textproduct: Honolulu

SYNOPSIS

Mostly dry and pleasant trade wind weather will continue into Monday, with brief passing showers favoring windward and mauka areas. A significant pattern change is expected by Tuesday as an upper disturbance and surface low pressure evolve northwest of the islands. This system is forecast to draw deep tropical moisture northward over the state, leading to an extended period of southerly winds, increased chances for widespread rainfall, and renewed flooding concerns from from the middle of the week through the weekend.

DISCUSSION

This afternoon, relatively quiet weather continues across the islands with only a few light showers embedded within moderate trade winds noted in regional satellite and radar imagery. In addition, the typical daytime increase in cloud cover has occurred over the Kona slopes of the Big Island, though little to no shower activity has been observed there today. Moderate easterly trade winds will continue to focus mainly light showers along windward and mauka slopes, while leeward areas remain mostly dry through Monday.

By Monday night, trade wind flow will be disrupted as the surface ridge to the north erodes and a broad surface trough develops northwest of the islands. As a result, weaker background surface winds will begin to veer out of the southeast to south from Tuesday through Wednesday. Guidance indicates that this also will mark the beginning of a series of shortwave troughs that are expected to dig southeastward towards the main Hawaiian Islands in what will be a notable amplification and shift in the large scale pattern across the Central Pacific for the rest of this week. As background flow becomes more southerly, models remain in good agreement that deep tropical moisture will be drawn northward once again over the islands as early as Tuesday but especially by Wednesday. This increasing moisture combined with daytime heating, sea breezes, and a potent shortwave trough moving through may even trigger some heavy showers over interior areas on Tuesday afternoon.

From Wednesday through the end of the week, potential for more widespread rainfall and flooding will return. Upper-level forcing is expected to strengthen as a strong shortwave rounds the base of the trough, potentially enhancing lift over the region while low pressure organizes west of the islands. The combination of deep moisture, increasing large-scale forcing for ascent, and southerly winds (which could be gusty at times) all point towards yet another widespread and organized rainfall event for the state. In addition to the rainfall concerns during this period, some strong to severe thunderstorms along with gusty downsloping winds for north and windward sides of the islands can't be ruled out.

There remain some differences among global models by next weekend, but the general consensus is that the active pattern could linger, with the potential for rain across at least portions of the state persisting through the weekend and beyond where the deep moisture axis anchors.

As previous discussion have 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 period of high rainfall rates could quickly lead to enhanced runoff, renewed rises in streams and reservoirs, and localized flash flooding concerns.

Some tweaks have been made with this afternoon's forecast updates, primarily for QPF and timing. This system will continue to be monitored closely over the coming days, and additional adjustments to timing, duration, and impact messaging are possible as details become more clear.

AVIATION

Mild to locally breezy trades will continue to prevail through the period. Periodic shower activity along windward sites may briefly bring conditions down to MVFR, however widespread VFR conditions are generally expected. Winds are expected to shift more east and even southeast during the day tomorrow across the islands.

No AIRMETS are in effect and none are expected through the next 24 hours.

MARINE

Moderate to locally fresh trades will persist for the next day or so as high pressure north of the islands moves slowly eastward. Winds will ease and shift southeasterly Monday night through Wednesday as a front stalls out and merges with a trough just west of the islands. The winds will turn more southerly and increase to moderate and strong levels on Thursday as a storm system develops along the trough west of the state.

Small northwest swells will keep some small surf along north facing shores through Monday. A moderate long-period northwest swell will fill in Monday night and Tuesday, giving more of a boost to north shore surf late Tuesday through Wednesday.

A small southerly swell will keep some small surf in place along south facing shores through Monday. A slightly larger, long-period south swell, will fill in Monday night. This swell will give a more noticeable increase to south shore surf Tuesday into next weekend. Strengthening southerly winds could make for choppy seas by late next week.

East shore surf will remain small and below the seasonal average during the next 7 days due to a lack of strong trade winds over and upstream of the islands.

HFO WATCHES/WARNINGS/ADVISORIES

None.