marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 854 PM EDT Mon Aug 21 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 stationary front extending roughly NE to SW across the central and southern nt2 waters, with high pres ridging elsewhere over the offshore waters. Latest available ascat hi-res and ascat passes from this morning show 5 to 15 kt winds over the offshore waters. Lightning density product data at 23z shows a few small areas of isolated showers and tstms over the outer zones of the southern nt2 waters.
Models...the medium range models are in good overall agreement across the offshore waters through the forecast period, and the only exception to this is the strength of the S to SW flow over the Gulf of Maine for late Tue and Tue night. Gem/GFS 30m are stronger with the S to SW flow and have Max winds of 30 to 35 kt, while ECMWF/GFS 10m/UKMET are weaker and have Max winds of 25 kt. Given the stability considerations, will be continuing to favor the weaker models for late Tue and Tue night. Am also continuing to watch Blake Ridge and the waters just S of the nt2 area, since the ECMWF/GFS/UKMET all suggest formation of a weak tropical low in this area for Sat and Sat night. Gem is different than the other models since it wants to form the low along the E coast of Florida. Overall the ECMWF/GFS 10m are in the best agreement, so will be using a representative 50/50 blend of these models for the wind grids during 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 with only minor differences noted. A 50/50 blend of the wna wavewatch/ECMWF wam will be used for the sea height grids through Sat night, in order to smooth out the model differences and maintain consistency with the preferred GFS/ECMWF models.
Extratropical storm surge guidance...N/A.
----------------------------------------------------------------- Previous discussion...
At 18z a stationary front was located over the southern nt2 waters. A 1025 mb high was located over the northern nt2 waters with its ridging extending into the nt1 waters with a 1023 mb high just east of the southern waters with its ridge extending into that region. Latest ascat high resolution data revealed 5-15 kt winds across both the nt1 and nt2 waters which matched well with values in the ongoing forecast for the today period heading into tonight.
12z guidance was in good agreement over the offshore waters through roughly late Thursday into Friday with differences noted by Saturday, again in regards to possible tropical activity moving into the southern nt2 waters at that time. As a result favored 50/50 blend of the 12z GFS/ECMWF winds over the period. Stationary front weakens over the southern nt2 waters tonight into tomorrow with high pressure ridging expanding to cover the entire region. Guidance continues to remain in good agreement concerning the next cold front expected to move east across the waters with frontal passage expected late Tuesday night into Thursday night across the nt1 waters with the southern end of the front hanging back over the nt2 waters into at least early Saturday. Anticipate strongest winds ahead of the front to be over the over the nt1 and northern nt2 waters but with southerly flow thinking winds maxing out around 25 kt is reasonable.
Regarding the guidance in reference to any possible tropical features, right now guidance is in agreement in developing something off of eastern FL with it moving towards or possibly just into the far southern nt2 waters at the end of the period. The 12z GFS/ECMWF/UKMET/CMC all hold this feature just south of the southern nt2 waters through Saturday though impacts from the northern quadrant or semicircle of the feature may impact the water. In the case of the UKMET/CMC this solution is slower as compared to yesterday's 12z runs. With that noted in terms of the latest model trend heading into the weekend, forecast won't mention hint of any possible tropical development for now and will show the southern end of the front just clearing the southern waters per using a 50/50 blend of the 12z GFS/ECMWF.
.Seas...Favored 12z nww3/ECMWF wam guidance over the 5 day period given choice to use 50/50 blend of the 12z GFS/ECMWF winds.
.Extratropical storm surge guidance...N/A.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Scovil/Holley. Ocean prediction center.