marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 942 am EDT Fri Sep 21 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The 12z opc preliminary surface analysis shows high pressure analyzed over Nova Scotia with a ridge extending across the nt1 and northern nt2 waters. The pressure gradient between this ridge and distant low pressure southeast of Bermuda is producing northeasterly flow across the mid-Atlantic waters with winds to 20 kt over the outer nt2 zones and 25-30 kt well east of our offshore waters.
The 06z GFS came in consistent with its previous run, so there are no changes to the forecast reasoning this morning. The near term grids were updated to clean up a bit along our coordination borders. Based on our 12z sea state analysis, will populate with the 00z ECMWF wam for seas through tonight.
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The GOES infrared satellite imagery shows an area of low pressure to the east of the southern nt2 offshore waters near Bermuda, and the imagery along with the lightning density product indicate the convection has been decreasing overnight. The latest outlook from NHC indicates the low has a 10 percent chance of tropical cyclone development over the next 2 days and 20 percent over the next 5 days as dry air along with upper level shear as shown by the models and the satellite imagery are expected to limit development. Further N, the water vapor satellite imagery indicates an upper ridge building along the coast this morning. The latest NCEP surface analysis indicates a tightened pressure gradient in between the associated surface ridge and the low, with the strongest part of the gradient just se of the northern outer nt2 waters. The 01z-02z ascat wind retrievals from last night showed up to 30 kt just se of the outer nt2 waters, and current surface observations show up to 20 kt in the offshore waters this morning. The 00z GFS first sigma winds are initialized well when compared with the data over the unstable areas in nt2, and the model indicates that the low will move further S and away from the offshore waters today as the ridge continues to build further se across the offshore zones. The 00z ECMWF/UKMET/NAM/Gem models all agree well with the 00z GFS today on the progression of the synoptic features, so planning on starting out with the 00z GFS first sigma winds in the next forecast. However, will use the 10m winds in stable areas to account for the shallower mixed layer.
On Sat, the 00z models all indicate a strong mid level trough will pass over the northern nt1 offshore waters while a strong cold front passes through the area at the surface. The models have all been showing strong southerly winds developing ahead of the front below a strong low level jet with GFS/ECMWF 925 mb winds increasing to 55 kt over the Gulf of Maine. However, model soundings show a fairly strong inversion developing over the cooler SSTs in nt1, with mixed layer winds barely reaching 30 kt. The previous forecast indicated sustained winds up to 30 kt in the Gulf of Maine as confidence was low with anything higher than that, and confidence remains low for the next forecast as the models have not shown anything different. The GFS and ECMWF remain in fairly good agreement on the timing and intensity of the front and winds, so planning on staying with the 00z GFS into sun with this system.
Further out and into the medium range, the 00z models all indicate that the low currently near Bermuda will drift back to the W and weaken just south of the offshore waters by Mon while pulling a trough over the southern zones. The models also show the aforementioned front weakening across the offshore waters as an area of strong high pressure builds over New England and se Canada. The interaction between the high and the low/front is expected to tighten the pressure gradient across the offshore waters again, which should produce a moderately strong easterly fetch across the Gulf Stream in nt2. The models continue to agree fairly well on the timing, though the GFS is slightly stronger with the winds in the easterly flow across the Gulf Stream. This makes a bit more sense as the models normally underforecast wind intensity in this type of scenario, so again favoring the 00z GFS through the remainder of the forecast period as it seems more representative of the intensity and is in decent agreement on the synoptic features.
Seas...both the 00z wavewatch and ECMWF wam have been initialized well when compared with the current observational data. The models agree fairly well throughout the forecast period, so planning on using an even blend of the two as a compromise solution.
Extratropical storm surge guidance: no significant positive surge events are expected during the next few days. However, the 00z estofs shows a 1+ ft surge along the mid Atlc coast Sun night into Tue as the previously mentioned easterly flow persists across the nt2 waters. This seems reasonable, but the 00z etss is 50 percent lower and seems a little underdone.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster reinhart/Kells. Ocean prediction center.