marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 836 PM EDT sun Aug 20 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 roughly NE to SW across the Gulf of Maine, with a cold front extending NE to SW over the northern and central nt2 wtrs, then becoming a stationary front near Cape Hatteras. The analysis also shows a high pres ridge over the southern nt2 waters S of the front. Latest available ascat hi-res and ascat passes from this morning show 5 to 15 kt winds over the mid Atlantic, Carolina coast and se coastal areas, and also across the far NE nt2 waters and the Georges Bank zones. Lightning density product data at 2310z shows isolated showers and tstms over the southern nt2 waters.
Models...the medium range models are in good overall agreement across the offshore waters during the forecast period, except the Gem/UKMET bring an extremely strong tropical low N and NE over the nt2 waters on Fri and Fri night. The ECMWF and GFS also indicate a tropical low emerging into the Blake Ridge area by Fri or Fri night, but they are significantly weaker and slower with the system. Overall it looks like the ECMWF and GFS 10m solutions are in the best agreement, so for now a representative 50/50 blend of the ECMWF/GFS will be used for the wind grids through the forecast period. Am not planning to make any significant changes to the current forecast trend.
Seas...the wna wavewatch and ECMWF wam both initialized well over the offshore waters, and are in good overall agreement through the forecast period with only minor differences noted. A 50/50 blend of the wna wavewatch/ECMWF wam will be used for the sea height grids over the entire forecast period in order to smooth out the model differences.
Extratropical storm surge guidance...N/A.
----------------------------------------------------------------- Previous discussion...
Per the 18z opc surface analysis a cold front was located across the far northeastern nt2 waters, extending southwest across the central nt2 waters to a point just off the North Carolina coast. The latest ascat high resolution pass had data mostly over the inner nt1 and nt2 waters where winds of 15 kt or less were generally noted with the lightest winds in the waters off the VA/NC coast and over anz835 in the southwest nt2 waters.
The 12z global guidance was in good agreement over much of the nt1 and nt2 offshore waters into Wednesday with differences noted heading into Thursday, more so over the nt2 waters in regards to some guidance developing low pressure, possibly tropical in nature, east of FL Tuesday/Tuesday night with it moving northeast across the nt2 waters thurs into Friday. Through Thursday high pressure builds into the waters behind the departing cold front, with the high moving east of the waters Tuesday as a second cold front nears the region. Guidance is in good agreement concerning frontal passage across the nt1 waters Wednesday into Thursday, and over the nt2 waters Wednesday into Wednesday night. Anticipate winds increasing over the nt1 waters and northern nt2 waters ahead of the cold front Tuesday but nothing to warning level criteria.
As mentioned earlier guidance then begins to differ Thursday. The 12z UKMET/CMC develop low pressure east of FL in the Tuesday to Wednesday time frame, strengthening the system as it moves northeast across the southern and central nt2 waters Thursday/Thursday night into in late Friday, then into the northern nt2 waters Friday night. The 12z GFS/ECMWF also develop a similar feature in the same area but are slower than the UKMET/CMC solutions with development taking place east of FL Friday night right at the end of the period or early Saturday, just beyond the end of the period.
With all that noted will favor a 50/50 blend of the 12z GFS/ECMWF through Thursday 00z. For the remainder of the period with the differing solutions between the 12z GFS/ECMWF/UKMET/CMC feel its reasonable to continue with a persistence forecast. This will will consist of a 50/50 blend of the previous 00z GFS/ECMWF run resulting in keeping the southern end of the second cold front over the nt2 waters on days 4 and 5. Concerning the scenarios shown in the UKMET/CMC and GFS/ECMWF camps later in or just beyond the end of the forecast period in regards to low pressure or a possible tropical system, will continue to monitor the situation over the next several days and evaluate future model trends.
.Seas...Favored a 50/50 blend of the 12z nww3/ECMWF wam through Thursday 00z, transition to a persistence forecast beyond resulting in a 50/50 blend of the prior 00z nww3/ECMWF wam model runs.
.Extratropical storm surge guidance...N/A.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Scovil/Holley. Ocean prediction center.