marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 1010 PM EDT Sat Aug 18 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
For the evening update we will attempt to highlight the threat for locally very strong winds, potentially near or exceeding gale force, and very rough seas in and near strong thunderstorms currently noted S and E of Long Island, se of Cape Cod, E of Georges Bank, and in a broad line extending from just E of the NE northern nt2 waters near 41n 65w, southwest to the North Carolina coast, just N of Cape Hatteras. Additional strong thunderstorms are occurring over coastal South Carolina and Georgia this evening. Sref guidance suggests that the overall threat for thunderstorms will persist over and near these waters tonight, with the general thunderstorm threat shifting slowly southward over the offshore waters through sun into early next week as a cold front moves slowly S over the waters. For the evening update, we will alter grids to fit current conditions and to fit with nearby coastal WFO and TAFB grids over the next few days, but otherwise not make major changes to the ongoing opc forecast at this time. The 18z GFS is suggesting the potential for patchy gale force SW winds over the se nt1 and NE outer nt2 waters by later Wed and Wed night. For now, we will not add gales to the ongoing forecast as we prefer to fully evaluate the 00z guidance once it arrives overnight, before making any adjustments to the ongoing forecast, if necessary.
Seas...sea heights will be adjusted slightly to fit current conditions and in deference to nearby coastal WFO and TAFB grids and forecasts over the next few days. Sea heights ranged from 8 feet or so off the North Carolina and virgina coasts to around 3 feet closer to the Delmarva, mid Atlantic and New England coastal waters according to the 00z ra1 opc seastate analysis.
----------------------------------------------------------------- Previous discussion...
In the near- and short-term parts of the forecast, winds should remain below hazards with the exception of areas in and near deep convection. As a cold front moves south from the far northern Gulf of Maine through the nt1 waters and to off of the coasts of Delaware and New Jersey by Sunday morning, scattered-to-numerous showers and thunderstorms will occur, with the highest coverage and strongest storms currently expected from off of the Virginia and North Carolina coasts east-northeast across the mid-Atlantic offshore waters, in vicinity of the Gulf Stream. The hrrr WRF and sref thunderstorm probabilities show the greatest concentration of thunderstorms there by later this evening, supported by GFS model soundings showing cooling and moistening aloft. High-resolution arw guidance would suggest pockets of gale-force winds tonight into Sunday along the front and across anz825, anz828, anz830 east across anz910 to the northern part of anz930. These types of winds should be associated with deep convection, as no other guidance shows it in the mean wind fields. For this forecast, will keep the winds below gales, but take the opportunity to note winds and seas higher in and near thunderstorms for several mid- Atlantic zones overnight into Sunday.
In terms of hazard potential regarding wind, the forecast gets interesting Wednesday into Thursday. Overall, the ECMWF, UKMET, and GFS all portray developing surface low pressure over the lower Great Lakes Wednesday, moving northeast toward the end of the week. The details between the guidance, though, are different. The GFS, over the last couple of runs, has been consistently slower with the progress of the low and slightly stronger, with a correspondingly sharper and slightly more amplified mid-level low over southeast Canada by late Wednesday afternoon. The UKMET has a stronger but less amplified mid-level low centered more to the northwest, with surface low pressure centered well to the northwest compared to the GFS and the ECMWF. Currently, the ECMWF seems to be a decent blend with a surface low slightly weaker and moving slightly faster compared to the GFS, and along a similar track, with a slightly more progressive mid-level trough. The stronger GFS would support 925mb winds reaching around 45kt in the far eastern mid-Atlantic offshore zones late Wednesday afternoon, with mixing on model soundings supporting gales there, while the ECMWF has winds at that level increasing to 40 knots right on the far eastern edge of the mid-Atlantic offshore waters Thursday morning where the gradient is tightest on that model then between a cold front, and high pressure to the southeast. The current thought is that evidence is starting to build toward a short period of gales in the eastern mid-Atlantic offshore zones sometime between Wednesday afternoon and Thursday morning. Unfortunately, timing differences in the guidance do not support enough certainty at any one time to introduce them now, but it would appear the best chance would be in the eastern part of anz905 southwest into anz920 for a short period between Wednesday afternoon and Thursday morning.
Afterward, a stalled front with weak high pressure to the northwest, and broad high pressure to the southeast, results in a slackening gradient and low potential for hazards outside of any showers and storms. In summary, for this forecast followed the ECMWF winds, adjusted slightly upward. Limited the winds to values just below gale in the far eastern mid-Atlantic zones overnight Wednesday into Thursday to allow later forecasts to refine more the timing and location of any gales.
.Seas...Wavewatch guidance in particular seemed to be underdoing the waves in the very near-term part of the forecast off of the mid-Atlantic and Carolina coast. The wam wave guidance appeared better, so leaned toward that for this forecast continuing throughout the short- and long-term. Differences in timing of higher waves during the middle of next week are noticeable as per the above discussion, and the wavewatch had greater maximum waves with its higher winds which, if gales occur, could actually develop. Opted to adjust the wam output higher about a foot or two in the far eastern mid-Atlantic offshore zones for waves around 12 feet in the eastern parts of anz905 and anz910 Thursday morning.
.Extratropical storm surge guidance...The etss and estofs models would provide for surge values around a half-foot in vicinity of the New Jersey and Delaware coasts Sunday night into Monday. Otherwise, no significant surge is expected. Monitor the latest forecasts from coastal National Weather Service offices.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Mills/figurskey. Ocean prediction center.