textproduct: Honolulu

SYNOPSIS

In the large scale, a surface kona low pressure system continues to move slowly northeastward away from the Hawaii Islands, producing additional periods of moderate to heavy rain, thunderstorms, and elevated threats for flash flooding statewide. A combination of low level forcing and passing upper level troughs will drive smaller scale heavy rain bands and thunderstorms. These smaller scale cloud bands are less predictable and will produce the heaviest rainfall rates in the 2 to 4 inches per hour range. Additional weather threats include strengthening south to southwest kona winds today that may produce down sloping wind gusts in the 30 to 45 MPH range along steep north and east slopes of island mountain ranges lasting through tonight. Improving weather trends will develop from west to east on Monday as we transition to a wet trade wind weather pattern lasting through the end of next week.

SHORT TERM UPDATE

Issued at 918 AM HST Sat Mar 21 2026

Showers and thunderstorms continue this morning, with the thunderstorms staying off the coast for the most part. As of 9:10 AM HST, the largest area of showers was located over Maui County, with lighter showers moving through Oahu and along the southeast coast of the Big Island. We still expect additional showers and isolated thunderstorms today, some heavy, and the Flood Watch remains in effect for all islands. The most likely areas for heavy rain today are Oahu and Maui County. Finally, the Winter Storm Watch for the summits on Big Island has been upgraded to a Warning, and runs from 6 PM HST today to 6 PM Sunday.

PREV DISCUSSION

Issued at 329 AM HST Sat Mar 21 2026

This mornings satellite imagery showers a long lived smaller scale low level convergence band slowly drifting eastward through Maui County. The heaviest showers and thunderstorms are currently developing along this convergence band within 50 miles north and south of Molokai early this morning. This band will continue the be the primary driver of heavy rain over Maui County today as it slowly drifts eastward into Maui later this morning. For Oahu, a broad band of enhanced clouds are moving towards Oahu and are consolidating along a new low level smaller scale convergence band. Showers are already increasing across the island with deeper. cloud heights rising into the 25,000 to 30,000 foot range based on satellite cloud top temperatures and may lead increasing moderate to heavy showers over Oahu. Enhanced showers are also shown on radar imagery developing just upstream of Kauai and the Big Island. We continue to watch this fluid situation closely as the heaviest rainfall impacts will evolve on an hour by hour timeline. Stay vigilant this weekend.

Focusing on the larger scale for a moment, we continue to see two surface lows within the larger kona low structure drifting towards the northeast passing just north of the island of Kauai. The highest upper level forcing from this kona low will develop over Oahu and the islands of Maui County into Sunday. The Big Island will likely see additional large scale forcing by early Sunday morning into Monday with this system.

By Monday, the pattern begins to change for the western islands as cool northeasterly trades blow into Kauai and Oahu. Returning trade winds will signal improving weather trends as the deep moisture and unstable atmosphere will be driven towards the south, away from the islands. By Tuesday, deep unstable moisture will clear up over Maui County and the Big Island of Hawaii with improving weather trends. Upper level troughing will linger across the Hawaii region next week. However, heavier showers will diminish as the weather pattern shifts back to a more typical wet trade wind weather pattern. Passing light to moderate showers will favor the typical windward and mountain areas with this easterly trade wind pattern lasting through next weekend.

AVIATION

Issued at 918 AM HST Sat Mar 21 2026

A band of showers and embedded thunderstorms continues to move over Maui County this morning. The ongoing kona low will support further rain bands across Oahu and Maui County through the period. Rainfall may spread to Kauai and the Big Island as well, however confidence is lower for those areas. Winds will remain primarily out of the south, potentially veering more southwest as the day progresses. Expect widespread MVFR conditions over Maui County and Oahu at times through the forecast period, going down to locally LIFR conditions under showers.

AIRMET Sierra for mountain obscuration remains in effect for Molokai is expected to continue through the period. Lanai remains under IFR as low cigs near the surface continue to prevail there, and may persist due to widespread showers in the area. The western half of Maui has now been included in the IFR AIRMET for heavy showers. Oahu is in a rainfall lull and has been removed from the mountain obscuration AIRMET for the time being.

AIRMET Tango also remains in effect for moderate turbulence downwind of terrain for Oahu and Maui County below 090, and is expected to continue through at least the day Saturday as gusty south to southwest winds are observed across both islands.

Light icing in layer 140-FL260 will also persist so long as high clouds envelop the islands due to this ongoing kona low.

MARINE

Issued at 918 AM HST Sat Mar 21 202

Moderate to locally strong south/southwesterly flow will prevail today. Bands of heavy showers and a few thunderstorms will move over portions of the coastal waters, especially the waters from Oahu through Maui County today, as a kona low continues to impact the state. The low will lift north on Sunday allowing moderate northerly winds to develop around Kauai, which will then veer to northeast and spread southward. Additionally, due to wave heights around Niihau, Kauai and Oahu in the 8-12 foot range, a Small Craft Advisory remains in effect for those waters until 6AM Sunday.

A northerly swell is building today and will create advisory- level surf for north facing shores of Niihau, Kauai, and Oahu that will peak this afternoon, then slowly decline tonight. As such, a High Surf Advisory (HSA) remains in effect for north facing shores for Niihau, Kauai, and Oahu until 6AM Sunday. Additionally, a slight bump is expected from a small, building west-northwest swell that will peak Sunday.

The south swell is slowly declining and will continue to trend down through the weekend into early next week. Smaller surf is expected along south facing shores through the remainder of the week. Surf along east-facing shores remains elevated due to wrap from the north swell, and will be near HSA thresholds. This swell will slowly diminish through the weekend.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through Sunday afternoon for Big Island East-Big Island Interior-Big Island North-Big Island South-Big Island Southeast-Central Oahu-East Honolulu-Ewa Plain-Haleakala Summit- Honolulu Metro-Kahoolawe-Kauai East-Kauai Mountains-Kauai North- Kauai South-Kauai Southwest-Kipahulu-Kohala-Kona-Koolau Leeward- Koolau Windward-Lanai Leeward-Lanai Mauka-Lanai South-Lanai Windward-Maui Central Valley North-Maui Central Valley South- Maui Leeward West-Maui Windward West-Molokai Leeward South- Molokai North-Molokai Southeast-Molokai West-Molokai Windward- Niihau-Oahu North Shore-Olomana-South Haleakala-South Maui/Upcountry-Waianae Coast-Waianae Mountains-Windward Haleakala.

High Surf Advisory until 6 AM HST Sunday for Kauai East-Kauai North-Koolau Windward-Niihau-Oahu North Shore-Olomana.

Winter Storm Warning from 6 PM this evening to 6 PM HST Sunday for Big Island Summits.

Small Craft Advisory until 6 AM HST Sunday for Kaiwi Channel- Kauai Channel-Kauai Leeward Waters-Kauai Northwest Waters-Kauai Windward Waters-Oahu Windward Waters.