marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 313 am EST Thu Dec 14 2017
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Earlier ascat imagery from around 00utc showed a small area of winds near 50kt in the eastern portion of anz905. Reviewing guidance for that time showed that even the most unstable wind field failed to show winds that high. However, interpreted from model soundings, the very low level of the atmosphere had such a steep lapse rate, greater than dry adiabatic with the sea surface temperature at least into the 60s if not possibly in the lower 70s in parts of that zone, that mixing to the surface of the higher winds toward 925mb indeed occurred. The core of these stronger winds should now be east of the northern waters but with only a short respite from wind hazards.
This forecast will maintain the previous forecast headlines in very large part. Water vapor imagery shows the compact mid-level wave associated with a clipper system moving over the mid- Atlantic states, and this will move over the offshore waters early today. Surface low pressure appeared to be over western Pennsylvania at 07utc and this will move east across the offshore waters in vicinity of 40n through the afternoon. In an unstable air mass from just north of 40n south through areas off of the North Carolina coast, gales are expected. 925mb winds become strongest in the eastern portions of anz905 and anz910 around 18utc, to 50 to 60kt from both the GFS and ECMWF, such that storm-force conditions have a good chance of occurring there, either with sustained wind or associated with scattered deep convection. Gusts with any deep convection there could approach 65kt.
The surface gradient slackens overnight tonight and into Friday, ahead of another low that should move off the North Carolina coast on Friday afternoon. In the modest gradient and instability ahead of the low, gales are expected from roughly anz930 northeast into anz910 late Friday and Friday night, then gales are possible Saturday into Saturday night in the cold-air advection behind the low and mainly over much of the mid-Atlantic and New England offshore waters. The low strengthens as it moves east of the offshore waters maintaining a decent northwest gradient until the low moves far enough away and high pressure builds in from the west overnight Saturday.
Through the weekend, guidance from the GFS, UKMET, and ECMWF are fairly similar. Some of the raw 10-meter wind guidance is below the current forecast, but when factoring in instability between 10 meters and 30 meters, the result is very similar to the ongoing forecast. Made some minor adjustments to the gridded forecast output to ensure continuity with previous headlines. Really took a look at the expected wind in anz815 where no hazard will exist in the near term, but there was no guidance that would provide for gales there, so will not extend headlines into that zone for today, though it will be close. Later, the ECMWF is a little farther north with the track of surface low pressure Friday night compared to the GFS, the difference being, in deference to the ECMWF and because it was already close on the GFS, ensuring some gales along and ahead of the low in anz910 Friday night.
Uncertainty increases early next week, and for this forecast will cap winds just below gales after the weekend through the end of the seven-day forecast. The ECMWF has definitely trended toward the weaker UKMET/GFS blend with regard to any low pressure developing near or east of the offshore waters by Tuesday. The ECMWF is also farther east. The UKMET would not provide for any gales in the offshore waters, albeit close to the far eastern mid-Atlantic zones late Monday night into Tuesday. The GFS would have spotty gales spatially and temporally mainly over the northern waters Tuesday night into Thursday, while the ECMWF would forecast widespread gales from Tuesday night through Wednesday night over the mid-Atlantic and northern waters due to its stronger low and tighter gradient. The ECMWF is also similar to the UKMET Monday night, just a little farther north and west encroaching gales into the central eastern offshore waters then. Considering all of the guidance and the pattern, there is reasonable likelihood that some gales will be introduced in a northwest flow under cold-air advection by Wednesday, mainly from the mid-Atlantic waters north.
.Seas...For this forecast, will lean toward a blend of the wavewatch and ECMWF wam. Most of the guidance is close through the weekend. However, the wave forecasts depart from one another toward the end of the seven-day forecast due to uncertainty with low strength and track. While not currently preferring waves out to Wednesday as high was what the ECMWF wam would indicate, given the persistence of the west and northwest flow, currently think the wavewatch could be underdone particularly near the Gulf Stream toward anz905 and anz910 by almost a factor of two.
.Extratropical storm surge guidance...With a predominantly offshore flow, etss and estofs surge values are a foot or less throughout the forecast period. Values on the estofs are highest in the Gulf of Maine, between a half-foot and a foot, late Tuesday.
.Nt1 New England waters... .anz805...Georges Bank west of 68w... gale today. Gale possible Saturday into Saturday night. .Anz900...Georges Bank east of 68w... gale possible Saturday into Saturday night. .Anz810...South of New England... gale today. Gale possible Saturday into Saturday night. .Anz815...South of Long Island... gale possible Saturday.
.Nt2 mid-Atlantic waters... .anz820...Hudson Canyon to Baltimore Canyon... gale today. Gale possible Saturday. .Anz915...Hudson Canyon to the great South Channel... gale today. Gale possible Saturday into Saturday night. .Anz920...Baltimore Canyon to the great South Channel... gale today. Gale Friday night. Gale possible Saturday. .Anz905...The great South Channel to the Hague line... storm today. Gale possible Saturday into Saturday night. .Anz910...East of the great South Channel and south of 39n... storm today. Gale Friday night. Gale possible Saturday into Saturday night. .Anz825...Inner waters from Baltimore Canyon to Cape Charles Light... gale today. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale today. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale today. Gale Friday night. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale today. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale today. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale today. Gale Friday night.
.Forecaster figurskey. Ocean prediction center.