Marine Weather for HS 100

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.Marine weather discussion for N Atlantic Ocean .NWS ocean prediction center Washington DC 215 am EDT Sat 25 Mar 2017

.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.

The most recent ascat overpasses across the offshore waters from 0050z to 0230z indicated southwest winds to 30 kt across the northern nt2 zones and to 25 kt elsewhere north of Hatteras Canyon. Although the most recent model guidance all initialized some winds to 30 kt, ascat shows more widespread 30 kt than initialized. However, the higher 00z GFS first sigma level winds are too high. As a result, and after some testing some different model blends, came up with a 2:1 00z GFS first sigma level and 00z ECMWF boundary layer wind blend that best matched the ascat. At 06z the cold front had moved off the New England coast and the 00z models are in good agreement that the front will slowly move south across the nt1 and northern nt2 waters today into tonight, before stalling off the Delmarva Peninsula late tonight into early Sunday. Models are also consistent that the west to southwest widns south of the cold front will gradually diminish through the day today, to less than 25 kt later this evening. Then as high pressure builds off the New England coast tonight into Sunday night, the 00z GFS/NAM/Canadian are somewhat stronger than the 00z ECMWF/UKMET with the easterlies just poleward of the stationary front across the southern nt1 and northern nt2 waters Sunday into Sunday night. This discrepancy in the models' wind speeds north of the front persists late Sunday night into Monday night as the front lifts northeast back across the nt1 waters as a warm front. Opc preference continues with these slightly higher winds to 25 kt. For wind grids will use the blend noted above through tonight, before transitioning to the 00z GFS Sunday through Monday.

In the medium range, the 00z models diverge greatly with a series of weak upper shortwaves moving through the northeastern US and off the northern mid Atlantic and/or New England coasts Tuesday night and Wednesday. The GFS has been consistently stronger with the associated developing low and cold front forecast to move offshore late Tuesday and Wednesday. There also continued to be farily widespread differences with the timing of this system. The 00z models then become more consistent later Wednesday night into Thursday that the aformentioned shortwaves and approaching northern stream trough will consolidate into an upper low across southeastern Canada and Maritimes. Given model discrepancy, have decided to use a blend of the 00z GFS and 00z ECMWF for wind grids Tuesday night through Wednesday night, which offers a compromised timing with the front. However did adjust these blended winds upward by about 10 percent to get some associated winds to 25 kt. We have above average forecast confidence that winds across the nt1 or nt2 offshore waters should remain below gale through at least midweek.

.Seas...The 00z wavewatch iii is slightly better initialized than the 00z ECMWF wam with wave heights across the west Atlantic this morning. However, the 00z ww3 may not be high enough across the nothern nt2/southern nt1 zones where there are still some 30 kt winds occurring. Am preferring to use an even blend between this guidance through the forecast period.

.Extratropical storm surge guidance...N/A.

.Warnings...Preliminary.

.Nt1 New England waters... None.

.Nt2 mid-Atlantic waters... None.

$$

.Forecaster Clark. Ocean prediction center.

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