Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
883 FXHW60 PHFO 130656 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 856 PM HST Mon Aug 12 2024 .SYNOPSIS... Persistent high pressure far north of the islands will support locally breezy trade winds through the weekend. Passing trade showers will periodically dampen most windward areas, with some showers occasionally spreading leeward, particularly overnight. && .DISCUSSION... The arrival of 00Z model guidance offers no big changes to the ongoing forecast philosophy. In the short term, radar and satellite images show a break in the nearshore low cloud field has led to decreased showers this evening, but island orography has still been able to spawn a few showers. Overnight cloud top cooling combined with the gradually digging trough aloft N of Kauai is expected to lead to increased shower coverage later tonight. && .PREV DISCUSSION ISSUED 313 PM HST... Afternoon visible imagery reveals mostly cloudy skies over windward and mauka zones with appreciable leeward cloud cover on most islands as well. This is likely due to the combination of a subtle increase in low-level moisture coupled with a weakening inversion. 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 cigs and SHRA to windward and mauka locations through the forecast period. Brief MVFR conds can be expected within SHRA but VFR should generally prevail. No AIRMETs are currently in effect, however AIRMET Sierra may be needed for mtn obsc for windward locations overnight into the early morning hours with the increase in SHRA. && .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 remains 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 small, medium period, background south swell will linger through tonight and bring small surf along south facing beaches. Another small, long- period, south swell is expected to arrive mid-day Tuesday and peak Wednesday. This swell will produce another small boost in surf along south facing shores before declining through early this weekend. Guidance currently introduces a 3 foot, long- period, southerly swell from Sunday into early next week, originating from a gale low, which currently resides east of New Zealand. This will likely generate surf above August averages. Seasonally flat to tiny surf expected through this week for north facing shores. Breezy trade winds will bring choppy surf conditions to east facing shores through the remainder of this week. && .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. && $$ UPDATE...TB PREV DISCUSSION...JVC AVIATION...SMW MARINE...JT