textproduct: Honolulu

SYNOPSIS

Mostly dry and pleasant trade wind weather will continue today, with brief passing showers favoring windward and mauka areas. A significant pattern change is then expected by Tuesday as an upper disturbance and surface low pressure evolve northwest of the islands. This system is expected 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 the middle of the week through the weekend.

DISCUSSION

The forecast has been nudged closer to the National Blend of Models, with some fine tuning of things like the QPF for the Wednesday through Friday time span.

A digging shortwave trough east of Midway will push rapidly to the east-southeast down the Northwest Hawaiian Islands today. The high resolution ensembles show the trough aloft should be close enough to begin locally enhancing showers over and near Kauai tonight in the southeast flow. A few of these might become briefly heavy as this leading shortwave gets close and mid-level temperatures cool for a while, but instability will still be limited at this point.

High pressure to the north will continue to bring some moderate trades to the islands today, but a cold front northwest of the islands that is moving to the east will help to push the ridge closer to the islands. The result will be weakening winds that will begin to veer to a southerly direction. Tonight or tomorrow morning, a broad surface trough is expected to develops northwest of the islands, helping to reinforce the background southeast to south winds from Tuesday through Wednesday.

This is about the time a series of shortwave troughs, like the one mentioned above, will begin to dig towards the islands. 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. The increased 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.

From Wednesday through the end of the week, there remains the potential for more widespread rainfall and flooding. 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.

Differences remain in the global models for next weekend, but the general consensus is that the active pattern could linger in the area. The ECMWF clears out much of the state of the bulk of the rain, but the GFS continues the potential for rain across at least portions of the state through the weekend and beyond, where the deep moisture axis anchors.

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

This system will continue to be closely monitored over the coming days, and additional adjustments to timing, duration, and impact messaging are possible as details become more clear.

AVIATION

A ridge north of the islands continues to bring moderate trades to the islands. As the ridge slowly sinks southward, in response to a front far to the northwest pushing to the east, trades will weaken and veer to a more southerly direction. Passing showers being carried in on the trade wind flow could bring brief MVFR conditions, however VFR conditions are generally expected

AIRMET Sierra is in effect for clouds and showers impacting the east side of Oahu, but anticipate this to be dropped shortly after daybreak. AIRMET Tango is in effect for upper level turbulence over the islands.

MARINE

Moderate to locally fresh trades will hold through this afternoon as high pressure far north of the islands moves slowly eastward. Winds will ease and shift southeasterly tonight through Wednesday as a front stalls just west of the islands. The winds will turn more southerly and increase to moderate and strong levels on Thursday as a low pressure system develops along the stalled frontal boundary.

Small northwest swells will keep some small surf along north facing shores through today. A moderate long-period northwest swell will fill in tonight and Tuesday, giving more of a boost to north shore surf late Tuesday through Wednesday. A small, short period north northeast swell will fill in during the day Tuesday before peaking Wednesday and subsiding Thursday. This overlap will make for some rough choppy conditions.

A small southerly swell will keep some small surf in place along south facing shores through today. A slightly larger, long-period south swell, will fill in tonight. This swell will give a more noticeable increase to south shore surf Tuesday and possibly reaching advisory levels by Wednesday before slowly easing Thursday into the weekend. Strengthening southerly winds could make for rough choppy surf 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, except for some wrap from the northerly swells.

HFO WATCHES/WARNINGS/ADVISORIES

None.