Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
631 FXHW60 PHFO 231955 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 955 AM HST Mon Sep 23 2024 .SYNOPSIS... Moderate trade winds will strengthen somewhat through Tuesday, and remain locally breezy through the week. Lighter winds are possible by the weekend/ Passing showers will favor windward areas, with the potential for some briefly heavy showers today due to a nearby disturbance aloft. && .DISCUSSION... A closed mid-level cutoff low centered about 100 miles N of Kauai is producing scattered thunderstorms near its core, and has destabilized the atmosphere in its vicinity. An overnight sounding was not available from Kauai due to ongoing equipment challenges, and a more stable air mass was sampled by the overnight Hilo balloon, but radar detecting heavier-than-normal rain cores over/near Kauai and Oahu helps to confirm this instability. Meanwhile, light to moderate trade winds are being driven by a low- level pressure gradient supported by a pair of high pressure systems centered NW and NE of the islands. A weakness in the ridge far N of the islands is due to a passing front, while the low aloft has also induced a surface trough just N of the islands, and trade wind speeds are on the lighter side. Visible satellite indicates limited low clouds immediately upstream of the islands, with scattered small cumulus elements approaching windward Kauai and Oahu, and even fewer low clouds near Maui and Hawaii counties. The remainder of the morning should feature a few windward showers, mainly on Kauai and Oahu, while the light trades allow lee-side sea breezes to drive the formation of clouds and a few showers. Some of these showers could be briefly heavy. The leading edge of an area of increased showery low clouds about 50 miles E of the Big Island will arrive over windward Big Island late this afternoon, fueling increased shower coverage there before clearing by early Tuesday morning. The mid-level low is forecast to move W at a fairly good clip over the next several days, with a mid-level high moving over the area from the E. This will lead to increased stability and a reduction in PWAT. Trade winds will strengthen subtly over the next couple of days as the nearby surface trough moves W, and the surface highs consolidate into a single center NNW of the islands. The resulting weather pattern is expected to provide only minimal showers, mainly windward, into next weekend. Lighter winds are possible by next weekend as a low passes N of the islands. && .AVIATION... Moderate trades will usher low clouds and showers into windward and mauka areas, with a few showers occasionally spilling over to leeward sides. Sea breezes will develop within some wind- protected areas this afternoon leading to low cloud and shower development over those areas. Then trades will strengthen slightly heading into tomorrow. Pockets of MVFR anticipated within any showers, otherwise VFR prevails. No AIRMETs in effect. AIRMET Sierra for mountain obscuration may be needed tonight for the eastern end of the state as a plume of moisture rides in on the trades. && .MARINE... A weak trough of low pressure passing just north of Kauai today is producing isolated thunderstorms over the NW offshore waters this morning. The threat for thunderstorms will continue to exist over the western offshore waters through Tuesday as the trough moves westward. With the trough moving away, the pressure gradient will tighten over the state and will lead to the trades strengthening on Tuesday. Moderate to fresh trade winds should hold through most of the week as a weak high remains anchored far north of the state. Winds will be borderline SCA thresholds each day for the typical windy waters of Maui County and the Big Island. A moderate long period northwest swell (320 to 330 degrees) is expected to steadily fill in on Tuesday and peak late Tuesday then gradually decline the rest of the work week. A smaller north- northwest swell should provide a boost this weekend. Surf along south facing shores will remain small but rideable, due to a series of small long-period southwest swells throughout the week. East facing shores should remain small through much of the week due to the lack of strong trades locally or upstream of the state. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Birchard AVIATION...JVC MARINE...Kino