runs of the ECMWF, which is also close to the latest wpc medium range winds.
.Seas...The 12z ECMWF wam appears to be somewhat better initialized with the west Atlantic wave heights this afternoon. The wave height grids will generally rely on this guidance through the forecast period. We increased wave heights associated with the Fri/Fri night gales by about 10 percent.
Extratropical storm surge guidance...N/A.
.Nt1 New England waters... .anz800...Gulf of Maine... gale today. .Anz900...Georges Bank east of 68w... gale today. Gale possible Sunday night into Tuesday.
.Nt2 mid-Atlantic waters... .anz915...Hudson Canyon to the great South Channel... gale possible Sunday night. Gale possible Tuesday into Tuesday night. .Anz920...Baltimore Canyon to the great South Channel... gale Friday night into Saturday. Gale possible Sunday night into Monday. Gale possible Tuesday into Tuesday night. .Anz905...The great South Channel to the Hague line... gale today. Gale possible Sunday night into Tuesday night. .Anz910...East of the great South Channel and south of 39n... gale today. Gale possible Sunday night into Tuesday night. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale Friday night into Saturday. Gale possible Sunday night into Monday. Gale possible Tuesday night. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale Friday night into Saturday.
.Forecaster Scovil/Clark. Ocean prediction center.