Marine Weather for HS 100

Forecast

marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 750 PM EDT sun Apr 23 2017

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

A stationary front persists W to E across the S central nt2 waters which in combination with a high pressure ridge across the srn nt1 waters is maintaining a moderately strong (20-25 kt) Ely gradient immediately N of the front. Max seas throughout the waters are in this gradient and are likely in the 6-9 ft range which are being handled reasonably well by the 18z wavewatch iii and 12z ECMWF wam models at the moment.

Over the short term, the latest models agree that the front will lift slowly N as a warm front tonight into Mon while maintaining the moderately strong Ely gradient to its immediate N. Then later Mon in response to a closed upper low forecast to move towards the se coast, the models remain consistent in forecasting a surface low to lift N into the far SW nt2 waters and merge with another low off the S Carolina coast with ESE gales developing. Then Mon night into Tue night the models generally agree that the surface low will become vertically stacked with the upper low and track slowly NE to off the mid Atlantic coast with the associated gales spreading NE across the nt2 waters. Versus its previous 12z run, the 18z GFS has trended slower with its forecast track of this low which brings it into better agreement with the 12z ECMWF track. The previous forecast package used a mix of 12z GFS winds for this system. So these winds still look representative, but will now go ahead and make some additional generally minor edits in deference to the 18z GFS/12z ECMWF.

In the long range, as the models forecast the upper low to weaken, they all forecast the associated surface low to weaken as it moves towards the srn New England coast Wed night with associated winds becoming subgale. Therefore will continue to make some additional minor edits then to the previously populated 12z GFS winds in deference to the 18z GFS/12z ECMWF. Then late Fri/Fri night, versus its previous 12z run the 18z GFS has trended slower with a cold front moving into the nt1 waters which brings it into better agreement with the previously populated 12z ECMWF solution. So do not plan on making any major long range changes in the next offshore forecast package.

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

A late season Nor'easter will bring widespread high-end gales to mainly the southern and central nt2 offshore waters Monday into Wednesday night.

Over the short term, a nearly stationary front extends northeast from near Charleston, South Carolina, with high pressure centered just south of Long Island. Low pressure was centered over far northern Georgia with another low over far southwest Florida per the preliminary 18z NCEP surface analysis. These low pressure areas will combine near the South Carolina coast later Monday, with the low then tracking north and northeast along or just off the East Coast Monday night through Wednesday night with the low weakening near eastern Long Island Wednesday night. Further north, the high will move east away from the waters Monday as a cold front moves southeast toward eastern Maine. The front will dissipate Monday night just north of the waters as high pressure builds over southeast Canada. Overall, the 12z models are in very good agreement over the region through Wednesday night. The 12z GFS has remained quite consistent over its past few cycles, and is in excellent agreement with the 12z ECMWF. As a result, we will rely on the 12z GFS and continue to use the smart tool which places the stronger first sigma layer winds over the unstable areas and somewhat lower 10 meter winds over stable areas tonight through Wednesday night over the waters. Widespread gales appear likely over southern and central nt2 waters Monday into Wednesday night, with above normal confidence. We will manually trim back gales for Tuesday from the far northwest nt2 waters, or zone anz820, for this package which will keep the headlines consistent from the previous few forecasts. We will continue to monitor future model guidance and trends to see if the gales need to be expanded into northern nt1 zones over the next day or so. Otherwise, widespread shower and thunderstorm activity will occur near the low as it tracks over the region tomorrow through Wednesday night. Currently, most of the scattered thunderstorms are located near the stationary extending east-northeast from near Charleston. Wind gusts exceeding gale force and very rough seas can be expected in or near any of the heavier thunderstorms that develop over the waters during the next few days.

Over the long term, the 12z models remain in good agreement over the waters Thursday into Thursday night. The low will weaken near the New England coast Thursday into Thursday night as a high pressure ridge builds west toward southern nt2 waters. We will remain close to the 12z GFS 10 meter winds for Thursday and Thursday night as a result, with no headlines expected over the waters during this time period. For Friday and Friday night the 12z models begin to diverge. The 12z GFS appears to be a little too fast in bringing the next cold front east and southeast over the waters Friday and Friday night, with the 12z ECMWF closer to the model consensus and ensemble mean forecasts. As a result, we will trend the forecast toward the 12z ECMWF for Friday and Friday night over the region, with confidence levels near average. This will result in the cold front passing southeast over nt1 waters Friday night, with the ridge dominating from southern nt2 waters eastward. This idea is also supported by the latest medium range forecast from wpc.

Seas...both the 12z wavewatch and ECMWF wam are both initialized well over the waters per the latest altimeter data and observations. For the afternoon package we will continue to use a 50/50 blend of the two wave models tonight through Thursday night, and then to match the wind forecast trend the forecast toward a nearly 100 percent wam model output for Friday and Friday night.

.Extratropical storm surge guidance...The 12z estofs model is indicating a surge of about 1 to 2 feet so along the southeast and mid-Atlantic coast by Tuesday into Wednesday with the persistent easterly fetch ahead of the Nor'easter moving up the coast. The 12z etss is a bit lower with the surge. At this time a compromise between these two surge models is preferred over the region into the middle of the week, as we continue to rely on the GFS model guidance for the wind forecast over the region.

.Warnings...Preliminary.

.Nt1 New England waters... None.

.Nt2 mid-Atlantic waters... .anz920...Baltimore Canyon to the great South Channel... gale Tuesday. Gale possible Tuesday night into Wednesday. .Anz910...East of the great South Channel and south of 39n... gale Tuesday. Gale possible Tuesday night into Wednesday night. .Anz825...Inner waters from Baltimore Canyon to Cape Charles Light... gale Tuesday. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale Monday night. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale Monday night into Tuesday. Gale possible Tuesday night. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale Monday night. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale Monday into Monday night. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale Monday into Tuesday. .Anz835...Inner waters from Cape Fear to 31n... gale Monday. .Anz935...Outer waters from Cape Fear to 31n... gale Monday.

$$

.Forecaster vukits/Mills. Ocean prediction center.

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