Marine Weather for HS 100


marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 826 am EST Thu Nov 23 2017

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

GOES visible satellite imagery indicates a strong low pres system centered NE of the area over the Gulf of St Lawrence, and cold air cumulus streaming across the offshore waters in the cold advection behind the associated cold front now E of the area. The ascat winds from last night indicated gales in the cold advection over the offshore waters, but the strongest cold advection has moved off to the NE and current surface reports show up to 20 kt in the NE zones. 06z GFS 10m winds show a few remaining 25 kt winds over the eastern Gulf of Maine at 12z, but the models all indicate the winds will decrease early today as a high pres ridge builds over the W Atlc today into tonight. The previous forecast started out with the 00z GFS for today which is supported by the 06z run, so planning on staying with it in the update package.

The satellite also indicates low pres over the NE Gulf of Mexico, and the 00z GFS/ECMWF indicate the low will move NE into the srn nt2 offshore waters late this afternoon into tonight along the tail end of the aforementioned frontal boundary. The models all indicate the winds will increase N of the low as it moves over the Gulf Stream, with the GFS showing few gales tonight in the enhanced NE flow. The 00z ECMWF is a little slower with the track of the low, but overall agrees well with the GFS on taking it over the Gulf Stream. It is slightly weaker with the winds, and indicates up to 30 kt. However, confidence with the gales is low as the models all indicate only about 30 kt at 925 mb, so the likelihood of gales mixing down is low. Also, there is still a fair amount of disagreement on the low center itself, with the GFS a bit faster than the 00z ECMWF/UKMET/Gem. There is also a strong signal in the 06z gefs members for a slower solution. As a result, planning on staying with the previous grids which were based on the 00z ECMWF solution. Will also cap winds at 30 kt in the NE flow.

In the medium range, the GFS/ECMWF come into better agreement on the low as it moves well NE of the area. The models indicate strong cold advection behind the front by sun into Sun night over nt1 and nrn nt2. The GFS has been a bit stronger with the winds in the cold advection, and shows a few gales in the first sigma winds. However, it is not supported well by the rest of the 00z guidance with only the 00z Gem indicating a brief period of marginal gale force winds. The previous forecast stayed with the 00z ECMWF through the medium range period, so planning on staying with it as it shows a brief period of 30 kt in the cold advection. Otherwise, not planning on any major changes through the remainder of the forecast period.

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

The latest GOES satellite imagery, observations and preliminary 06z opc-NCEP surface analysis shows a deepening 990 mb low over central Nova Scotia moving NE away from the offshore waters with high pressure centered over the Ohio Valley moving E. An ascat overpass from last evening indicated gale force winds moving E from the far NE nt2 and eastern nt1 waters associated with the deepening low. The threat for gales over the offshore waters is expected to remain rather low for the next 3-5 days.

Over the short term, the 00z models are in very good agreement over the region today with a high pressure area building E into the offshore waters as the low mentioned above tracks NE away from the region. A low currently over the NE Gulf of Mexico will move E and NE over the next few days, and allow conditions to deteriorate over southern nt2 waters by tonight. For today, we will stay close to the 00z GFS guidance, and use the smart tool which places the higher first sigma layer winds over the unstable areas, and slightly lower 10 meter winds over the stable locations. For tonight through sun, we will then trend the forecast more toward the 00z ECMWF guidance as it is quite similar to the 12z Wed ECMWF that was used for the previous package. This will also maintain continuity as the GFS remains rather unstable at this time, beyond today. Therefore, we still expect low pressure to move into the far southern nt2 waters later tonight into Fri, continue organizing over the se nt2 waters Fri night, and then pass just E of the central and northern nt2 waters Sat. A moderate to strong cold front will approach the waters from the W and NW Sat. Any gales that develop with the low are expected to remain E of the offshore waters at this time, with chances for gales over the offshore waters not high enough to add to the forecast at this time. There will be an increasing threat for thunderstorms mainly over southern nt2 waters tonight into Fri night, with local wind gusts near or exceeding gale force and rough seas possible in and near of the heavier thunderstorms. The low will then lift NE away from the waters Sat night into sun as the cold front crosses the waters from NW to se, with cold air advection beginning over the waters at that time. High pressure will then begin to slowly build E toward the area during sun.

Over the medium range, we see no need to deviate too much from the 00z ECMWF guidance for Sun night through Mon night over the region, with the 00z ECMWF remaining quite consistent. The high will slowly overspread the waters from W to E Sun night through Mon night with improving conditions likely over the region. Any gales associated with the rather strong cold air advection will likely remain E of the nt1, and northern nt2, waters Sun night into Mon evening.

Seas...once again the 00z ECMWF wam appears to have initialized slightly better when compared to the current data over the offshore waters. In order to fit the sea heights forecast with the wind forecast will we stay fairly close to the 00z ECMWF wam through Mon night over the region, which will tend to result in little change from the past few forecasts. We will make only a few manual adjustments to fit sea height grids to initial conditions, and to fit with nearby coastal WFO and TAFB grids.

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


.Nt1 New England waters... None.

.Nt2 mid-Atlantic waters... None.


.Forecaster Kells/Mills. Ocean prediction center.

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