textproduct: Honolulu

SYNOPSIS

An increasingly active weather pattern unfolding across the Hawaiian Islands will persist through Friday as low pressure strengthens northwest of the state. Deep tropical moisture being drawn northward into the islands will support periods of heavy rainfall, strong southerly winds, isolated strong to severe thunderstorms through Friday. Although some brief improvement for portions of the state is possible by Saturday, another active period appears possible from Sunday through early next week.

DISCUSSION

Forecast confidence continues to increase in a high-impact weather event evolving across the state today through Friday, with the primary concerns being heavy rainfall capable of producing flash flooding, strong southerly winds, and strong to severe thunderstorms.

Model guidance remains in good agreement depicting a deep upper trough and embedded shortwave energy digging southeastward toward the islands while surface low pressure deepens northwest of the state. As this system strengthens, the local pressure gradient will quickly tighten through the day, resulting in increasingly strong south to southeast winds across the island chain.

This flow will transport a deep tropical moisture plume northward over the state, with PWs hovering around or above 2 inches. Combined with increasing large-scale ascent and strengthening low- level convergence, conditions favor widespread and organized heavy rainfall and isolated strong to severe thunderstorms (despite the best instability remaining off to the northwest).

The greatest rainfall impacts will most likely begin over Kauai and Oahu today before spreading eastward across the remainder of the state tonight through Friday. Confidence remains highest across the western end of the island chain for the most significant rainfall totals and flash flooding concerns during the initial phase of the event.

Wind impacts are also becoming increasingly concerning. Guidance supports strong and gusty southerly winds across Kauai and Oahu today through tonight, which has led to an upgrade from the High Wind Watch to a High Wind Warning for north and east facing slopes.

Farther east, guidance also indicates gusty southerly winds across exposed portions of the Big Island during the same time period. While winds there are not currently expected to reach warning criteria, speeds may near Wind Advisory thresholds by later today or Friday, particularly across wind-prone and terrain-enhanced areas.

For the Big Island summits, gusty winds along with a wintry mix supports the Winter Weather Advisory and High Wind Advisory into Friday.

Although forecast confidence decreases somewhat beyond Friday, some gradual or temporary improvement is anticipated Friday night into Saturday as upper heights rise and some drier air advances down the island chain from the west. Guidance, however, shows another shortwave dipping southeastward into the area late in the weekend, likely supporting another active weather period across the state that may extend through early next week.

AVIATION

Shower activity continues this morning across the islands. Latest radar imagery shows another broad expanse of showers approaching from the southwest, expected to arrive around sunrise. This will be followed by what is expected to be a more potent band arriving at Kauai around the early afternoon hours, spreading eastward across the islands through the overnight hours heading into Friday. With this band, gusty winds from the south will follow, making for periods of IFR or worse conditions under heavy showers and even isolated thunderstorms. Until this band moves through, however expect borderline MVFR conditions as light rain and low ceilings prevail across the islands.

AIRMET Sierra remains in effect for mountain obscuration above 020 ft across all islands. Some relief is possible as showers decrease in coverage this morning, but confidence in exact timing is low, so expect Sierra to persist through the TAF period.

AIRMET Tango also remains in effect for moderate turbulence between FL200 and FL350 across the entire islands. This is also expected to remain in effect through the TAF period.

AIRMET Zulu is also in effect for moderate icing as this ongoing weather pattern envelops the islands with extensive cloud cover. Expect this to remain in effect through the period as well.

MARINE

South to southeast winds are expected to steadily increase to strong to gale force speeds later this morning through tonight, as a low pressure system develops just west of the state and lifts northward. A Gale Warning has been issued for the western half of the coastal waters and the offshore waters due to the strengthening winds today. Small Craft Advisory (SCA) is in effect for most of the remaining waters for today.

For tonight, Gales are expected to continue over the western half of the coastal waters through the first half of the night then gradually shift eastward throughout the night. A Gale Watch remains in effect for the eastern half of the coastal waters. As the low pressure system further deepens today into Friday, we will likely see a greater coverage of thunderstorms and gusty winds especially tonight into Friday, where we could potentially see severe thunderstorms over the coastal waters capable of producing waterspouts and damaging winds. Very rough boating conditions are also expected later today into Friday as seas quickly rise in response to the strong to gale force winds. South facing harbor entrances will also be greatly impacted due to rough conditions.

The current south swell that peaked during the day yesterday has dropped below advisory thresholds this morning. But due to the strong to gale force southerly winds that are expected to quickly develop today over the western half of the state, a High Surf Advisory remains in effect for Kauai County and Oahu through tonight. Large disorganized surf is expected along south facing shores of Kauai and Oahu through at least tonight. Depending on how the low pressure system evolves today, the High Surf Advisory may need to be expanded towards Maui County tonight.

Along north facing shores, a moderate long-period northwest (310-320 degree) swell peaked during the day yesterday and will steadily lower today and dissipate to background levels on Friday. However a smaller short-period northwest swell generated by the low developing just west of us will fill in over the weekend into early next week.

Surf along east facing shores could see an increase later this week due to the strengthening southeasterly winds especially for southeast exposures.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through Friday afternoon for all Hawaii islands-

High Surf Advisory until 6 AM HST Friday for Niihau-Kauai Leeward-Waianae Coast-Kauai South-East Honolulu-Honolulu Metro- Ewa Plain.

High Wind Warning until 6 AM HST Friday for Kauai Mountains-Oahu North Shore-Olomana-Central Oahu-Waianae Mountains-Kauai North- Koolau Windward-Koolau Leeward.

Winter Weather Advisory until 6 AM HST Friday for Big Island Summits.

Wind Advisory from noon today to 6 PM HST Friday for Big Island Summits.

Small Craft Advisory until 6 AM HST early this morning for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel-Oahu Leeward Waters.

Gale Warning until 6 AM HST Friday for Kauai Northwest Waters- Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters.

Small Craft Advisory until 6 PM HST this evening for Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters- Alenuihaha Channel.

Gale Watch from this evening through Friday afternoon for Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters- Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 PM HST this evening for Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.