880 FXHW60 PHFO 210134 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 334 PM HST Wed Nov 20 2019 .SYNOPSIS... High pressure north of the islands will bringing increasing trade winds to the region through the weekend. A wet trade wind pattern is expected, with some rain reaching leeward areas at times with the stronger winds. && .DISCUSSION... No major changes to the forecast in the near term. Have boosted the QPF values for the first 18 hours or so to better reflect some of the higher values we are seeing. Have also modified PoPs/Sky/Wx and Wind grids for Monday night through Wednesday to bring the forecast in line with a blend of the GFS and ECMWF which includes southeasterly winds and increased PoPs. High pressure north of the islands will be moving to the east through tomorrow, and is helping trades to build back in across the region. The high is expected to remain to the northeast through the weekend, with a strong pressure gradient over the islands. As such, expect winds to increase and then remain strong through much of the weekend. It is quite possible a Wind Advisory will be needed for some areas as early as Friday morning. Will continue to monitor future model runs, particularly of the higher resolution models to nail down timing and location. The ECMWF and GFS both show an upper level trough at 250mb dropping down over the islands over the next 24 hours or so, with the GFS developing a low near Kauai at that level. Heading into the weekend, the models differ on the placement and strength of the trough/low, but both lift it out to the northeast by Sunday afternoon. The models have some reflection of this upper trough at the mid levels near 500mb, with a 500mb low moving to the west during the weekend. High pressure remains at the surface throughout, with strong trade winds. Anticipating a fairly decent trade wind inversion to set up, with the upper level features helping to enhance the trade wind showers. At this time, not expecting any additional impacts. Longer range, the GFS and ECMWF both indicate a cold front with associated upper level trough approaching the islands from the northeast next week. The GFS continues to be faster with the system, and has a stronger system that gets closer to the islands than the ECMWF. This set up could bring another round of thunderstorms and/or heavy rain to the area. The current forecast does reflect an upward trend in precipitation during this time, but will need to make further refinements as the even draws closer. As mentioned above, have made some adjustments to use a blend of the models for Monday night through Wednesday, but given the model differences, this may be washing out some of the details, which still need to be worked out. && .AVIATION... East northeast trades will strengthen this evening as surface high pressure continues to build north of the state. This will act to concentrate most shower activity along the windward coasts and slopes. Only isolated activity expected for the leeward sides. Similar conditions will ensue late this evening and into tonight as clouds and showers increase, especially along the windward areas. Thick low clouds and greater SHRA coverage may lead to MVFR conditions at times with periods of mountain obscuration or low visibility. No AIRMETs are currently in effect. && .MARINE... Trade winds are beginning to strengthen and will become breezy to strong tonight into the weekend in response to strong high pressure building far north and northeast of the islands. Combined seas are expected to exceed the 10-foot Small Craft Advisory (SCA) threshold in most coastal zones tonight into Thursday due to a building northwest swell and/or strong trade winds. An SCA is in effect through Friday at this time, but will likely be extended for many areas due to strong winds continuing. Gale conditions are possible in some channels toward the end of the work week. A new long-period northwest swell is beginning to fill in this afternoon at the Waimea nearshore buoy, but is running a bit slower to arrive than expected. However, buoys 51001 and 51101 northwest of the islands are now caught up to and following WW3 guidance, so expect the same here locally later this evening. A High Surf Advisory (HSA) for north and west shores of the smaller islands is in effect through Thursday night. Due to the northwest buoys following guidance at this point and WW3 running a bit smaller for the past few model runs, have adjusted the swell and surf heights down slightly, though still well above advisory threshold. Another, slightly smaller, NW swell arriving this weekend may require a brief HSA. Strong trade winds will likely drive a steady increase in short-period wind waves tonight into the weekend, eventually leading to an HSA for east facing shores. No other significant swells are expected. See the Oahu Surf Discussion (SRDHFO) for details on swell sources. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 AM HST Friday for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward Haleakala. Small Craft Advisory until 6 PM HST Friday for all Hawaiian waters except Maalaea Bay- && $$ DISCUSSION...M Ballard AVIATION...Thomas MARINE...TS