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000 AGNT40 KWNM 260100 MIMATN Marine Weather Discussion for N Atlantic Ocean NWS Ocean Prediction Center Washington DC 900 PM EDT Tue Jul 25 2017 .FORECAST DISCUSSION...Major features/winds/seas/significant .weather for the North Atlantic Ocean W of 50W from 30N to 50N. Update...The 18z ncep surface analysis shows a weak 1016 mb low centered near 38N71W in the northern nt2 waters, with a cold front extending SW from the low to Cape Hatteras, and a warm front extending NE from the low over the NE part of the nt2 area. The analysis also shows a low pres trough extending W across the Gulf of Maine, with a second trough SW to NE across the far southern nt2 zones. Latest available ascat hi-res and ascat passes from this morning show 25 to 30 kt winds in SW flow over the southern and central nt2 waters, with mainly 15 to 25 kt winds across the rest of the offshore waters. Lightning density product data at 23z indicates scattered showers and isolated tstms over much of the nt2 area and mainly with the cold front. Models...The main forecast problem during the forecast period involves the strength of the SW flow in the outer zones of the nt2 area ahead of a cold front, and the strength of the NE flow in the S of New England waters for Sat and Sat night, since the medium range models are in poor agreement regarding the strength of the winds here. The gem has winds to 40 kt, the gfs 30m has winds to 35 kt, the ecmwf and gfs 10m have winds to 30 kt, and the ukmet has winds to 25 kt. Although a boosted version of the ecmwf was previously used for the wind grids on Sat and Sat night, the gfs 30m and gem do support gales. However, an additional problem is the medium range models are in fairly poor agreement with the timing of the main weather features for the Sat through Sun timeframe. For continuity purposes will keep the gale warnings up for Sat and Sat night but with low to moderate confidence. Otherwise, the models are in good overall agreement across the offshore waters, so the ecmwf will continue to be used for the wind grids over the entire forecast period. Seas...The wna wavewatch and ecmwf wam both initialized well over the offshore waters. The models are in close agreement for tonight into Fri, but then differences develop and the ecmwf wam builds up the seas a few feet higher in the nt2 waters on Sat and Sat night. Since the ecmwf is being used for the wind grids over the entire forecast period, the ecmwf wam will be used for the sea height grids throughout as well, in order to be consistent. Extratropical Storm Surge Guidance...N/A. ----------------------------------------------------------------- PREVIOUS DISCUSSION... With a frontal boundary located from off of the Carolina coast northeast to east of Nova Scotia, the primary hazard for the near-term part of the forecast will be scattered-to-numerous showers and thunderstorms in vicinity of the stalling, weakening front. To the north, in advance of the next front and in warm air advection, there should be some areas of low visibility over northern waters by Thursday. Into Friday, the surface gradient in general should be slack, resulting in winds well below hazards. As has been consistently noted recently, the primary forecast challenge comes toward the end of the week and into the weekend. The latest guidance offers departures from previous runs. In this case, the GFS and the ECMWF both have a closed 500mb low moving southeast over southeast Canada, but lag a trailing low at that level moving southeast out of the Great Lakes toward the mid- Atlantic. The result is low pressure that develops and moves off of the coast more slowly and farther south. The NAM is somewhat in agreement with this, while the UKMET is even slower aloft and keeps low pressure much farther north along the coast and into the far northern waters late in the weekend. GFS ensemble clustering is quite tight into Saturday, with surface low pressure emerging off of Delmarva Saturday morning, but then with good spread thereafter with a consensus track a little north of east. The deterministic ECMWF is similar. If the GFS and ECMWF verify, or verify close, it would seem two areas of gales are quite possible. The first would be in the tight gradient to the north of low pressure, off the coast from southern MA to off of DE, primarily ANZ810, 815, and 820, into 915 before the low starts to fill a little and move northeast. The other area would be near the Gulf Stream from ANZ910 southwest to ANZ930, where 850mb winds on both the GFS and ECMWF increase to 35 to 45kt. Given the guidance variability of late, confidence in the timing and location of gales is only modest, but confidence is growing that an area, or areas, of gales will occur Saturday into Sunday. Some refinement is possible in any headlines over time. Overall, for this forecast leaned toward the ECMWF winds with modifications, particularly Saturday into Sunday. .SEAS...Based on the wind forecast above, leaned toward the ECMWF WAM wave output, modified especially for expected conditions Saturday into Sunday. The primary increase in waves from that guidance was in vicinity of the Gulf Stream Saturday and Saturday night, in the axis of higher southwest winds. For this forecast, waves should have a maximum near 15 feet east of the Delmarva coast. As with the wind forecast described above, modifications are possible if the variability in the guidance continues. .EXTRATROPICAL STORM SURGE GUIDANCE...N/A. .WARNINGS...Preliminary. .NT1 New England Waters... .ANZ810...South of New England... Gale Possible Saturday. .ANZ815...South of Long Island... Gale Possible Saturday. .NT2 Mid-Atlantic Waters... .ANZ820...Hudson Canyon to Baltimore Canyon... Gale Possible Saturday. .ANZ915...Hudson Canyon to the Great South Channel... Gale Possible Saturday into Saturday night. .ANZ920...Baltimore Canyon to the Great South Channel... Gale Possible Saturday. .ANZ910...East of the Great South Channel and south of 39N... Gale Possible Saturday into Saturday night. .ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon... Gale Possible Saturday. .ANZ930...Outer Waters from Hatteras Canyon to Cape Fear... Gale Possible Saturday. $$ .Forecaster Scovil/Figurskey. Ocean Prediction Center.

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