marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 1027 am EDT Tue Sep 25 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
Per the 12z NCEP surface analysis a strong 1041 mb high, located over Nova Scotia, extended its ridge down across the nt1 and nothern nt2 waters. A stationary front was located over the central nt2 waters while a 1015 mb low, being monitored by the NHC for possible tropical development, was located over the se nt2 waters near 32n76w. Numerous thunderstorms associated with this low were well represented in the latest satellite and lightning data this morning, with the majority of this activity in zones anz930, anz935 and anz835. The latest NHC tropical weather outlook, gives this system a 50 percent chance of tropical development within the next 48 hours and through day 5 and continues to mention the system could become a tropical depression later today as it moves northwesward before a deep upper trough moves through the Great Lakes tonight and Wed with increasing upper level southwest winds offshore. Mariners should keep abreast of the latest forecasts.
Main concern with this intermediate forecast update will be the near term in regards to the aforementioned low pressure over the se waters and its forecast movement through the waters into tomorrow, while monitoring it for aforementioned possible tropical development per NHC. Reviewing the 06z GFS/NAM, those runs were fairly similar to the prior 00z ECMWF/UKMET through roughly tomorrow night. The overnight forecast package populated the grids through Wednesday with a blend of the 00z ECMWF/arw, with winds adjusted based on the available ascat data at that time. With this morning intermediate update I feel it's reasonable to not make major adjustments to the ongoing forecast, barring any changes to the tropical outlook per NHC. Hence, will keep maximum winds at 30 kt across the east quadrant of the low as it tracks north and northeast across the nt2 waters Wed into Thu, which is closest to the ECMWF. Confidence with the forecast scenario on this system is below average. Additionally this results in winds remaining no greater than 30 kt across parts of the northern nt2 and south central nt1 waters today into tnight, and across the Gulf of Maine tonight into Wednesday morning.
Despite no warnings hazards for sustained winds, winds occassionaly reaching gale force or above in gusts are possible within and in the vicinity of any areas of thunderstorms.
See previous discussion for additional information.
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The 0120z and 0220z ascat overpasses across the offshore waters returned an expansive area of easterly 25 to 30 kt winds extending from 70w to the coast south of New England to just south of Baltimore Canyon. There were also 25 to 30 kt winds across the outer nt2 zones south of Hatteras Canyon, north of the developing surface low just south of the area. The previous wind grids, which were based on a GFS/namnest blend with the stronger winds then adjusted upward by about 10 percent, were well initialized compared to these ascat winds. The convection associated with the aformentioned surface low is well north of the center and the elongated surface low has nearly moved into the southern nt2 waters this morning. The 00z models continue to offer very differenct scenarios with any possible tropical development. There should be a narrow window of time today where the system could become a tropical depression or storm before the deep upper trough moves through the Great Lakes tonight and Wed with increasing upper level southwest winds offshore. The high resolution guidance including the hiresw-arw and nmm, and CMC-reg all develop at least tropical storm force with the low across the nt2 waters already by late this evening. Over the past day or so, the ECMWF and UKMET have backed off from respective previous runs. Based on the weaker model consensus, we will likely maintain forecast continuity keeping maximum winds to 30 across the east quadrant of the low as it tracks north and northeast across the nt2 waters Wed into Thu, which is closest to the ECMWF. Our confidence with the forecast scenario on this system is below average. Regardless of whether gales develop, there will be widespread showers and thunderstorms associated with the low as it moves across the offshore waters today through Thu, and likely brief gale force winds within these stronger thunderstorms.
The same high resolution models mentioned above as well as the 00z namnest are also showing some marginal gales developing across the Gulf of Maine tonight as the warm front lifts northeast toward the strong high pressure ridge. Even given the anomalously warm sea surface temperatures across the Gulf of Maine, we anticipate that the low levels will not be sufficiently unstable to allow any gales to mix to the surface. There will likely be sustained gales observed at the higher Mt desert and Matinicus Rock platforms, and a fairly widespread area of 25 to 30 kt at the surface across nothern portions of the Gulf of Maine tonight.
The 00z models are then generally consistent that a cold front will move southeast through the New England waters Wed night and across the northern mid-Atlantic zones Thu. The models indicate the front should slow or stall across the northern mid Atlantic later Thu, before developing low pressure along the front pulls the weakening feature further south across the waters at the end of the week. Winds are expected to be 20 kt or less through the medium range, and mainly for continuity our wind grids will be based on the 00z ECMWF.
Of note, is the rapid deepening and strong liklihood of hurricane force winds developing with Post-tropical cyclone Leslie across the central Atlantic over the next few days. The global models are all indicating that the strong baroclinic forcing will result in the system deepening into the low 970's or upper 960's millibars at a latitude between 35n and 37n. The gale and storm force wind radii will also rapidly expand late today through Thu, with the powerful non-tropical system slowly turing back west tonight and Thu. The models are also showing the system will likely attain sub-tropical or even tropical characteristics again late in the week and over the weekend. Mariners should keep abreast of the latest forecasts of this intense system.
Seas: buoys from south of Long Island to off the New Jersey and Delmarva coasts continue to report 8 to 10 ft wave heights this morning. The higher 00z ECMWF wam is better initialized than the 00z wavewatch this moring. However, even the ECMWF wam is a couple feet low here. We will also be adjusting the wave heights 1 to 2 ft higher than the guidance across the waters south of Cape Fear with NOAA buoy 41002 reporting 8 to 9 ft the past several hours. We generally relied on the ECMWF wam through the forecast period with manual adjustments through the short term.
.Extratropical storm surge guidance...The strong easterly flow has generated a surge around 1 to 1.5 ft from western Long Island Sound to along the New Jersey coast, Delaware Bay and the Delmarva Peninsula. The higher estofs appeasr to have a much better handle than the etss with this surge. However, recent station hydrographs are indicating that even the estofs is a slightly low this morning. Consult coastal National weather services offices for their latest coastal flood advisory information.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Holley/Clark. Ocean prediction center.