marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 952 PM EDT Fri Sep 30 2016
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The Sat img indc area of high pres over the far nrn waters but mostly cloudy over the rest of the region with dense lightning mainly to the srn half. The sref model has 30 percent prob of tstms over the waters S of 40n and continues to show 20 percent chance of tstms thru sun. The latest synoptic sfc observations including earlier ascat pass at 1530z show higher winds over the nrn waters with Max winds just barely in gale force range over waters from south of New England to Hudson Canyon including Baltimore Canyon. At 00z the NCEP map has low pres 1018 mb over the Hudson Canyon with a warm front stretching ENE from the low through the baltmore canyon. High pres just N of the region extends a ridge into the Gulf of Maine. The pres gradient has relaxed over the nrn waters and has remained slack over the srn waters. Seas are still highest over the Hudson Canyon with peak to 14 ft. Seas range between 9 and 12 ft over the nrn waters and they range between 2 and 6 ft over the rest of the waters. Nww3 fits well with the 00z observed seas pattern but underdone by 2 ft over the Hudson Canyon. Ecmwfwave has initialized well with the seas pattern and is also underdone by a foot over Hudson Canyon. Otherwise both wave models show decent agreement in the short term but differences are vivid in the extended period especially over the srn waters. For this update will continue with nww3 initially then transition to ecmwfwave similar to the earlier solution.
The nwp models GFS/CMC/ecmwfhr/ukmethr/NOGAPS/jma have intialized fairly well with the 00z observations and they are ij decent agreement in the short term. There are some differences in the extended period mainly over the srn waters as hurc Matthew apporoaches from the S. GFS and CMC are still faster to bring the storm into the waters while ukmethr is the slowest. For this update will continue with the GFS initially and then switch to ecmwfhr. In the short term winds will diminish to below gale force and so will not have gale warnings in the forecast. Will keep monitoring the onset of higher winds over the srn waters as hurcn Matthew gets closer to the waters form the S. For more on hurcn Matthew see the latest NHC advisory.
----------------------------------------------------------------- Previous discussion...
Combination of a lopres and warm front draped across the central opc waters NE from near Nags Head then E along 38-39n -- with a strong surface high over Maine -- continues to generate persistent east to northeast onshore flow across much of the mid-Atlc and northeast. The core of strongest winds reside N of Cape Henlopen with areas of gale force winds embedded within the pressure gradient squash zone confirmed by recent metop-a/b ascat overpasses after 1530z. In addition to the satellite sensed ascat winds model output from 12z GFS is well supported by current observations - most notably buoys 025 and 017 both reporting 31g39 as of 19z. Near term guid from 12z GFS along with regional guidance from 12z 4km NAM and 18z hrrr all suggest winds finally waning below gale criteria just after 00z this evening perhaps closer to 01z or 02z. With the aftn fcst issuance will cont to err on the side of caution and continuity and keep Gale Warning hoisted until 03z tonight.
Aftn grid preference: 12z GFS into 18z Tue...then 12z ECMWF thereafter.
Overall afternoon guidance remains in good agreement throughout most of the forecast period over the next 4.5 days...with differences amplifying towards the very end of the forecast period with respect to Hurricane Matthew. Broad cutoff upper low over the Ohio Valley will remain nearly stationary tonight into Saturday...before slowly lifting northeast and gradually filling Sunday into Monday. To varying degrees of agreement GFS and ECMWF then suggest another Rex block as upper high cutoffs the low once again...Tuesday into Wednesday...this time mcuh weaker and in the waters E of Nantucket and south of Nova Scotia. Sensible surface weather: after winds abate below gale criteria tonight another weak low will form along the warm front Sat...then slowly move NE across the nrn waters Sat night and sun. Return S-SW flow will spread across most of the offshore zones sun into Sun night ahead of approaching surface trough or forming weak cold front...which will slowly move across the waters Sun night and Mon as parent low meanders E along / between 38n-40n. Strong high pres will filter from the N over the nrn waters Mon night and Tue...then cont to spread S Tue night into Wed night. E-NE gradients will increase Tue into Wed night over the nrn waters in a similar scenario to what has plagued the nrn zns and mid-Atlc coast as of late...however winds should remain below gale criteria and over a wider areal extent but further offshore. Deference to ECMWF soln late the forecast period provides the grids the slower forward / north speed of approaching Hurricane Matthew. At 18z Oct 05 latest NHC guidance forecasts position at 25.0n 76.0w.
Hurricane Matthew is currently churning over the south-central Caribbean and is forecast to move WNW over the next 2-3 days before banking hard right sun...clipping eastern Jamaica Mon...eastern Cuba Mon night into Tue...and moving to a position over the Bahamas by early Wed aftn. All marine interests especially over the southern zones should remain vigilant in monitoring the latest forecasts from the National Hurricane Center.
Seas...significant wave height obs match well with ww3 and wam guidance and based on 18z regional sea state analysis Max values to 15 ft are occuring within the area of E-NE gales. Will be using the ww3 thrut most of the forecast...except tail end where preference remains with ECMWF soln. Will be trending grids towards the ECMWF wam accordingly.
Extratropical storm surge guidance...current 12z estofs guidance conts to output positive surge values of 1-2 ft from VA beach northward to near Nantucket. Combing through station data this morning and early aftn off the mid-Atlc coast...it appears that some of these values may be as much as a foot on the low side. Surge values will gradually wane late tonight and into the weekend as the onshore flow weakens.
.Nt1 New England waters... .anz815...south of Long Island... gale tonight.
.Nt2 mid-Atlantic waters... None.
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.Forecaster musonda/musonda/Collins. Ocean prediction center.