marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 410 PM EDT Wed Jul 18 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The 18z opc surface analysis depicts low pressure over Georges Bank with a cold front extending southwest across the mid- Atlantic waters to near Cape Fear. Widely scattered convection persists across the nt2 waters this afternoon, primarily south of Cape Lookout. Earlier ascat retrievals between 14z-16z supported 20-30 kt winds across much of the outer nt2 zones with marginal gales in anz910 over the Gulf Stream. The hi-res 12z WRF-arw is performing significantly better than the global models this afternoon, particularly over the Gulf Stream where the unstable boundary layer is allowing some gales to reach the surface. In fact, the last two arw runs picked up on these gales, while the GFS winds remain 5-10 kt too low. Therefore, will populate with the 12z arw winds tonight through Thu and raise a brief Gale Warning for anz910 tonight until the front clears the area.
For Thu night and beyond, the 12z ECMWF was the preferred model choice for this forecast package. The 12z ECMWF did come in weaker with the frontal wave passing east of the outer waters Thu and Thu night, which is more consistent with the 12z GFS/UKMET/NAM. So, the grids will no longer show any gales associated with this feature east of our waters. The weekend is more of a challenge as the guidance is still struggling with coastal low pressure that develops from a shortwave trough tracking north near the mid-Atlantic coast. The 12z ECMWF remained consistent with its previous run and lies between the faster/weaker GFS and the slower/stronger UKMET solutions. Using the ECMWF winds maintains continuity with our previous forecast and caps sustained winds at 30 kt as the low moves up the coast Sat/Sat night. Gales certainly cannot be ruled out, but confidence remains below average at this time. A more seasonable pattern will develop by the end of the period as Bermuda high pressure gradually builds west towards the waters.
.Seas...Earlier altimeter data indicated the wavewatch and ECMWF wam were running several feet too low with the strongest winds over the Gulf Stream in the outer nt2 zones. At 18z, Max seas were analyzed to 13 ft within the gales over anz910. For tonight through Thu, manually boosted the previous wave grids to account for the GFS/ECMWF winds running too low. Then, transitioned to the 12z wam seas from Thu night through Mon night.
.Extratropical storm surge guidance...There is the potential for a positive surge event this weekend associated with low pressure tracking northward up the mid-Atlantic coast. The ECMWF winds were preferred over the GFS in the medium range, so the 12z estofs/etss guidance is not consistent with our wind forecast. Therefore, the exact timing and magnitude of such an event is relatively uncertain at this time.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... .anz910...east of the great South Channel and south of 39n... gale tonight.
.Forecaster reinhart. Ocean prediction center.