textproduct: Honolulu

SYNOPSIS

Moderate trades deliver showers windward and mauka through the period.

SHORT TERM UPDATE

Moderate trades ushering shallow moisture through the area this morning resulting in some enhanced shower coverage windward and mauka. Per the usual, this shower activity will diminish with peak heating giving rise to another mostly dry fair weather day. Additional moisture advances through the area tonight bringing similar conditions to most areas. Worth noting that tonight is modeled to bring slightly deeper saturation up to 700mb/10kft and flow within this layer is generally southeasterly. Guidance hints at some island interaction with convergence downstream of Maui County potentially bringing enhanced showers to Leeward Oahu by very early Wednesday morning. Too small in scale to be reliably modeled, observations will be worth monitoring this afternoon into tonight.

PREV DISCUSSION

Issued at 345 AM HST Mon Jul 20 2026 A quiet and fairly dry start to Monday as upper ridging engulfing the central Pacific provides the greater subsidence and stability required in maintaining drier weather conditions. Higher moisture advancing toward us from the east will likely boost statewide shower frequency and coverage the next couple of days. This morning's slightly below normal precipitable water values of 1.1" to 1.2" will be on the rise back to near normal levels of around 1.4" by tomorrow in response to greater mid layer moisture advection. Higher low to mid layer humidity will be the impetus to increased shower frequency from later today through Wednesday. A subtle lift in modeled boundary layer thickening, or lifted inversion heights, from this morning's 6.5 kft to 7 kft heights to Tuesday's near 10 kft depth signals higher probabilities of more areawide early week rainfall. Drier air following this moist air mass will be noticed in lessened late week shower activity. After several days of light to moderate trades, a more favorable orientation of the pressure gradient back from a large surface high far north of the area will keep the central Pacific gradient tight enough to support more fresh late week trades.

Tropical Storm Fausto, currently located several hundred miles west southwest of southwestern Mexico, is steadily strengthening. Fausto is expected to achieve hurricane strength this evening with a northwest becoming more westerly track through Wednesday. Deterministic model guidance has Fausto reaching 140W in about a week with it weakening along its northwesterly track as the cyclone encounters western ridging and cooler waters. If TC Fausto stays on a northwesterly track, localized trade flow will be interrupted with much weaker trades expected early to mid next week.

AVIATION

Issued at 345 AM HST Mon Jul 20 2026 Moderate trades will continue through the forecast period. VFR conditions will prevail except isolated MVFR conditions over windward and mauka areas in clouds and showers.

No AIRMETs are in effect and none are anticipated through the forecast period.

MARINE

The trade winds will hold at mostly moderate speeds today through Tuesday, then restrengthen back to fresh and locally strong speeds Wednesday through Friday as high pressure strengthens north of the island chain.

The current long-period south swell is bringing advisory level surf to some south facing shores this morning. The swell is expected to hold through at least Tuesday, which is expected to keep surf at lower-end advisory levels for which a High Surf Advisory is now in effect. This swell with then slowly fade during the middle of the week, but will be followed by a new long-period south swell arriving on Thursday, which will keep surf elevated above the summertime average through the weekend.

Surf along east-facing shores will remain slightly below seasonal levels today through Wednesday as the trades ease upstream of the islands. East shore surf should then build back close to seasonal levels late this week and through the weekend as stronger trades return over and upstream on the islands. Additionally, swell generated by former tropical cyclone Elida may give a slight boost to east shore surf during the middle of the week.

Surf along north-facing shores will remain at flat to tiny levels through the weekend.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 PM HST Tuesday for all south-facing shores.