Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
016 FXHW60 PHFO 190151 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 351 PM HST Wed Sep 18 2024 .SYNOPSIS... A dry weather pattern under gentle to breezy trade winds will prevail the next several days. Occasional showers will focus along windward-facing terrain with more frequent shower activity occurring during the overnight hours. There are increasing chances for more statewide rain early next week as an upper low develops in the vicinity of the state. && .DISCUSSION... A 1030 mb high centered approximately 1,000 miles north of the Hawaiian Islands is the main driver in producing a tight enough pressure gradient toward the state in the maintenance of this week`s generally gentle to breezy trade winds. Little movement nor weakening of this high the next couple of days guarantees similar trade behavior through tomorrow (Thursday). The mid-level trough about 600 miles west of Kauai is continuing on its western trek and is taking 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. Trade showers will follow the script the next couple of days...that of mainly windward and upper terrain periodic rain with more frequent overnight activity. Other than leeward valley mist and the occasional windward shower spilling atop the ridgelines, the only other daily rainfall caveat will be the daytime cloud build and higher rain chances over Big Island`s Kona region. Satellite-derived moisture data and a slight 24 hour drop in surface dew points are evidence that there is a drier air mass moving in east of the state. This drier lower and mid level air will advect across the islands through late Friday. This has the near term forecast trending on the drier side of late summer traditional trade flow. Regional weather will likely undergo change later this weekend with the consolidation of a couple of weak upper lows or troughs over or in the vicinity of the islands. Ensemble guidance has lowering heights over the state next Sunday and Monday. As upper heights begin to fall this weekend and mid to upper levels cool and moisten, there should be enough available moisture to produce more statewide shower activity. While deterministic guidance is a bit out out of phase with this lower height scenario, a signal has been noted and this has increased the early week wet bias. Hence, the forecast will lean toward a more wet next Sunday through the middle of the week as a potential upper low forms in close proximity to the islands. && .AVIATION... Moderate to breezy trade wind flow will prevail for the next several days. Stable conditions will result in only limited areas of low ceilings and temporary MVFR conditions over windward and mountain locations. Afternoon clouds that developed over Kona slopes of the Big Island should clear this evening. VFR conditions are expected to prevail for most locations. AIRMET Tango remains in effect for low-level turbulence over and downwind of island terrain. This AIRMET will likely remain in effect at least for the next several days. && .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. East shore surf will hold near seasonal levels through Thursday morning, then slowly lower Thursday evening 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 around Tuesday or Wednesday of next week as a slightly larger long-period southwest swell moves through. North shore surf will remain very small through the weekend until Tuesday and Wednesday of next week when a 3 to 5 foot medium to long-period northwest swell could arrive. && .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...Birchard/Tsamous