Residents in Hawaii are bracing for a wave of weather extremes, as the National Weather Service in Honolulu forecasts a plume of tropical moisture expected to deliver heavy rain and thunderstorms through tomorrow. According to the NWS, this phenomenon will be particularly impactful over Maui and the Big Island, citing early morning satellite imagery that shows a weak surface trough approximately 200 miles southeast of the Big Island, which could lead to potential flooding.
With around 1.77 inches of precipitable water already recorded in Hilo, this measure sits uncomfortably close to the 90th percentile for this time of year, raising concerns about an intensification of these already daunting conditions. The surface trough, coupled with a passing upper-level trough from the west, is generating a statewide stir of instability that increases the risk of severe weather across the eastern islands. Meanwhile, trade winds are expected to strengthen today due to the trough inflating the local pressure gradient, leading to more showers and possible thunderstorms. A Flood Watch is currently in effect through tomorrow for the already saturated areas of Maui and the Big Island.
The weather pattern developing for the middle of the week suggests a slight decrease in trade winds tomorrow, coinciding with the approach of a cold front from the northwest. As per the NWS Honolulu discussion, this front has the potential to bring heavy downpours to Oahu and Kauai before the weather system loses momentum. Despite this possibility, confidence remains low, preventing the extension of the Flood Watch to these islands.
Forecasting models are split post-Wednesday—some, like the ECMWF, predict the cold front will push through the islands, while others foresee it stalling near the islands before dissipating entirely by Thursday. With variance in predictions, the general consensus points toward a resurgence of breezy to locally windy conditions later in the week as a new high pressure system builds to the north of the islands, providing a faint glimmer of respite to weather-weary islanders.
In terms of aviation, moderate to locally breezy trades are taking the main stage today, and with an uptick in shower activity, expect MVFR and isolated IFR conditions to be a possibility. The warning from NWS notes that "AIRMET Sierra is in effect for mtn obsc for windward Big Island," a clause that could extend to other regions depending on unfolding weather scenarios.
For marine interests, the current passage of a weak surface trough is expected to tighten the regional pressure gradient, subtly strengthening trade winds that will affect the waters near the Big Island and Maui County, as reported by the NWS. Consequently, boat operators should heed the Small Craft Advisory effective through tomorrow. Regarding surf conditions, a High Surf Advisory remains in place as a moderate-sized northwest swell gradually subsides. There may be an increase in surf along east-facing shores later in the week if trade winds strengthen as forecasted.
