Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
217 FXHW60 PHFO 121919 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 919 AM HST Mon Aug 12 2024 .SYNOPSIS... Persistent high pressure far north of the islands will support locally breezy trade winds this week. Passing trade showers are nearly a certainty for most windward areas as showery low cloud clusters periodically move quickly through. Some showers will occasionally spread leeward, particularly overnight. && .DISCUSSION... The first visible images of the morning reveal mostly cloudy skies over most windward and mauka zones with the exception of the Big Island. Here, the mountain breeze has temporarily halted any onshore moisture push. Immediately upstream there resides a shallow and stable, but fairly extensive, corridor of low level moisture. The remnants of T.S. Emilia, centered about 1300 miles ENE of the islands, immediately trail this moisture field as it shallow circulation advances westward within prevailing trades. Satellite trends suggest the stable upstream moisture reservoir is gradually peeling apart within the trades and individual cloud clusters are encountering a slightly less stable environment over the islands. This has led to a subtle uptick in trade wind shower activity during the last 12 hours or so. This trend will continue and even amplify somewhat during the next few days as broad scale height falls settle over the islands bringing weak large scale ascent and mid-level cooling. Temperatures at 700mb, currently averaging 10-11C across the island chain, will fall by several degrees to 7C or slightly lower by late Tuesday into Tuesday night. As a proxy for the trade wind inversion, this strong indicator that stability will steadily diminish during the next 36-48 hours. As such, stable upstream cloud elements peeling toward Hawai`i will tend to grow in depth resulting in increasing coverage of showers and high confidence that showers will bring beneficial rainfall to windward and mauka zones this week. Any locally deep pockets of moisture will bring periodic light rain to leeward zones, particularly overnight. The remnants of T.S. Emilia will graze the northernmost islands of Oahu and Kauai in the Wednesday-Thursday period leading to another uptick in trade wind shower activity. PWATs will not be particularly impressive, around 1.4", as these remnants sweep past the island chain, but this will largely be due to the shallow nature of the remnant circulation. High saturation within the lowest levels is anticipated leading to high confidence in a brief period of wetter trades for at least Kauai and Oahu late Wed-Thurs. Locally breezy trades continue through the period, with trade winds temporarily easing toward the moderate range as Emilia`s remnant trough passes north of the area during mid-week. && .AVIATION... Moderate to locally breezy trades will deliver low clouds and brief showers to windward areas today. VFR conditions will prevail with isolated MVFR CIG over windward areas. AIRMET Sierra has been issued for mtn obsc due to low cigs and SHRA. Otherwise, no other AIRMETs are in effect. && .MARINE... An area of surface high pressure north-northeast of the state will remain nearly stationary over the next several days, producing moderate to locally strong trade winds across the local waters. A Small Craft Advisory has been extended through Wednesday morning for the typically windier waters around Maui County and the Big Island and will likely be extended through late this week. A 2 to 3 foot south swell is lingering this morning at the nearshore buoys, with the most concentrated energy in the 12-14 second range. As a result, south shore surf will likely remain slightly elevated this morning, before gradually declining this afternoon. Another small, long-period south swell is expected to arrive mid-day Tuesday and peak Wednesday into Thursday morning. This swell will produce another small boost in surf along south facing shores before declining through the rest of the week. Seasonally flat to tiny surf expected through this week for north facing shores. Near shore buoys indicate swell energy out of the east over a wide part of the period spectrum. The 6-9 second energy is due to the breezy trades and will continue to bring choppy east shore surf through much of the week. Another batch of energy in the 10-11 second range could be due to recent tropical cyclone activity in the East Pacific and looks to linger for another day or so before fading. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...JVC AVIATION...Vaughan/Shigesato MARINE...TS