textproduct: Honolulu

SYNOPSIS

A powerful kona storm will impact the Hawaiian islands this week through the weekend, bringing numerous hazards statewide. A combination of considerable flash flooding, damaging southerly winds, and strong to severe thunderstorms will threaten the state through the weekend. The risk for flash flooding will begin today for Kauai, tonight for Oahu, Wednesday for Maui County, and Wednesday night for the Big Island. The potential for strong to severe thunderstorms and damaging southerly winds is expected to increase significantly statewide late in the week and into the weekend.

DISCUSSION

At the surface, a low pressure system is located about 1,100 miles to the northwest of Kauai. An associated trough with heavy showers and embedded thunderstorms resides just to the west of Kauai and Niihau. These surface features will drive much of the impactful weather that will affect the islands over the next several days. Winds today will strengthen and veer southerly for the western half of the state and southeasterly for the eastern half. A broad swath of deep tropical moisture with precipitable water values (PWats) ranging from 1.5 to 2 inches will be drawn up in this southerly flow (currently within the trough seen to the west of Niihau) and move over the western end of the state today into Wednesday. The mid and upper levels of the atmosphere will make this weeks event particularly threatening for the state. Large scale, potent troughing aloft with smaller shortwaves rotating around the broader trough will support large scale ascent across the area. The broad upper trough will move southeast towards our area and push the associated surface trough and moisture east.

Together, these ingredients will support periods of heavy rainfall with embedded thunderstorms and gusty winds. Impacts today and tonight are expected to be focused primarily across the western half of the state. By Wednesday, the moisture will spread to Maui County, and then the Big Island by Wednesday night, where heavy showers and thunderstorms will be moving into the south and southeast slopes.

Flooding concerns will then continue statewide as the week progresses, particularly where the heavier rainfall persists over the same areas for multiple days. By late week into the weekend, the upper level trough to the northwest will continue to sharpen and take on a negative tilt as a jet streak on its western flank strengthens and another shortwave trough will round the base of this trough. The axis of deep tropical moisture will spread out across the island chain, with model guidance showing PWats exceeding 2 inches. Prior to this, soils will already be saturated, increasing the likelihood that additional rainfall will produce rapid runoff and dangerous flooding conditions.

While thunderstorms are expected today through Thursday, the environment will become increasingly favorable for severe thunderstorms Thursday night through Saturday morning as the thermodynamics and kinematics (MU CAPE of 1200 to 2300 J/kg, 850-mb winds of 50 kt with up to 40 kt or more of 06 km bulk shear) become more supportive. Model guidance is also consistently showing significant upper level divergence, which increases the chances of strong to severe thunderstorms. These environmental conditions will support strong rotating updrafts (mesocyclones).

In addition to the rainfall threat, southerly winds are forecast to strengthen through the week, with the strongest winds expected Friday into the weekend. This pattern will lead to damaging downsloping winds on north through east sections of the islands, capable of downing trees and power lines. Damaging winds will also be possible with any storms that develop as well.

For the summits of the Big Island, periods of heavy snow with significant accumulations will become possible later in the week through the weekend, likely requiring the issuance of a winter storm watch/warning.

Long range guidance indicates that unsettled weather could potentially continue into next week. Residents and interests across the state should continue monitoring forecasts as the weather evolves.

AVIATION

Heavy showers with embedded thunderstorms are approaching Kauai early this morning as a powerful kona low develops northwest of the islands. These bands will overspread Kauai through the day and spread to Oahu tonight, bringing periods of MVFR conditions with brief IFR or LIFR CIGS/VSBYS possible in the heavier showers and storms. These impacts are expected to gradually spread eastward down the island chain by midweek. Light to moderate southeast winds will gradually veer southerly and strengthen over the western end of the state through today as the pressure gradient tightens.

AIRMET ZULU remains in effect for light icing over Kauai and Oahu. AIRMET SIERRA remains in effect for mountain obscuration along the east through south slopes of Kauai and Oahu, but will likely be updated to include all of Kauai later this morning. Additionally, AIRMET TANGO for moderate turbulence may become necessary later today.

MARINE

A powerful kona storm is expected to bring heavy rain, thunderstorms, and rough seas to the islands and adjacent waters beginning today and lasting at least into the weekend. Winds will veer southerly and increase to fresh to locally strong speeds through late this week as the kona storm approaches. A Small Craft Advisory (SCA) will go into effect for waters north and west of Kauai this morning. Later this week, possibly as early as Thursday night, winds will approach, or possibly reach, gale force for portions of the area. These strong winds will slowly creep eastward heading into the weekend.

A series of small, medium- to long-period west-northwest swells will bring small surf to north and west facing shores through the rest of the week. The first of these will build today and peak on Wednesday, resulting in a slight bump in surf along north and west-facing shores. Another small, medium-period north swell is also possible late this week.

Choppy surf along east-facing shores will gradually trend downward, falling below seasonal levels by mid-week as winds veer southerly. Surf along south-facing shores will remain small for the next couple of days, though will become rough and choppy as southerly winds increase. South shore surf will rapidly build late this week as southerly winds increase even more and a series of small, long-period south swells move through.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through Saturday afternoon for Niihau-Kauai-

Flood Watch from this evening through Saturday afternoon for Oahu-

Flood Watch from Wednesday morning through Saturday afternoon for Lanai Mauka-Kahoolawe-Maui Windward West-Maui Leeward West- Haleakala Summit-Molokai-Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley North-Maui Central Valley South- Windward Haleakala-Kipahulu-South Maui/Upcountry-South Haleakala.

Flood Watch from Wednesday evening through Saturday afternoon for Kona-Kohala-Big Island Interior-Big Island South-Big Island Southeast-Big Island East-Big Island North.

Small Craft Advisory until 6 AM HST Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters.