Marine Weather for HS 100


marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 929 PM EST Mon Dec 10 2018

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

The GOES water vapor satellite imagery shows a pair of mid-level vorticity maximums over the nt2 offshore water this evening, and a strong shortwave trough along the se coast about to move offshore. The latest surface analysis indicates a strong low pressure center over the ern nt2 waters associated with the vorticity maximums. The models indicate that the low will move off to the east quickly tonight as a second low forms to its SW as a result of the shortwave trough moving in from the SW as the right entrance region of a strong upper level jet streak strengthens over the outer central nt2 zones. The 18z GFS and 12z ECMWF both indicate that the second low will move E across the area late tonight into Tue night while strengthening due to the forcing from the aforementioned upper level features over the warmer waters in the Gulf Stream. The previous forecast started out with widespread gales across the central nt2 offshore waters this evening, and ascat wind retrievals from 14z this morning confirmed the stronger winds but closer to the coast. Current surface observations show up to 25 kt at 00z right along the srn mid Atlc coast, though the strongest conditions are now further E near the low center where there are currently no observations. The infrared satellite imagery shows strong cold advection in NE flow across the Gulf Stream,which is normally underforecast in the guidance. With the latest guidance showing gales in nt2 this evening, confidence with the previous gale headlines is above average. In addition, the 12z ECMWF which is in decent agreement with the bulk of the 12z guidance outside of the GFS which has been an inconsistent outlier solution has been indicating strong development of the second low as the shortwave moves into the area and strengthens the low. The model shows storm force winds for a brief period in the outer nt2 waters Tue afternoon before it passes E of the area while continuing to strengthen. This seems to be a very likely scenario and is supported by the 18z GFS which is starting to trend towards the model consensus. As a result, am planning on also continuing the previous storm headline with above average confidence.

Beyond that, the GFS/ECMWF both indicate a weak low will pass se across the nt1 waters on Wed afternoon as a somewhat weak northern stream shortwave trough moves across the area. The models then indicate another strong low approaching the coast on Fri, before passing through the offshore waters on Sat while pulling a cold front over the area. The GFS/ECMWF agree somewhat on the approach of the low, but the GFS diverges going into Sat and is faster and NW of the ECMWF track. There is quite a bit of spread on the track when comparing the rest of the 12z global models, but there is a moderately strong signal in the 18z gefs members for a track similar to the 12z ECMWF. Wpc medium range guidance also favors the ECMWF, which was the model of choice in the previous forecast. As a result, am favoring the ECMWF as well. As far as the winds and headlines are concerned, the models are in better agreement on the timing and intensity of the associated cold front and winds ahead of it. The ECMWF is indicating gales mainly in the unstable environment over the Gulf Stream. This supports the previous gale headlines in nt2, so planning on not making major changes to them. Otherwise, will only be making minor edits to the previous grids in the update package.

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

Over the short term, no significant adjustments to the ongoing opc forecast appear needed for the afternoon forecast package. An ascat overpass from this morning around 1510z indicated widespread gales occurring over southern and central nt2 waters which matched up quite well with the opc grids. The 12z models are generally in good agreement over the waters into Wed night, with the 12z GFS still a bit of an outlier as it strays from most of the guidance as it keeps low pressure moving NE from the southern nt2 waters on Tue a little too far S, and likely not deep enough as nearly all guidance deepens the low well in the 970 mb range, with the GFS into the 990s E of the waters, near 40n 60w, early Tue. A large high pressure area will then build se toward the region Tue night into Wed as the low intensifies further and moves S of the Canadian Maritimes. For the afternoon package, with good agreement from the 12z ECMWF, we will only make minor adjustments to the previous grids for tonight through Wed, mainly in deference to nearby coastal WFO and TAFB grids with no change to the previous hazards. Storm force winds still appear rather likely on Tue for the outer zone of anz930, with some chance for storm force winds expanding into nearby zones. This potential will be addressed in the next few forecasts. Outside of this zone, gales are likely through Tue into Tue night. Gales will then re-develop in the strong cold air advection occurring over the waters between the departing intensifying low and high pressure building se from eastern Canada, Wed into Wed night. Forecast confidence is near average over the short term, into Wed night.

Over the medium range, for Thu into Sat night we will rely on the previous grids which were close to the 00z ECMWF winds for Thu through Fri night as the 12z ECMWF is in very good agreement with the 00z ECMWF from last night during this time frame. Gales will end Thu as high pressure overspreads most of the region as a front forms near the se U.S. Coast. The front will lift N over the waters as a warm front Fri into Fri night as low pressure slides E and NE over the Gulf states. We will maintain the previous gale force wind hazards for Fri night off the se U.S. Coast for now as the 12z ECMWF also indicates the potential for gale force winds over these waters. For Sat and Sat night we will transition toward an even blend of the previous grids and 12z ECMWF winds, with the low pressure area tracking from near the Delmarva coast E and NE over northern nt2 waters. Gales will spread N toward northern nt2 waters Sat and Sat night, and we will continue to cap winds at 40 kt until better overall model agreement develops in the coming days. The opc forecast once again fits well with the latest wpc medium range guidance, and with the ECMWF model remaining consistent we feel confident enough to add gales to the nt2 waters for Sat into Sat night. These gales could be even more widespread with some potential for storm force winds developing over the weekend. Please closely monitor the latest opc forecast.

Seas...sea heights appear to be running up to 3-6 feet above both the 12z wavewatch and 12z ECMWF wam model output off the North Carolina coast according to the latest observations and altimeter data which shows maximum seas around 27 feet over this region early this afternoon. This area is close to the west wall of the Gulf Stream, and with the counter-flow occurring in this area we would expect these higher seas in this region. For the afternoon package, as we remain close to the previous wind grids over the next few days we will also stay close to the previous sea height grids, except to add up to 25 percent or so to the wave height grids in this general region into tonight. By Sat through sun to match the winds we will blend in the 12z wam guidance to the previous grids and as we cap winds at 40 kt as noted above, cap sea heights at 14 ft to match these capped winds.

.Extratropical storm surge guidance...In the strong easterly fetch off the mid Atlc coast this afternoon, the 12z estofs and etss both show a positive surge along the coast from Cape Hatteras to Cape May with highest values still near 1.5 ft along the Outer Banks into this evening. This still seems reasonable considering the strong flow, though might be a tad underdone. The models then show another surge of about a foot on Fri into Sat in the strong southerly flow ahead of the next strong low pressure system. However, since the GFS was deemed a bit too fast, the surge is still more than likely too fast to come as well. Interests along the mid-Atlantic and southeast U.S. Coasts should monitor the latest advisories or warnings, and forecasts from coastal National Weather Service offices.


.Nt1 New England waters... None.

.Nt2 mid-Atlantic waters... .anz920...Baltimore Canyon to the great South Channel... gale Tuesday. Gale Wednesday. Gale possible Saturday into Saturday night. .Anz905...The great South Channel to the Hague line... gale Tuesday night into Wednesday. Gale possible Wednesday night. Gale possible Friday night. .Anz910...East of the great South Channel and south of 39n... gale Tuesday into Wednesday. Gale possible Wednesday night. Gale possible Saturday night. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale possible Saturday. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale tonight into Wednesday. Gale possible Saturday into Saturday night. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale tonight. Gale possible Friday night into Saturday. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale tonight. Gale possible Friday night into Saturday. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale tonight. Storm Tuesday. Gale possible Friday night into Saturday. .Anz835...Inner waters from Cape Fear to 31n... gale tonight. Gale possible Friday night into Saturday. .Anz935...Outer waters from Cape Fear to 31n... gale tonight into Tuesday. Gale possible Friday night into Saturday.


.Forecaster Kells/Mills. Ocean prediction center.

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