The National Weather Service posted the following at 3:55 pm today (Tuesday, 3/10):
A powerful kona storm will impact the state through the weekend, bringing multiple hazards to the islands including significant flash flooding, damaging winds, and strong to severe thunderstorms. Flooding concerns will continue over Kauai this evening and spread to Oahu late tonight, then gradually expand eastward across the island chain through the week. The potential for strong to severe thunderstorms and more widespread flash flooding will increase late in the week and into the weekend.
A wide band of deep moisture with widespread showers and embedded thunderstorms on the east side of a broad low far northwest of the state will continue to impact Kauai and spread eastward to Oahu late tonight. Rainfall rates have remained manageable so far, with most areas of Kauai receiving only a few hundredths to half an inch of rain since early this morning.
CIMSS MIMIC Layer Precipitable Water imagery shows this moisture band contains precipitable water values ranging from 1.5 to 2.25 inches this afternoon. This plume will gradually spread eastward, reaching Maui County by Wednesday and potentially the Big Island by Wednesday night, where heavy showers and thunderstorms will increasingly affect south and southeast slopes. In addition to the abundant moisture, a shortwave rounding the base of a potent upper-level trough northwest of the state will provide increasing instability, while favorable upper-level jet dynamics support large-scale ascent. These ingredients will support periods of heavy rainfall with embedded thunderstorms and gusty winds. Surface winds will continue to veer southerly across the western half of the state and south-southeasterly across the eastern half tonight, strengthening to breezy levels through Wednesday. This flow will enhance showers along south-facing slopes, with the greatest flooding concerns through Wednesday focused over the western half of the state.
By late week into the weekend, the upper-level trough northwest of the islands will continue to sharpen and take on a negative tilt as a strengthening jet streak develops along its western flank and another shortwave rotates through the base of the trough. Deep tropical moisture will spread across the island chain during this time, with model guidance continuing to show precipitable water values exceeding 2 inches. While thunderstorms are expected tonight through Thursday, the environment will become increasingly favorable for strong to severe storms Thursday night through Saturday morning as instability and wind shear increase. Strong upper-level divergence will further support organized convection. By this time, soils may already be saturated, increasing the likelihood of rapid runoff and dangerous flooding.
In addition to the rainfall and severe thunderstorm threat, southerly winds are expected to strengthen through the week, with the strongest winds likely from Thursday night into the weekend as the pressure gradient tightens. This pattern may produce dangerous winds at Haleakala and the Big Island summits, along with damaging downslope winds across north through east sections of the islands capable of downing trees and power lines. Damaging winds may also occur with stronger thunderstorms. A High Wind Watch has been issued for Haleakala and the Big Island summits from Thursday night through the weekend.
Colder air associated with the upper trough combined with increasing moisture may also bring periods of heavy snow to the Big Island summits later in the week and into the weekend, potentially requiring winter storm headlines. Long-range guidance suggests unsettled weather could persist into next week. Residents and interests across the state should continue monitoring forecasts as this potentially impactful weather pattern evolves.
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