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000 AGNT40 KWNM 290029 MIMATN Marine Weather Discussion for N Atlantic Ocean NWS Ocean Prediction Center Washington DC 829 PM EDT Fri Apr 28 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 low pres trough extending ne to sw across the nt1 and nrn/central nt2 waters into the cape hatteras area. The analysis also shows a cold front approaching the long island and new jersey coasts. Latest available ascat and ascat hi-res passes from this morning show 15 to 25 kt winds in over the ne part of the nt2 area, with 10 to 15 kt winds indicated over the gulf of maine, georges bank and the srn nt2 waters. Lightning density product data at 23z shows scattered showers in zones 905/910/920 over the ne part of the nt2 area, with scattered showers and tstms over zone 925 in the central nt2 waters. Models...The medium range models are in good agreement across the offshore waters for tonight into Sun, so the representative gfs 10m/30m solution with the stability smart tool will be used for the wind grids over this timeframe. The gfs is just a bit faster than the other models with the warm front for Mon and Mon night. The models are still split into two camps concerning the strength of the s to sw flow ahead of the cold front for late Mon through Tue, with the gem/gfs offering a stronger assessment with gales and the ecmwf/ukmet showing a weaker solution. The stronger assessment has been favored and we have gale warnings up for Tue in the nt2 waters around the gulf stream, with average confidence. Will use the gfs 10m solution for late Sun through the rest of the forecast period, but time shifting it 3 hours slower in deference to the somewhat slower ecmwf/gem/ukmet models. Am not planning to make any significant changes to the current forecast trend. Seas...The wna wavewatch and ecmwf wam both initialized well versus latest observations over the offshore waters. The models are in good overall agreement through the forecast period, with only minor differences noted. Will use a 50/50 blend of the two models for the sea height grids over the entire forecast period, but with a 3 hour time shift on the wna wavewatch to account for the time shifted gfs winds on Tue. Extratropical Storm Surge Guidance...N/A. ----------------------------------------------------------------- PREVIOUS DISCUSSION... The latest GOES visible satellite imagery and the lightning density product indicates a line of showers and tstms moving through the nrn NT2 and SE NT1 waters this afternoon ahead of a low pres trough over the area. The Ascat wind retrievals from 1400 UTC this morning indicated up to 25 kt in the SW flow over the Gulf Stream, and up to 15 kt to the N and W of it. Current surface observations indicate winds only up to 15 kt in the offshore waters, though there are none near the trough. The 12Z GFS winds are well initialized over a majority of the offshore waters when compared with the current data, and although they are slightly below the 25 kt Ascat winds over the Gulf Stream. The 12Z models all indicate that the trough will pass east of the offshore waters tonight before a cold front that is quickly approaching the New England coast follows closely behind the trough. The models all indicate the 2 features will remain fairly weak while over the area with winds generally less than 25 kt. The 12Z GFS 10m winds seem reasonable over stable areas while the first sigma winds better reflect the deeper mixing expected over less stable areas. As a result, planning on starting out with that combination over the forecast area while making minor edits to account for the higher winds currently over the Gulf Stream. The biggest forecast problem is with the next system starting on Sun. The GFS has been a bit faster with a warm front through moving NE across nrn NT2 and NT1, and the ECMWF/UKMET/GEM have all been about 3 to 6 hours slower with the front for the past few days now, and is also a little slower with the associated cold front. In addition, there are differences on the intensity of the winds in the southerly flow ahead of the associated cold front. The GFS and GEM have been consistently stronger with the winds and indicate gales, and both continue to show them in their respective 12Z runs. The ECMWF/UKMET have been consistently weaker, and indicate 25 kt with a few small areas of 30 kt. However, both the 12Z ECMWF and UKMET have trended a little stronger in their 12Z runs and have expanded the areal coverage of the higher winds despite maintaining 30 kt as the maximum value at the surface. The ECMWF had also been weaker with the low level jet ahead of the front, though it has been trending slightly stronger over the past few runs. Confidence with gales developing over the offshore waters is near to just above average as a result of the consistent forecast of the GFS and the stronger trend of the 12Z ECMWF. Also, the known low bias of the ECMWF/UKMET surface winds would indicate a stronger result. The previous forecast capped winds at 30 kt, so at this point will go with the GFS 10m wind and introduce marginal gales in NT2 for Tue in the less stable environment over the Gulf Stream. However, will time shift the GFS grids 3 hours slower in deference to the timing of the 12Z ECMWF/UKMET solutions. Otherwise will continue with the 12Z GFS 10m winds for the remainder of the forecast period. Seas...The 12Z Wavewatch and ECMWF are both initialized within a ft or so of the current observations over the offshore waters. The models are in generally good agreement through the forecast period, with only minor differences noted mainly related to the small timing differences noted above. Will use a 50/50 blend of the two models for the wave height grids over the entire forecast period, but will first use a 3 hour time shift on the Wavewatch to account for the slightly too fast GFS winds on Tue. .Extratropical Storm Surge Guidance...N/A. .WARNINGS...Preliminary. .NT1 New England Waters... None. .NT2 Mid-Atlantic Waters... .ANZ920...Baltimore Canyon to the Great South Channel... Gale Possible Tuesday. .ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light... Gale Possible Tuesday. .ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon... Gale Possible Tuesday. .ANZ830...Inner Waters from Currituck Beach Light to Cape Hatteras... Gale Possible Tuesday. $$ .Forecaster Scovil/Kells. Ocean Prediction Center.

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