Marine Weather for HS 100

Forecast

marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 840 PM EDT Fri Jun 23 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 warm front extending E and se across the nt1 area, with a low pres trough approaching the middle Atlantic coast. The analysis also shows a high pres ridge over the central and srn nt2 waters. Latest available ascat hi-res and ascat passes from this morning show 10 to 15 kt winds over the offshore waters, although the models suggest that 15 to 20 kt winds are likely over the nt1 and nrn nt2 waters where there is a data void in the ascat passes.

Models...the medium range models are in very good overall agreement across the offshore waters through Wed night. The main weather feature in the near term will be a cold front approaching the coast tonight, then sweeping se over the offshore waters Sat and Sat night. Although the models differ somewhat regarding the strength of the S to SW flow ahead of the cold front, the GFS 10m solution looks like an excellent median model solution with maximum winds of 30 kt in the SW flow. The GFS has been showing very good run to run continuity over the past couple of days, so will use the GFS 10m solution for the wind grids over the entire 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 coastal and offshore waters. The models are in good overall agreement in the wrn Atlantic during the forecast period, with only minor differences noted, so a representative 50/50 blend of the wna wavewatch/ECMWF wam will be used for the sea height grids for the entire forecast period.

Extratropical storm surge guidance...N/A.

----------------------------------------------------------------- Previous discussion...

A 1530z ascat overpass returned maximum winds to 20 kt off the New Jersey coast. The 12z models are in very good agreement across the west Atlantic for the next few days and are consistent that the south to southwest winds across the offshore waters will gradually increase tonight through Saturday evening. The models share a similar timing with the approaching cold front that is forecast to reach the New England and northern mid Atlantic coast early Saturday, slowly pass southeast across the nt1 and northern nt2 zones later Saturday into Sunday, then stalling central nt2 waters Sunday night. To varying degrees, the 12z models all forecast Post-tropical Cindy will become absorbed by the cold front as both move offshore early Saturday. The 12z ECMWF is the only model to indicate that the winds associated with this low along the front will increase to 30 kt across the southern New England zones Saturday. We are favoring the weaker GFS/NAM/UKMET consensus across these waters which indicate winds to 25 kt. However, across the northern outer nt2 waters over the Gulf Stream there remains a slight potential for winds to briefly reach gale force Saturday afternoon and evening. The 4km NAM which usually handles the low level stability across the Gulf Stream better than the GFS, is only showing marginally unstable low level lapse rates here. And the models only depicting a 40 to 50 kt low level jet developing, do not see enough evidence to upgrade the winds to gale. We will continue to limit winds across these outer nt2 zones to 30 kt. Through Sunday, will populate the wind grids with the 12z GFS, which is generally very well supported by the other 12z models.

Once the front stalls across the nt2 waters Sunday night/Monday, the 12z models all forecast a series of weak lows to track east northeast along the front Monday through Tuesday, with the cold front finally passing east of the nt2 waters Tuesday night. The 12z GFS/UKMET are only slightly faster than the 12z ECMWF with the front. The guidance is then still consistent through midweek in forecasting a high pressure ridge to reach the coast Wednesday and move offshore Wednesday night. The 12z GFS winds look reasonable and will continue to populate our wind grids with it through the forecast period.

.Seas...The 12z wavewatch iii and 12z ECMWF wam are both well initialized with the west Atlantic wave heights this afternoon. There are only minor wave height differences (2 ft or less) between the models through the forecast period. Plan to populate wave height grids with a 50/50 blend of the 12z ww3 and 12z ECMWF wam.

.Extratropical storm surge guidance...N/A.

.Warnings...Preliminary.

.Nt1 New England waters... None.

.Nt2 mid-Atlantic waters... None.

$$

.Forecaster Scovil/Clark. Ocean prediction center.

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