A powerful Kona storm is expected to impact the state this week and into the weekend, bringing numerous hazards across the islands. A combination of the potential for considerable flash flooding, damaging winds, and strong to severe thunderstorms will threaten Hawaii from Tuesday through at least Saturday. The risk for flash flooding could begin as early as Tuesday and Tuesday night over Kauai and Oahu as deep tropical moisture spreads into the western end of the state. This threat will expand eastward across the state by the latter half of the week, with the likelihood for severe thunderstorms increasing late this week and into the weekend.
A significant transition in the large-scale weather pattern is expected to unfold across the Hawaiian Islands over next couple of days and continuing through the upcoming weekend. The current dry and relatively stable regime, characterized by moderate east- southeast flow, will persist today. Conditions will begin to deteriorate tonight into Tuesday as a deep upper-level trough amplifies northwest of the islands.
Deterministic models and their respective ensembles remain in strong agreement regarding the evolution of this system. A powerful jet streak with core winds of 100 to 140 kt at 250 mb is forecast to dig southward along the western flank of the trough. As this occurs, the trough will gradually acquire a negative tilt while expanding toward the island chain through the week. The islands will increasingly fall beneath the jet streak’s left-exit region, supporting enhanced divergence aloft and large-scale ascent across the region.
At the surface, model guidance depicts a broad area of low pressure consolidating northwest of the islands in response to strong upper-level height falls. Central pressures could fall to near 990 mb while remaining well northwest of the state. The resulting pressure pattern will shift winds across the islands out of a southerly direction, drawing a plume of deep tropical moisture northward across the region.
Precipitable water values are expected to increase into the 1.5 to 2.0 inch range beginning Tuesday across the western islands, then spread eastward across the remainder of the state through midweek. This pattern will then persist through next weekend, with PWAT values potentially rising into the 2.0 to 2.3 inch range statewide later this week.
The combination of strong upper-level forcing for ascent, deep tropical moisture, and persistent southerly flow will create a favorable environment for periods of heavy rainfall with embedded thunderstorms. Repeated rounds of convection may occur as smaller disturbances rotate through the broader upper trough, increasing the potential for training rainfall bands along terrain-favored areas.
Flooding concerns are expected to increase statewide as the week progresses, particularly where the heavier rainfall persists over the same areas for multiple days. By late week and next weekend, soils will already be saturated, increasing the likelihood that additional rainfall will produce rapid runoff and dangerous flooding conditions.
In addition to the rainfall threat, strengthening south to southwest winds may develop during the latter portion of the week and upcoming weekend as the pressure gradient tightens between the developing low to the northwest and high pressure to the east. If this materializes, strong and damaging Kona winds and localized downslope winds capable of downing trees and power lines will be likely.
While thunderstorms are certainly possible earlier in the event, the environment will become increasingly favorable for severe storms by midweek as as the thermodynamics and kinematics (MU CAPE of 1500 to 2500 J/kg, 50 kt 850 winds with up to 40 kt or more of 0–6 km bulk shear) become more supportive.
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.
While uncertainty remains regarding the exact timing and location of the heaviest rainfall and storms, confidence continues to increase that the islands will experience a prolonged period of unsettled and potentially impactful weather this week. Residents and interests across the state should continue to monitor forecasts as details regarding the timing and location of impacts become clearer.
East southeast winds should weaken today. Low cigs and some SHRA are possible along southern and windward slopes. MVFR conds possible but VFR looks to prevail. A Kona low is expected to impact the whole state starting Tuesday through the weekend moving west to east through the week. Widespread heavy RA, severe TSTMS and damaging winds can be expected. LIFR to MVFR conds will be possible at times.
AIRMET Sierra for mtn obsc is in effect above 020 for windward locations of Kauai, Oahu, and Maui. This should improve later this morning. AIRMET Sierra will most likely be needed this week for mtn obsc and IFR conds.
A powerful Kona storm is expected to impact the state as heavy rain, thunderstorms, and rough seas are expected to accompany this storm system starting Tuesday into the weekend. Currently, strong high pressure far northeast of the state will drift east as fresh to locally strong east to east southeast winds slowly decline through the day. Winds will begin to veer southerly and increase to fresh to locally strong speeds Tuesday through late this week. The Small Craft Advisory (SCA) has been scaled back to windier waters and channels around Maui County and the Big Island through this afternoon. An SCA will likely be issued for western waters later this week as this Kona low develops.
Surf along east facing shores will gradually trend downward today then fall below normal levels Tuesday through late this week. A series of small long period west-northwest swells will keep some small surf in place along north and west facing shores during the forecast period. A small medium period north swell is possible late this week.
Surf along south-facing shores will remain small, with the exception of areas exposed to wind swell wrap. Rough and choppy conditions will develop Tuesday and continue through late week as southerly winds increase in advance of a front. A series of small long-period south swells will impact shorelines through late this week.
Flood Watch in effect for Niihau and Kauai beginning Tuesday morning and on Oahu beginning Tuesday evening.
Small Craft Advisory until 6 PM HST this evening for Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
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