textproduct: Honolulu

SYNOPSIS

Weakening trade winds will produce light rainfall over mainly windward areas tonight. Low pressure will develop north of the state on Wednesday, causing winds to shift out of the south and allowing sea breezes to produce a few interior showers in the afternoon. Deep southerly winds will draw tropical moisture over the portions of the island chain Wednesday night through at least Thursday, bringing some potential for flooding rainfall. The low will weaken several hundred miles northwest of Kauai Friday and Saturday then lift away on Sunday, though a few heavy showers may linger through the weekend under light winds. Light trade winds are expected to return early next week.

DISCUSSION

Stable and somewhat dry trade winds continue to slowly weaken this afternoon as a cold front 800 miles northwest of Kauai pushes a surface high eastward. A slowly eroding mid level ridge over the islands is maintaining stable conditions, and rainfall has been modest and focused over windward areas.

Winds will weaken considerably tonight and begin to shift out of the east to east-southeast as surface low pressure develops along the stalling front roughly 800 to 900 miles north of Kauai. No significant rainfall is expected, and land breezes could push most showers offshore in the predawn hours.

The low will intensify and drift to about 700 miles north of Kauai on Wednesday. Since this feature will be somewhat far away from the islands, local winds will shift out of the southeast to south but are only expected to be at gentle to moderate strength. As a result, expect afternoon sea breezes to overpower the background flow along north shores and generate isolated interior showers in the afternoon. Stability will decreasing, though heavy showers are not expected at this time.

A humid southerly flow will bring increased chances for widespread rainfall Wednesday night through at least Thursday. The low be at its strongest and closest to the islands at this time, possibly within 600 miles north of Kauai. Moderate southerly flow will draw up an area of deep tropical moisture over the islands while a jet stream entrance region along the eastern flank of the low provides some moist ascent. This could lead to a period moderate to locally heavy rainfall favoring the western half of the island chain but possibly extending as far east as Maui. Due to the southerly flow, leeward areas, which tend have a lower threshold for flooding, could be exposed to potentially heavy rainfall. High resolution guidance yet to produce a concise picture of a flood threat, though the highest chances would be late Wednesday night and early Thursday. We have held off on issuing a Flood Watch until we see better model agreement. While gusty downsloping winds are typically a concern in this setup, the flow does not appear to be strong enough for winds to reach advisory levels.

The low will stall and begin to gradually weaken roughly 700 miles northwest of Kauai on Friday. The jet stream on its eastern flank will retreat north of the islands. This will reduce upper level support for widespread heavy rainfall. However, light southerly winds will persist, with daytime sea breezes supporting spotty afternoon showers and dew points hovering in the low to mid 70s.

The humid flow will gradually shift out of the southeast on Saturday. The GFS and ECMWF suggest that the highest rainfall chances will be along a weak surface convergence zone near Oahu and Kauai, and ridging aloft will build in from the east and provide increased stability for at least Big Island. The low is expected to fill as it lifts off to the northeast Sunday. Aside from the possibility of variable winds on Kauai, conditions should be similar to Saturday.

Light trade winds are favored to return Monday.

AVIATION

Weak trade wind pattern will give way to an increasing chance of rain by Thursday (see main forecast discussion for more details). Winds will weaken and veer southerly by Wednesday. Scattered showers may increase Wednesday night, especially over mountain areas, with periods of MVFR ceilings and visibility possible.

No AIRMETs are currently in effect. Localized mountain obscuration will be possible later this afternoon and tonight as moisture and cloud cover increase, but widespread conditions are not expected at this time.

MARINE

Winds will veer to the east southeast this afternoon and tonight as a low pressure system and an associated cold front develops north of the state and approaches from the northwest. A trough extending from this low is forecast to move into the northwest offshore waters by Wednesday afternoon. The low will stall north of the islands and draw moisture out of the tropics and over the area through the second half of the week, so expect an increase in showers with moderate south to southeast winds. Thunderstorms will be possible over the waters around Kauai and Oahu, mainly Wednesday night into Thursday morning. The trough could potentially linger into the weekend as it dissipates.

Long period south swell energy will continue filling in today, keeping surf elevated but just below advisory levels this afternoon. By Wednesday, this swell will begin to fade, but surf heights will change little this week, as a mix of new, small south swells will keep surf elevated near or just above seasonal norms. Another long period swell then looks to fill in this weekend, bumping surf back up to just below advisory levels.

A long-period northwest swell will continue building today, pushing surf for north-facing shores up tonight to near moderate levels on Wednesday. Small to moderate surf is then forecast through the rest of the week even as swell energy becomes shorter period, since it will be reinforced by a low to the north- northwest of the Hawaiian Islands. A larger moderate swell could arrive early next week, bringing north shore surf to just below advisory thresholds. Surf along east facing shores will remain small as local winds remain fairly weak.

A small, long-period west swell originating from Typhoon Choi- wan is forecast to reach the islands Thursday into the weekend, which may produce moderate surf for west-facing shores.

FIRE WEATHER

Gentle to moderate breezy trades will continue a slow decline tonight, with humidity just above the critical threshold this afternoon. Fire weather risk diminishes through Friday as southerly winds bring increased moisture and humidity. The inversion range from 6 kft to 8 kft through the evening and will deteriorate Wednesday.

HFO WATCHES/WARNINGS/ADVISORIES

None.