textproduct: Honolulu

SYNOPSIS

The kona low continues to move rapidly towards the northeast away from the state and a high pressure system builds in quickly as the low departs. An upper level subtropical jet stream and deep unstable band of tropical moisture lingers over the Big Island today. This larger scale moisture band will drift eastward as high pressure builds in from the northwest, allowing more stabilizing trade winds to spread down the island chain. These cooler moderate to locally breezy trade winds will lower humidity levels and continue blowing across the islands through next weekend with passing showers favoring windward and mountain areas.

DISCUSSION

Trade winds are returning faster than predicted with northeasterly winds observed this morning as far east as the islands of Molokai and Lanai. This mornings water vapor satellite imagery channel shows a Sub Tropical Jet over Maui County and the Big Island. Enough instability lingers near Lanai for thunderstorms to develop along a trade wind convergence band just southwest of the island. These thunderstorms are not moving and are likely anchored along this down stream convergent feature, which makes it less likely to build over the islands in Maui County. A strong upper level divergent wind signature continues to produce scattered thunderstorms along the southwestern edge of the jet streams southern edge. These thunderstorms are developing on the tail of the deep tropical moisture plume, which may move into the Big Island later today. Heavy rain under this thunderstorm 'tail' tracking towards the Big Island is the primary reason we chose to extend the Flood Watch through the afternoon hours for Hawaii County. This watch will likely be cancelled for the Big Island once this last thunderstorm threat clears Hawaiian airspace.

The weather pattern changes back to northeasterly trade winds today and the threat for heavy rain diminishes. A passing upper level trough drifting southeastward down the island chain will keep periods of moderate showers in the forecast for all windward mountain slopes. Downward trends in shower activity will develop over each island as the trough axis passes and downward vertical motions (subsidence) strengthens as the upper ridge begins to build in aloft. The trough will pass through Kauai and Oahu through the overnight hours, then pass through Maui and Hawaii Counties by Tuesday. Cooler temperatures are expected to last into the weekend as moderate to locally breezy trade winds bring cooler air with lower humidity levels into the Hawaiian Islands from the north.

AVIATION

Residual moisture associated the departing kona low continues to produce showers with embedded isolated thunderstorms across much of the Hawaiian Islands, particularly from Molokai to the Big Island. These conditions are resulting in periods of MVFR ceilings and visibility. Trade winds have begun to reestablish over the western islands and are expected to gradually expand eastward across the state through the remainder of the forecast period.

AIRMET Sierra is in effect for Kauai and Oahu from N to E sections, and for Maui, Molokai, and Lanai for the entire area due to mountain obscuration from showers and low clouds.

Light icing remains possible in layer 130-FL200 from Molokai through the Big Island.

MARINE

Bands of scattered showers continue to impact waters surrounding Maui County and Big Island. A boundary northwest of the island chain will travel southeast across the state today. Gentle variable to southerly breezes downstream of this boundary will transition to strengthened north-northeast winds behind the boundary passage. Moderate trades will become better established early this week, along with widely- scattered showers, as the upper trough passes east of the state Tuesday.

The northeast swell that peaked yesterday will continue to decline today. A small north swell will reinforce an already established background north swell Tuesday and Wednesday. This will maintain elevated north- facing shore surf the next few days. Very small south southwest swell will keep south-facing shore surf from going completely flat the remainder of the week. East surf will stay slightly elevated today as result of a fading northeast swell and a little north wrap. East-facing shore short period wind waves will pick up response to strengthened trades Tuesday into Wednesday.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through this afternoon for the Big Island of Hawaii.