Marine Weather for HS 100


marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 936 am EDT Wed Oct 18 2017

.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.

The latest NCEP surface analysis indicates a high pres ridge across offshore waters, and a stationary front to se of the srn nt2 waters. Ascat from 02z overnight indicated 30 kt off the se coast in the moderately strong pres gradient between the front and the ridge, but current observations show only about 25 kt. The 06z GFS winds are initialized well when compared with the data, and indicates that the winds over srn nt2 will continue in the 25 to 30 kt range as a wave develops along the front. The 00z ECMWF agrees well with the GFS on the overall development, and both show the low developing into an intense system to the E of Newfoundland by Fri night. As far as the nt2 waters are concerned, winds are expected to decrease below 25 kt late tonight as the low and front moves E of the area. The previous forecast stayed along the lines of the 00z GFS with this system, so planning on staying close to previous wind grids as the 06z GFS has not changed much. Further N, the analysis also shows indicates a cold front moving into New England. The 06z GFS indicates the system will move offshore tonight and pass through the Gulf of Maine. The 00z ECMWF/NAM/UKMET/GFS all agree well with the 06z GFS and the previous wind grids, so planning on making only minor changes to update for current observations.

Otherwise, the models indicate high pres will dominate the pattern over the W Atlc through the forecast period, though they also indicate another weak cold front will pass through the Gulf of Maine Thu night into Fri. The GFS indicates SW flow with 10m winds ahead of the front up to 25 kt. This seems reasonable as the 00z ECMWF/UKMET agrees well on the intensity, and since am not expecting a deep mixed layer with the cooler SSTs N of the Gulf Stream helping to create a stable boundary layer. As a result, planning on staying close to the 06z GFS 10m winds for this system. In the medium range, the models all indicate another cold front will approach from the W Sun night. The GFS again indicates strong SW flow ahead of the front, and the 10m winds seem reasonable and well supported by the 00z ECMWF. The models have also come into better agreement on the timing of the front, so planning on staying close to the GFS for the remainder of the forecast period.

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

High pressure will persist to the west of the forecast waters and a relaxed pressure gradient across the region will keep winds below gale force threshold. Geocolor satellite images show mostly clear skies inland west of the forecast waters but mostly cloudy areas over the southern waters with some convection confined to the far southern and southeastern waters. This is in line with inland high pressure and a surface cold front across the southeastern and far southern waters. Winds are mainly from the northeast with weaker winds over the north areas and they increase southward with maximum winds to 30 kt over the southern waters in the vicinity of the front. NCEP map at 06z still has inland high pressure over the southern states that extends its ridge northeast across the mid Atlantic states into New England and that has northeast winds to the east which is mainly the forecast waters. Another high pressure 1030 mb to the east of the southern waters generates southwest winds to the east of the region. Low pressure 1003 mb northeast of the waters has its cold front that stretches southwest into another low pressure 1012 mb 240 nm east of the Baltimore Canyon, front continues southwest between the two highs, just east of the waters then into the southern waters. Latest scatterometer pass at 0244z this morning indicated areas of 30 kt winds confined to the far southern portion of zone anz935.

Models GFS/CMC/ukmethr/ecmwfhr/NOGAPS/NAM/gfsmean have initialized the 06z main synoptic features fairly well with just minor differences on the actual position for the central pressure value of the inland high and ukmethr is the only model that does not have a closed low just east of the Baltimore Canyon but a deep trough. Models have trended weaker and have pushed highest winds that were east of the forecast waters farther east. In the short term, models have a general agreement on keeping high pressure just west of the region and that will block strong frontal boundaries from reaching the waters and so the pressure gradient will remain relaxed over the waters through the forecast period. The models have also indicated some instability to persist over the southeastern waters as indicated by the persistent negative lifted index and that will sustain some mixing that will keep winds relatively higher to the southeast of the Gulf Stream with maximum winds over the far southern waters. Will continue with GFS for consistency and also that models are in generally good agreement.

.Seas...Seas are relatively smaller to the north of the Gulf Stream with 6 ft along the Gulf Stream and just 2 ft over the far northwest waters. Otherwise, seas mostly range between 6 and 9 ft except the southeastern zones where seas are over 9 ft with a peak at 13 ft. Jason 3 pass about 1908z returned peak significant wave height of 13 ft over the southeastern waters. Nww3 generally fits very well the observed seas pattern but just underdone by a foot on the peak value. Ecmwfwave matches the peak value but not so much in line with observations to the north as it extends its 6 ft line farther north of the Gulf Stream. As such, an equal blend of the two wave models is a better compromise and so will go ahead with a blend.

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


.Nt1 New England waters... None.

.Nt2 mid-Atlantic waters... None.


.Forecaster Kells/musonda. Ocean prediction center.

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