textproduct: Honolulu

SYNOPSIS

An upper level trough passes through the region today triggering periods of heavy showers and isolated thunderstorms mainly over the western half of the state today. The highest threats for flash flooding remain over the northwestern islands of Kauai County and Oahu. These flooding threats will begin to diminish later this afternoon. Shower activity will trend significantly lower by this evening as the upper trough lifts north of the islands. Trade winds will weaken this weekend with land and sea breezes developing from tonight through Monday afternoon. A building high pressure system far north of the state will bring in a shower band from the north affecting Kauai and Oahu by Monday night, and then spreading across the state through Tuesday. Trade wind speeds increase from Tuesday afternoon into Wednesday. Drier weather trends with lighter winds are forecast statewide from Thursday onward.

DISCUSSION

Looking into the big picture satellite imagery this morning, we see a negatively tilted upper level trough moving from west to east across the Hawaiian Islands. Divergence aloft and cold temperatures associated with this upper trough will continue to enhance shower activity in an unstable atmosphere surrounding the island chain. Low level convergence bands forming in the lee of island mountains will continue to anchor moderate to heavy showers over the islands of Kauai and Oahu today with elevated risks for flash flooding. The Flood Watch was extended in time through this afternoon for Niihau, Kauai and Oahu to cover these lingering flash flooding threats. The upper trough will lift north of the islands later this afternoon with more stable conditions and decreasing shower trends starting later tonight. Deeper moisture and colder temperatures with this passing upper trough will combine forces to produce a brief wintery mix of light snow and freezing drizzle over the Big Island summits above 12,000 feet elevation through this afternoon. A Winter Weather Advisory was issued today for the highest elevations of the Big Island summit zones.

The weather pattern evolves tonight through Sunday, as a nearly stationary low level trough will set up just north of the Hawaiian Islands. This trough will serve to significantly weaken the large scale trade winds with sea breezes developing along terrain sheltered leeward areas of each island lasting through Monday afternoon. High pressure building in from the north by Monday night will push clouds and showers along this low level trough back into the islands from the north, increasing shower trends, and producing moderate to breezy trade winds from Tuesday into Wednesday.

By Thursday onward trade winds will decrease yet again as a cold front approaching from the west weakens and drives the high pressure ridge directly over the Hawaiian Islands. Increasing large scale subsidence under this ridge will decrease inversion heights to around 5,000 feet and produce drying trends lasting into next weekend. Wind directions over the western islands will likely veer from a more southeasterly direction with returning daytime sea breezes by next week Friday.

AVIATION

Trade winds will ease today and become light and variable tonight as a surface trough develops over the island chain. Low clouds and showers will favor windward and mauka areas, although interior and leeward areas will see an increase in showers this afternoon through early this evening. A rumble or two of thunder will also be possible as a disturbance aloft moves overhead. VFR conditions will prevail for the majority of the forecast period at the TAF sites, although MVFR cigs/vsbys will be common in the showers, with brief IFR conditions possible.

AIRMET Sierra remains in effect for mountain obscuration across Kauai and Oahu, as well as windward sections of Molokai, Maui and the Big Island. Some improvement is possible later this morning.

MARINE

Fresh trades will ease over the weekend as a gale to the northwest lifts northward and away from the region. Winds will become light and variable with overnight land breezes and daytime sea breeze wind regime by tonight. Seas will respond by gradually lowering, likely falling below the 10 ft Small Craft Advisory level by this afternoon. In addition to the winds and seas, expect a wet pattern to continue with isolated thunderstorms due to an upper disturbance in the area.

Surf along exposed east-facing shores will remain rough this morning, but gradually ease later today as the winds diminish locally and upstream of the state.

Surf along exposed north- and east-facing shores will quickly build through the day Monday as a long-period north-northeast swell from a broad storm-force low evolves off the northwest Pacific coast this weekend. A large area of northerly gales between this system and a 1048 mb blocking high centered over the Aleutians will continue to expand southward through the weekend, with the head of the fetch reaching less than 1,000 nautical miles from the state. Heights will reach warning levels for exposed north- and east-facing shores by late Monday through Tuesday. Although a gradual downward trend is anticipated by midweek, additional pulses from this same system will keep surf above advisory levels for east-facing shores through much of the week. This swell direction will bring surf into some typically protected areas such as Kua Bay, West Maui, and exposed shores of Lanai.

Other impacts from this north-northeast swell direction could include significant erosion along some coasts and accretion at other locations as sand shifts opposite the typical swell direction, particularly along north-facing shores. Additionally, overwash along vulnerable sections of coastline and roadways will be possible beginning Monday night during high tide cycles. Mariners can also anticipate harbor surges at Kahului and Hilo.

Surf along exposed west-facing shores will rise early next week as a long-period northwest swell arrives from a recent broad storm- force low over the far northwest Pacific.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through this afternoon for Niihau, Kauai and Oahu.

Winter Weather Advisory until 6 PM HST this evening for Big Island Summits.

Small Craft Advisory until noon HST today for Kauai Northwest Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters- Kaiwi Channel-Maui County Windward Waters-Pailolo Channel- Alenuihaha Channel-Big Island Windward Waters.