Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
998 FXHW60 PHFO 192000 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 1000 AM HST Thu Sep 19 2024 .SYNOPSIS... Dry conditions under gentle to breezy trade winds will be the prevailing weather pattern the next few days. Occasional showers will primarily focus along windward-facing terrain with more frequent shower activity during the overnight hours. The formation of an upper level low in the vicinity of the islands this weekend will increase the chances for more widespread rain with possible isolated thunderstorms over Oahu and Kauai. && .DISCUSSION... Regional radar imagery is void of any significant shower activity this morning under mainly clear skies. Thicker cloud cover and weak isolated showers may occur across windward exposures and in the higher elevations with afternoon overcast and scattered showers developing over the Big Island Kona region. Current satellite imagery, a regional sounding profile displaying a stout 7k ft trade inversion and stratocu clouds passing by within well- established trade flow all evidence of a very stable environment. Trade showers will follow the script the next couple of days... that of mainly windward and upper terrain periodic rain with more frequent overnight shower activity. A 1029 mb high centered approximately 1,100 miles north of the Hawaiian Islands is the main driver in producing a tight enough pressure gradient toward the state in the maintenance of gentle to breezy trades. Little movement nor weakening of this high the next couple of days guarantees similar trade behavior through Friday. The mid-level trough several hundred miles west of Kauai is continuing on its slow western trek...taking convection and higher moisture with it. While the high to the north will subtly weaken and shift east from Friday into the weekend, overall winds will remain at similar magnitudes into early next week. Drier lower and mid level air will advect across the islands through late Friday. Along with overall subsidence brought on by weak upper ridging over the state, the near term forecast will trend on the drier side of traditional late summer trade behavior. A weak upper low south of 30N near 170W (as noted by convection flaring up along its northeast periphery) will move off to the southwest and allow another weak low to drift into the islands from the north this weekend. Lowering heights and resultant mid to upper level cooling will destabilize the resident island air mass enough this weekend to increase more statewide rainfall. As upper heights begin to fall this weekend and mid to upper levels cool and moisten to around -10C, there should be enough available moisture to produce more statewide shower activity. Ample cooling may also steepen lapse rates enough to initiate a couple of storms near Kauai and Oahu late Saturday or Sunday. The development of any weekend thunderstorm activity will be dependent upon the location, strength and proximity of the upper low to the island chain. Drier more stable trade wind weather conditions will return from Monday through the middle of next week as upper ridging to the north better roots itself over the area. && .AVIATION... Breezy easterly trade winds will continue for the next few days, with clouds and showers favoring north through east sections of the islands. Brief periods of MVFR ceilings and visibilities are possible as showers pass through. Otherwise, VFR conditions will prevail for most locations. Although there are currently no AIRMETs in effect, AIRMET Tango for low-level turbulence over and downwind of island terrain may be needed by the afternoon. && .MARINE... High pressure north northeast of the islands will keep moderate to locally strong trade winds in place through the weekend. A slight weakening of the high could decrease wind speeds early next week. A Small Craft Advisory (SCA) is now in effect for the typically windy waters around Maui and the Big Island through 6 AM Saturday. An upper level low could develop just northwest of Kauai this weekend, bringing the chance for isolated thunderstorms to the coastal waters around Kauai and Oahu. The development of any thunderstorms will highly depend on the strength of this developing upper low. North shore surf will remain very small through the weekend until mid-week when a 3 to 5 foot medium to long-period northwest swell could arrive. East shore surf will hold near seasonal levels today then slowly lower through early next week. A mix of southeast, south and southwest swells will keep small surf rolling into south facing shores through Monday. A slight bump in south shore surf is possible Monday night through mid week as a slightly larger long-period southwest swell moves through. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Blood AVIATION...Vaughan MARINE...Tsamous