Marine Weather for HS 100


marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 955 am EDT Thu Oct 19 2017

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

A weak stationary front remains draped just south and southeast of the southern waters this morning, with persistent east to northeast flow generally south of about 34n resultant from the combination between this front and high pressure ridging draped over the central waters. Previous overnight grids contained forecast winds to about 25 kt, which was based on good overnight model agreement, but only seeing winds to 20 kt with latest ship and buoy obs from 12-13z. Will make the initial downward adjustment accordingly, but still expect winds to increase to about 25 kt this afternoon as a frontal wave develops along the boundary well east of the area, but strong enough to sufficiently increase the pressure gradient over opc southern zones. This remains well supported from 06z GFS and NAM.

Farther north, strong surface low over far northern Quebec will move east and into the Labrador Sea this afternoon and overnight, pulling a cold front across the northern areas. Models remain in excellent agreement with timing and strength of boundary, and expect winds to increase to 25 kt by 21z for areas north of about 41n.

Friday onwards: as far as mid-October is concerned, relatively calm winds and flat seas expected beneath strong high pressure ridge over the W Atlantic. Southerly return flow forecast late Monday night as the high moves farther east, and deep upper trough begins to approach the East Coast. Overnight shift preferred the slower non-NCEP solution with respect to timing of the surface cold front, which was also supported by the overnight 00z wpc guidance. At this time see no reason to make any changes in the medium range. Will re-evaluate timing and strength of boundary with upcoming 12z suite of guidance.

----------------------------------------------------------------- Previous discussion...

The 06z NCEP surface analysis indicates a high pres ridge across offshore waters, and a stationary front to se of the southern nt2 waters. Ascat from 0224 in the evening indicated up to 25 kt in the outer waters off the se coast in the moderately strong pressure gradient between the front and the ridge, but current observations from 41002 which is west of the 25-kt ascat winds show around 20 kt. The ascat-b pass which was a bit earlier showed 25 kt farther W, and in fact 41002 at 01z showed winds 23 g27 kt. The 00z GFS first sigma winds are initialized well when compared with the data, and indicates that the winds over the southern nt2 area will continue in the 20 to 25 kt range as a wave develops along the front early today. The 00z ECMWF agrees well with the GFS on the overall development, and both show the low developing into an intense system to the E of Newfoundland Fri night. As far as the nt2 waters are concerned, winds are expected to slowly decrease below 25 kt by Thu night as the low and front move E away from the area. In addition, the analysis shows a ridge extending NE over the nt1 waters and the next cold front is well inland over the Great Lakes. The 00z GFS indicates the ridge will weaken over the waters today with a southerly flow developing in the nt1 waters ahead of the next cold front. The 00z GFS first sigma winds look reasonable over unstable areas in the short range, while the 10m winds seem more representative in the more stable environment N of the Gulf Stream, so planning on starting out with the 00z GFS in the next forecast.

Otherwise, the 00z models all indicate high pres will dominate the pattern over the W Atlc through the forecast period, though they also indicate another weak cold front will pass through the Gulf of Maine Thu night into Fri. The 00z GFS indicates SW flow with 10m winds ahead of the front up to 25 kt. This seems reasonable as the 00z ECMWF/UKMET agrees well on the intensity, and also makes sense since the mixed layer should be somewhat shallow with the cooler SSTs N of the Gulf Stream helping to create a stable boundary layer. As a result, planning on staying close to the 00z GFS for this system.

In the medium range, the models all indicate another cold front will approach from the W Mon and Mon night. The GFS again indicates strong southerly flow ahead of the front, and the 10m winds seem reasonable and well supported by the 00z ECMWF. However, the 00z GFS and UKMET are a bit faster than the 12z/00z ECMWF and the 00z Gem is a fast outlier. Also wpc medium range guidance favors the timing of the 12z ECMWF, so planning on switching to 12z ECMWF after 12z Mon.

.Seas...Both the 00z ECMWF wam and wavewatch are initialized within a ft or two of the current data in the offshore waters. The models are in reasonably good agreement into early Mon, so planning on using a 50/50 blend of the two models. Beyond that, am planning on transitioning to the 12z ECMWF wam exclusively to match the preferred timing of the 12z ECMWF winds rising ahead of the front on Mon and Tue.

.Extratropical storm surge guidance...Na.


.Nt1 New England waters... None.

.Nt2 mid-Atlantic waters... None.


.Forecaster Collins/Bancroft. Ocean prediction center.

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