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FXHW60 PHFO 040648 AFDHFO

Area Forecast Discussion National Weather Service Honolulu HI 848 PM HST Fri May 3 2024

SYNOPSIS
Breezy trade winds will continue into Monday as a high pressure system remains anchored far north of the state. An upper level low northwest of Kauai will slowly drift across the island chain enhancing shower activity mainly over windward and mountain areas of each island. By Tuesday the disturbance aloft weakens and drifts northeast of the state, briefly decreasing shower activity.
A low level cloud band will ride into the islands on the trade winds with support from an upper level trough from Wednesday to Thursday, increasing shower trends over all islands.

DISCUSSION
Satellite imagery water vapor channel shows an upper level low roughly 700 miles to the west-northwest of Kauai this evening. An upper level trough is also evident over the islands enhancing trade wind showers as shown on island radar. The subtropical jet stream lingers just south of the Hawaiian Islands and will slowly drift north over the next few days.

A 1030 mb high pressure center and subtropical ridge will remain anchored over the Central Pacific basin, far north of the Hawaiian islands through the weekend. This system will produce breezy trade winds across the Hawaii region into Monday. Trade wind speeds will slowly decrease to moderate levels from Monday into Tuesday as the high pressure center drifts eastward away from the island chain and a long wave trough moves into the Central Pacific. Easterly trade winds will continue to blow at moderate levels through Friday.

Expect continued enhanced showers across the state as the unstable upper level disturbance slowly drifts eastward across the island chain. Most of the rainfall amounts will favor windward and mountain areas as breezy trade winds build clouds up and over the windward mountain slopes. Down sloping trade wind flow over the leeward sides of island mountain ranges will tend to decrease shower activity for leeward areas of each island. Shower trends will tend to increase during our typical diurnal rainfall maximum in the overnight to early morning hours.

In the extended forecast we continue to see a long band of unsettled shallow clouds, remnants of an old Eastern Pacific cold front, drifts into the islands on the moderate trade winds from Wednesday morning into Thursday. This cloud band may produce wet weather over all islands for an 18 to 24 hour time period. The highest rainfall amounts will tend to favor windward and mountain areas, however many leeward sites may also see measurable rainfall totals with this next system. This fairly shallow cloud band will likely not be deep enough to produce heavy rainfall, rather more beneficial rain is forecast for all islands.

AVIATION
Breezy easterly trade winds will persist through the weekend, with clouds and showers favoring windward and mauka locations.
MVFR CIGs /VSBYs in showers are possible over these windward areas, especially overnight through the early morning periods.
Elsewhere, VFR conditions will prevail with an occasional shower making it to leeward sections of the smaller islands.

AIRMET Sierra is in effect for tempo mountain obscuration for windward portions of the Big Island. This AIRMET will likely need to be expanded to other windward areas later this evening.

AIRMET Tango is in effect for mod turb below 9000 feet downwind of island terrain, which will continue through the weekend.

MARINE
Consolidating high pressure far N of the islands will support strong E trade winds into Monday, with winds accelerating to near-gale force in the windier channels. The high will move E thereafter, leading to a gradual easing in trade wind speeds by the middle of next week. A Small Craft Advisory (SCA) for all waters will remain posted through at least Sunday.

A medium- to long-period NNW swell is building in island waters this evening, with NDBC buoys 51101/51001 NW of Kauai reporting swell heights a couple feet larger than anticipated by wave model guidance. If current trends continue, forecast surf heights for Saturday will be increased for N and W facing shores, but peak surf heights will remain below High Surf Advisory (HSA) levels.
This swell will gradually diminish Sunday, but another relatively small, moderate-period NW swell is expected Monday and Tuesday.

Increasing short-period wind waves along E facing shores will result in rough and choppy surf for at least the next several days. Elsewhere, the storm track in the S hemisphere recently became increasingly favorable for the generation of S swells, with these swells arriving on-and-off in the islands for at least the next week. The first of these long-period swells is expected this weekend and early next week, with peak surf heights remaining below HSA levels, although later swells may turn to be large enough to warrant a HSA.

HFO WATCHES/WARNINGS/ADVISORIES
Small Craft Advisory until 6 AM HST Monday for all Hawaiian waters-

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