textproduct: Honolulu

SYNOPSIS

A notable shift in the weather pattern is underway today as an upper-level trough and developing surface low organize northwest of the islands. This evolving system is expected to pull deep tropical moisture northward across the state, ushering in a prolonged period of strong southerly winds, widespread showers and thunderstorms, and elevated risk of flooding from midweek through the weekend.

DISCUSSION

Latest observations show an upper-level trough just northwest of the Hawaiian Islands slowly meandering eastward over the next several days while ridging at the surface lingers northeast of the state. Finally, a cold front well west of the islands is forecast to track eastward into the vicinity over the course of the next day or so. Winds across the islands remains light to moderate along the windward side of all islands, while even lighter winds/land breezes hold place along on the leeward areas, especially along the Kona coastline of the Big Island due to wind blockage.

A broad surface trough is forecast to develop just ahead of the aforementioned front prior to its approach, resulting in the veering of southeast winds to become more southerly across the Hawaiian Islands. This will cause tropical moisture to be pulled in northward in the southerly flow. Additionally, a moisture band associated with the trough will also develop along side of Kauai and Niihau early this morning, bringing a chance of heavy shower activity. Guidance also supports some heavier showers and thunderstorms developing over the interior of most islands, given the mixture of daytime heating, sea breezes, and the shortwave trough ahead of the front.

As the upper-level trough exits to the east, tropical moisture continues being advected poleward and may lead to periods of showers tonight through Wednesday morning. Upper-level forcing is forecast to strength again tomorrow as another vigorous shortwave pivots the base of the previous trough. This may enhance lift over the region as a new surface low consolidates just west of the islands. The deep tropical moisture overspreading the state, combined with the increasing large-scale lifting, supports another widespread and organized rainfall event for the entire state. Because of this, a Flood Watch has been issued for all islands beginning Wednesday morning and lasting through Friday afternoon.

A Winter Storm Watch has also been issued beginning Wednesday morning for a combination of freezing rain and snow on the summits of the Big Island above 12,500 feet, lasting through Friday afternoon as well.

There has been careful consideration for a Wind Advisory beginning Wednesday night as southerly winds begin to ramp up, especially in and around heavier showers and thunderstorms, but also in the vicinity of mountain ranges where downsloping gusts over northern and windward slopes can occur. Severe wind gusts (58+ mph) may be possible, though most areas will be in the range of 40 to 50 mph. Future shifts will monitor and refine the situation in the next 24 hours or so as the pattern evolves.

As the week comes to an end, model guidance seemingly appears to have come to an agreement compared to the previous run. The general consensus of the GFS and the ECWMF shows the latest pattern is expected to continue through the weekend and into early next week as the moisture band anchors over the state. Longer range models of the GFS finally show much of the rainfall clearing most of the islands by midweek next week.

Of important note, and as previous discussions have already mentioned, one important consideration with this week's rainfall is antecedent conditions. The islands experienced significant flash flood events in recent weeks, and despite several days of drier trade wind weather, soils remain vulnerable in many areas. As a result, any time period of higher rainfall rates could quickly lead to enhanced runoff, renewed rises in streams and reservoirs, and localized flash flooding concerns.

AVIATION

Southerly winds have spread across the region, and an area of rain is spreading over Niihau from the south. This area of rain in expected to move over Kauai shortly. Additional rain is expected today, particularly during the afternoon hours. While VFR conditions will start the day, MVFR and isolated IFR conditions are possible, particularly this afternoon.

Anticipate AIRMET Sierra being issued soon to account for mountain obscuration on Kauai. This will likely be expanded to additional areas later today.

MARINE

Southeasterly winds have spread across the islands with a front stalling to the west of the islands, and a surface right far to the north. Winds are expected to increase out of the south tomorrow and Thursday, as a low develops across the stalled front.

Showers, some locally heavy, will be possible through Friday as a result of the southerly winds bringing tropical moisture to the north over the islands. Thunderstorms have developed in the offshore waters to the northwest of Kauai, and thunderstorms are possible over the offshore waters through at least Friday.

A moderate long-period northwest swell will fill in today, giving more of a boost to north shore surf late today through Wednesday. A small, short- period north-northeast swell will fill in today before peaking Wednesday and subsiding Thursday. This overlap will make for some rough choppy conditions along north facing shores.

A slightly larger, long- period south swell, will increase today with a more noticeable increase tomorrow. Surf could reach advisory levels by Wednesday before slowly easing Thursday into the weekend. Strengthening southerly winds will make for rough and choppy surf by late next week.

East shore surf will remain small and below the seasonal average due to a lack of strong trade winds over and upstream of the islands, except for some wrap from the northerly swells.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from Wednesday morning through Friday afternoon for all Hawaii islands-

Winter Storm Watch from Wednesday morning through Friday afternoon for Big Island Summits.