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North Atlantic Marine Weather Discussion[Printable Version] [Alt Link/Previous Versions] [Glossary]
AGNT40 KWNM 180717 MIMATN .Marine Weather Discussion for N Atlantic Ocean .NWS Ocean Prediction Center Washington DC 215 AM EST MON 18 DEC 2017 .FORECAST DISCUSSION...Major features/winds/seas/significant .weather for the North Atlantic Ocean W of 50W from 30N to 50N. The 01 UTC Ascat still returned some northwest winds to 25 kt across the far outer ANZ905 and ANZ910 zones, while the subsequent pass even showed some 20 kt winds south of the stationary boundary as far west as about 80 to 150 nm southest of Cape Charles, over the Gulf Stream. The 00 UTC GFS first sigma level winds were the best initialized with these higher winds offshore. The 00 UTC models are consistent that relatively weak gradients and surface winds will occur across the offshore waters at least through the early part of today as the high pressure ridge is forecast to shift east of the offshore waters. Also, the models share a similar timing in with a developing surface low along the stationary front across the northern NT2 waters today. The developing low should pull the weak front south to near Cape Fear Mon before it again stalls and dissipates tonight/Tue. Based on the slightly higher winds seen in the Ascat, will be blending in some of the higher 00 UTC GFS first sigma level winds with the previous OPC wind grids, which were based on the ECMWF, through tonight. The latest models then share a similar timing with another cold front forecast to move off the northern mid-Atlantic and New England coasts Tue night. The GFS and ECMWF have become more consistent, and slightly stronger compared to their respective runs yesterday, with the post-frontal west to northwest winds across the eastern NT1 and northeastern NT2 zones Tue night into Wed. As a result, we have increasing forecast confidence that there will be a brief period of gales across the Gulf of Maine/Georges Bank and the outer ANZ905 waters at that time. For these winds, we again used an even blend of the GFS first sigma level winds and the ECMWF 10m winds. The gale hazards will remain consistent with the previous offshore forecasts. The 00 UTC models are then consistent that a southern stream low will develop and move off the southern Mid Atlantic coast late Wed and Wed night, with the 00 UTC GFS/ECMWF/UKMET all advertising associated winds as high as 30 kt across the waters south of Cape Hatteras. Of note, the 00Z GFS even trended stronger than previous runs, and now shows some marginal gales associated with the low Wed night. We are favoring the weaker model consensus here, with winds only as high as 30 kt, close to the more consistent ECMWF. Once the low passes east of the offshore waters early Thu, the models are then consistent that an inverted low pressure trough will linger along the Carolina coast Thu night and Fri. At the same time, a strong high pressure ridge will pass east across the NT1 and northern NT2 waters. The models then all show another cold front will approach the eastern seaboard late Fri and Fri night. However, the 00 UTC models vary as to the timing of the front, and vary widely with how amplified the supporting upper level trough is forecast. Versus its previous few runs, the 00 UTC GFS trended significantly faster, and also is a strong outlier with the pre-frontal winds Fri night into the weekend. OPC preference lies with the weaker slower non-GFS model consensus, which is also supported by the latest WPC medium range guidance. .Seas: The 00 UTC Wavewatch III and 00 UTC ECMWF WAM are equally well initialized with the wave heights across the west Atlantic this morning. Will be updating the previously preferred 3:1 ECMWF WAM and WW3 blend for the wave height grids through Thu night. Then, with the GFS being the strong outlier with the late Fri and Fri night winds, we will transition solely to the 00Z ECMWF WAM beginning Fri. Extratropical Storm Surge Guidance...N/A. .WARNINGS...Preliminary. .NT1 New England Waters... .ANZ800...Gulf of Maine... Gale Tuesday night. Gale Possible Wednesday. .ANZ805...Georges Bank west of 68W... Gale Tuesday night. .ANZ900...Georges Bank east of 68W... Gale Tuesday night. Gale Possible Wednesday. .NT2 Mid-Atlantic Waters... .ANZ905...The Great South Channel to the Hague Line... Gale Tuesday night. Gale Possible Wednesday. $$ .Forecaster Clark. Ocean Prediction Center.