Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
924 FXHW60 PHFO 080206 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 406 PM HST Tue May 7 2024 .SYNOPSIS... Trade winds will continue to weaken tonight through Thursday as a trough approaches the state. An unusually strong upper level low for this time of year will produce an unstable weather pattern late Thursday through around Saturday. Even though a wide spread heavy rain event is currently not expected, showers that do develop will bring the threat of flash flooding and isolated thunderstorms especially Friday afternoon through the evening. Weather should gradually improve from west to east this weekend, but some unsettled weather will likely linger over the eastern half of the state on Saturday. && .DISCUSSION... In the short term, the trade winds will continue to weaken further tonight into Wednesday with sea breezes expected over leeward areas by Wednesday afternoon. This will allow for scattered afternoon showers to develop tomorrow over interior and leeward areas. On the summits of the Big Island, wind observations continue to show wind gusts over 70 mph, thus the High Wind Warning has been extended through tonight. Winds should taper off by Wednesday but still may remain breezy at the summits throughout the day. Starting Thursday, the atmosphere will begin to change as an upper level low approaches the state. The surface winds will likely weaken further as a surface trough begins to develop over the state. Temperatures aloft will begin to cool, which will bring instability over the state. With the added instability, some low level moisture, and sea breeze convergence, showers should increase during the afternoon on Thursday over interior and mountain areas. Due to light winds in the lower half of the atmosphere, any showers that develop will be slow moving and will be capable of producing some downpours in the afternoon. Although some locally heavy showers will be possible Thursday afternoon, the main concerning period will be Friday through Saturday. An unusually cold upper level low will approach the state Thursday night and is expected to be over or very close to the state by Friday afternoon. 500 mb temperatures on the ECMWF and GFS is showing around -14C to -15C for Lihue, which is very unusual for this time of year. This will produce a very unstable weather pattern for Friday and will allow for any showers that do develop to become a heavy rain threat. Also, with temperatures that cold aloft, we will likely see isolated thunderstorms with some of the stronger storms capable of producing small hail. At this time, it is too early to tell exactly where the heavier showers and thunderstorms will develop, and it will highly depend on the position of the upper level low. Latest GFS and ECMWF has the upper level low centered just north of Kauai Friday afternoon. This will cause the most favorable area for thunderstorms and heavy rain to be from Oahu eastward. But if the upper level low was to shift slightly more west, the entire state could see the threat of heavy rain and isolated thunderstorms. Although the details are still uncertain, the main takeaway is that we are not expecting a widespread rain event. But, any showers that do develop on Friday will be capable of producing intense rainfall rates. With the potential for intense rainfall rates and the slow movement of any showers or thunderstorms, a Flood Watch could be issued in the coming days. For the weekend, there is still quite a bit of uncertainty on how quickly the upper level low will move through the state (from west to east) with the latest global models trending slower. Although the models have trended slower, there is fairly good agreement with the upper level low being east of Kauai by Saturday. The threat for flash flooding and isolated thunderstorms should decrease from west to east as the upper level low slowly progresses eastward. At this time, the eastern half of the state, sees the greatest chance for some unsettled weather this weekend. && .AVIATION... Moderate trade winds will gradually ease over the next twenty-four to thirty-six hours. Bands of light showers off the Pacific will continue to favor windward and mauka areas, and be most active during the overnight hours. Isolated moderate showers are likely over island interiors this evening. Brief MVFR CIG and VIS are to be expected in passing showers, otherwise VFR conditions will prevail statewide. && .MARINE... Trade winds will weaken over the next several days as high pressure northeast of the state moves further away and a surface trough builds over the island chain. Chances for heavy showers will increase Thursday through the weekend. An upper low will move across the island chain Friday through Sunday and also bring a slight chance of thunderstorms. The current northwest swell will decrease tonight and Wednesday. followed by a reinforcing a short-period north northwest swell Friday into the weekend. Forerunners from a larger and longer lived south-southwest swell will arrive Wednesday, and surf is expected rise near, or to, the High Surf Advisory level during the peak of the swell Thursday and Friday. This swell will decline through the weekend. Surf along east facing shores should remain small as winds remain light through the rest of the week. && .HFO WATCHES/WARNINGS/ADVISORIES... High Wind Warning until 6 AM HST Wednesday for Big Island Summits. && $$ DISCUSSION...Kino AVIATION...Bedal MARINE...Foster