Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
149 FXHW60 PHFO 240215 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 415 PM 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 some afternoon showers over the Big Island`s leeward slopes. && .DISCUSSION... A closed mid-level cutoff low centered less than 100 miles N of Kauai is producing isolated thunderstorms near its core, and has destabilized the atmosphere in its vicinity. Afternoon sounding from PHLI indicates a lack of a subsidence inversion, while a weak inversion near 8000` was noted at PHTO. Meanwhile, light to moderate trade winds are being driven 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, resulting in trade wind speeds on the lighter side. Visible satellite indicates limited low clouds immediately upstream of the smaller islands, with an area of increased low clouds moving into windward Big Island from the E. This area will fuel increased shower coverage there this evening before diminishing after midnight. Elsewhere, brief and passing showers will favor windward areas overnight and early Tuesday, with the instability potentially allowing a few showers to become briefly heavy over Kauai and Oahu. 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 through the week, and a brief period of increased high clouds late tonight and Tuesday. 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 new low passes N of the islands. && .AVIATION... Moderate trades remain on the drier side this afternoon with limited cloud cover in most areas and only very spotty showers. Deeper moisture east of the Big Island will advance westward across the area tonight bringing periods of MVFR to windward and mauka zones. Confidence in MVFR potential is highest over the eastern end of the state. Drier air fills in behind this batch of showers leading to high confidence in improving conditions and prevailing VFR by daytime Tuesday. No AIRMETs in effect. AIRMET Sierra for mountain obscuration may be needed for windward and mauka portions of Big Island through Oahu tonight. && .MARINE... A weak trough of low pressure passing north of Kauai today is producing isolated thunderstorms over the NW offshore waters this afternoon. 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 starting tomorrow Tuesday 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 into Wednesday morning 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 should see a series of small long- period southwest pulses through the weekend due to a series of storms near the Tasman Sea last week. Additionally, a fetch of strong southwest winds near 10N170W over the past 24 hours should bring an increase of short-period energy from the southwest Wednesday into Thursday. 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