marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 904 PM EDT Mon Jun 26 2017
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The NCEP surface analysis indicates weak low pres off the se coast along a weak stationary front across the nt2 waters, and a high pres ridge to the the se of the waters. The current surface observations indicate up to 15 kt in the offshore waters, though the infrared satellite imagery shows an area of weakening convection over far srn nt2 ahead of the front which may have a few gusts above 15 kt. The previous forecast indicated up to 20 kt over a majority of the offshore waters, and favored the 12z ECMWF into 18z Tue. The 12z/18z models all agree on the front remaining very while while the low off the se coast moves NE along it. Will only be making minor adjustments from the previous forecast based on current data for tonight and Tue.
The models stay in generally good agreement over the forecast period, except for the 12z Gem which becomes an outlier solution into the medium range period. The rest of the models indicate a relatively weak pattern over the next 5 days, and the GFS/ECMWF differ slightly on the track and timing of a few weak features over the next few days. The previous forecast favored the 12z GFS which is supported marginally better by the 12z UKMET. The 18z GFS has not changed much from the 12z run, so will only be making minor adjustments in the update package.
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This mornings offshore ascat overpasses returned winds to 30 kt and even some retrievals to marginal gale south of 31n which were all associated with the strong thunderstorms near 1000 fathoms from 31n to 32n occurring at that time. Elsewhere ascat showed some winds to 20 kt across the outer nt2 waters near the front. Based on the recent lightning data and warming cloud tops, this convection has weakened significantly since this morning. The sref based probability guidance did not seem to have a good handle with these thunderstorms, instead showing higher probabilities nearer the coast this morning. There will likely be another diurnal maximum in this thunderstorm activity around daybreak. Overall the 12z models are in very good agreement across the west Atlantic for the next several days. The 12z GFS/ECMWF/CMC/UKMET all forecast a few weak lows to develop across the southern/central nt2 waters tonight and move NE along the weak stationary front Tuesday into Tuesday night. The 12z ECMWF shows the associated wind reaching 20 kt and this is well supported by the higher resolution 4km NAM/hrrr. The 12z models then forecast a fairly amplified upper level trough will reach the coast late Tuesday, and support a cold front passing east across the waters Tuesday into early Wednesday. The guidance is consistent with the timing of the cold front passing east of the nt2 waters by early Wednesday, and then high pressure subsequently building to the coast by Wednesday evening.
Then with the low forecast to track east across New England and its associated front forecast to approach the New England coast late Thursday and Friday, some timing differences remain between the latest models. The 12z ECMWF is faster than the 12z GFS/UKMET/CMC in moving the cold front off the coast. The latest gefs and ECMWF eps both support the slower GFS/UKMET. The 12z GFS looks reasonable with the return flow to 25 kt Thursday night through Friday night, while the 12z parallel GFS continued to be overdone with 10m wind close to gale across the Gulf of Maine. The 12z GFS/UKMET are mostly consistent that the cold front will stall across the Gulf of Maine Saturday and lift back N as a warm front Saturday night. Meanwhile through the medium range, a high pressure ridge will persist across the nt2 waters.
.Seas...The 12z wavewatch iii and 12z ECMWF wam are well initialized with the offshore wave heights (4 ft or less) this afternoon. These models are generally within a ft or so of each other through the forecast period. To account for some minor differences will populate wind grids with a 50/50 blend of the two models through the forecast period.
.Extratropical storm surge guidance...N/A.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Kells/Clark. Ocean prediction center.