Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
706 FXHW60 PHFO 200141 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 341 PM HST Thu Sep 19 2024 .SYNOPSIS... Dry conditions under gentle to breezy trade winds will be the prevailing weather pattern going into the weekend. 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 or trough north of the islands this weekend will increase the chances for more widespread rain with possible isolated thunderstorms over Kauai and Oahu. && .DISCUSSION... Mostly sunny, dry and breezy. Today`s regional radar imagery has been void of any significant shower activity through the day. Thicker cloud cover and weak isolated showers may occur across windward exposures and in the higher elevations...afternoon warming creating overcast and isolated showers over Big Island Kona region/Manua Loa slopes. Current satellite imagery and this afternoon`s regional sounding profiles displaying a stout near 6k ft trade inversion and stratocu clouds passing by within well- established trade flow all evidence of a very stable environment. A more comfortable day will afternoon interior dew points in the 60s...drier vertical profile with slightly below seasonable norms at around 1.3 inch pwats. Trade showers will follow the script through early Saturday. That being of mainly windward and higher terrain periodic rain becoming more frequent overnight. A 1029 mb surface high centered approximately 1,100 miles north of the Hawaiian Islands has been 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 700 mb trough several hundred miles west of Kauai is continuing on its slow western trek and taking convection and higher moisture with it. While the high to the north will subtly weaken and shift east tomorrow (Friday) into the weekend, overall winds will generally remain low end breezy into next week. Drier lower and mid level air will continue to advect across the islands into the weekend. Along with overall subsidence brought on by weak upper ridging over the state, the near term forecast remains on the drier side of traditional late summertime trade behavior. A weak upper low south of 30N near 170W (as noted by convection flaring up along its eastern periphery today) will move 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 (around -8 C) while moistening up, there should be a uptick in areal shower activity. Ample mid-level cooling may also steepen lapse rates enough to initiate a couple of (isolated) storms and/or localized heavy rain on or around Kauai and Oahu late Saturday or Sunday. As of this afternoon, the highest thunderstorm indices (1.5k surface-based CAPE, sub-zero lifted indices) are focused far north and west of Kauai over the offshore waters. The development of any weekend thunderstorm activity will be dependent upon the location, strength and proximity of the upper low/ troughing 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 anchors 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. There are currently no AIRMETs in effect, and none are expected through tonight. && .MARINE... High pressure north northeast of the islands will keep moderate to locally strong trade winds in place through Saturday morning, then slightly weaker Sunday. A marginal 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 from Saturday evening through Sunday. 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 about the middle of next 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