textproduct: Honolulu

SYNOPSIS

Low pressure will draw deep tropical moisture over the state today. Waves of heavy rain capable of producing flash flooding are expected Wednesday through the end of the week. SE winds may become strong as well.

SHORT TERM UPDATE

Issued at 836 PM HST Tue Apr 7 2026

A band of heavy showers and thunderstorms is moving towards Maui County where it could bring a decent amount of rainfall. There is also heavy rain anchoring over the east facing slopes of Maui in Hana and the southeast slopes of the Big Island. Any heavy rain persisting tonight could lead to flash flooding, with a flash flood warning already in place over East Maui. Conditions look to be temporarily clearing up for Oahu and Kauai as the shower band moves east. Shifting gears to winds, the most recent high resolution guidance that just came in is indicating a slightly higher wind threat from Thursday morning through Friday afternoon. South to southeast winds could potentially gust in the 50 to 60 mph range for select downslope areas to the lee of terrain, with Kauai and Oahu at a greater risk. Otherwise the forecast remains on rack with significant hazardous weather ahead.

PREV DISCUSSION

Today through Wednesday night. The initial mid-level shortwave is advancing ENE while remaining north of the islands today. Forcing associated with this feature has maintained a line of moderate to heavy showers that reached Oahu during midday. The showers are maintained by a corridor of weak low- level convergence and are encountering an increasingly moisture-rich environment as they advance east. This has allowed the band to hold together for much of the day, but diminishing forcing through this evening should allow this activity to dissipate with time.

A much more potent shortwave is seen on water vapor imagery anchoring the southern extent of a longwave trough that extends NE to Alaska. As longwave troughing amplifies immediately upstream of the islands, downstream ridge amplification will suppress deep convection tonight through much of Wednesday as rather strong mid- level subsidence takes shape through midday Wednesday. ESE-SSE winds gradually strengthen during this time in response to upstream pressure falls. Meanwhile, southerly winds aloft carry deep moisture characterized by PWAT values around 2" into the forecast area during the day Wednesday. Showers will become increasingly widespread in the afternoon, mainly over the western end of the state. The instability axis will lie over and immediately west of Kauai. This area will escape much of the developing subsidence as it remains under cyclonic flow aloft for much of the next 24 hours. Forcing is maximized within this corridor as well making it the favored location for the heaviest rainfall Wednesday afternoon through Wednesday night.

Thursday. Little change during the day Thursday as the upper low deepens but remains about the same distance west of Kauai. As a result, the heavy rain axis will remain largely stationary this time either over Kauai or just west of the island. Otherwise, waves of warm advection within the decreasingly stable environment will bring periods of locally heavy showers through Thursday.

Thursday night through Friday night. Intense upper-level forcing develops. The combination of right entrance dynamics and curved flow will maximize upper-level divergence over the western end of the state by Thursday evening. Meanwhile, vorticity advection is augmented by the descending/left entrance branch of the jet bringing a stratospheric potential vorticity intrusion as far down as 500mb, a testament to the intensity of emerging dynamic forcing. A mature jet-front system takes shape west of Kauai as this forcing rapidly organizes and ramps up and showers and thunderstorms along the frontal zone begin to intensify. At the same time, the strengthening surface low will cause prevailing ESE/SE winds at the surface to increase to 25-30kts sustained. 925mb southerlies will strengthen to around 40 kts by Thursday evening. This is not a traditional downsloping direction, making potential over typical downsloping areas like Windward Oahu a tough call in terms of wind potential. The southerly direction may instead favor the northern coast of Kauai. Convection, however, will be capable of mixing down these stronger winds making localized wind gusts around 50 mph a possibility Thursday evening into Friday afternoon. At this time, it appears that a Wind Advisory may be needed for select zones on Thursday. The front and attendant heavy rain then surge eastward Thursday night bringing the heaviest rainfall to Kauai late Thurs night into early Friday, Oahu during the first half of Friday, and Maui County and the Big Island during the second half of Friday.

High-end advisory winds are likely on the Big Island Summits and possibly Haleakala Summit on Maui Thursday night through Friday. Snow potential on the Big Island Summits will be limited by marginal temperatures.

Saturday onward. The resident band of tropical moisture lingers over the islands for the foreseeable future, even as trades potentially return during the middle of next week. Weakly cyclonic flow aloft, especially over the western end of the state, will favor the occasional upper-level disturbance tracking over the area and subsequent shower development. This may yield periods of localized heavy rain, but that remains to be seen.

AVIATION

Issued at 836 PM HST Tue Apr 7 2026

Latest radar imagery shows a band of showers and isolated thunderstorms moving through Maui County, with orographic showers also impacting the southeast portion of the Big Island. Clearing conditions prevail behind this rain band for Kauai and Oahu. Under the current weather pattern, however, the islands are expected to be under the threat of showers for at least the next several days, potentially longer.

Hilo in particular may see prolonged degraded conditions as the weather pattern favors persistent orographic showers for the rest of the week and even into the weekend. As for the rest of the sites, confidence is low to moderate on when exactly the next wave of rain will occur, but high resolution weather models suggest Wednesday evening may see shower redevelopment. Winds are expected to be relatively light over the next 24 hours as this pattern weakens the trades.

AIRMET Sierra remains in effect for mountain obscuration for Maui, Molokai, Lanai, and Oahu. Oahu in particular is expected to improve overnight, however Maui County may see low ceilings prevail, and will likely need to consider extension of this AIRMET into SE Big Island given current observations.

MARINE

Issued at 836 PM HST Tue Apr 7 2026

South to southeast winds are expected to steadily increase late tomorrow into Thursday, as a low pressure system develops just west of the state. Showers, some locally heavy, will be possible through Friday as a result of the southerly winds bringing tropical moisture to the north over the islands. As the low pressure system further deepens Thursday into Friday, we will likely see a greater coverage of thunderstorms and winds are expected to increase to gale force across many coastal waters. Very rough boating conditions are expected late Thursday into Friday as seas quickly rise in response to the strong to gale force southerly winds.

Latest offshore buoys and near-shore buoys are showing a steady rise in the 18 to 22 second energy bands this evening and surf will continue to rise tonight and peak on Wednesday above advisory thresholds. A High Surf Advisory has been issued for all south facing shores of the Hawaiian Islands through Wednesday night. Along north facing shores, a new moderate long-period northwest (310-320 degree) swell continues to fill in this evening and will peak on Wednesday below advisory thresholds. A small, short- period north-northeast swell will also fill in this evening before peaking tomorrow and subsiding Thursday.

As the long-period south swell declines on Thursday, strong to near gale force southerly winds are expected to develop on Thursday and strengthen even further Thursday night into Friday. This increase of southerly winds will bring a rapid increase of short-period energy Thursday into Friday and will once again bring surf heights above the advisory thresholds. Large disorganized surf is expected along many south facing shores Thursday into Friday.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 AM HST Thursday for Big Island South- Big Island Southeast-East Honolulu-Ewa Plain-Honolulu Metro- Kahoolawe-Kauai South-Kauai Southwest-Kona-Lanai Leeward-Lanai South-Maui Central Valley South-Maui Leeward West-Molokai Leeward South-Molokai West-Niihau-South Haleakala-South Maui/Upcountry-Waianae Coast.

Flood Watch from 6 AM HST Wednesday through Friday afternoon for Big Island East-Big Island Interior-Big Island North-Big Island South-Big Island Southeast-Big Island Summits-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.

Winter Storm Watch from 6 AM HST Wednesday through Friday afternoon for Big Island Summits.