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Flash flooding on the Hawaiian island of O‘ahu

By Scott Bachmeier •

5-minute GOES-18 Infrared images centered on O’ahu, with overlays of GLM Flash Extent Density, Flash Flash Warning (red polygons), Flood Watch (dark green polygons), Flood Advisory (light green polygons) and Marine Weather Advisory (white polygons) in addition to METAR surface observations — from 0701 UTC on 20 March to 0401 UTC on 21 March [click to play MP4 animation]

5-minute CONUS Sector GOES-18 (GOES-West) Infrared images viewed using RealEarth (above) displayed deep convection (with intermittent GLM-indicated lightning activity) that moved across the Hawaiian island of O’ahu on 20 March 2026 — producing heavy rainfall and flash flooding.

The first Flash Flood Warning for O’ahu on that day was issued at 0857 UTC, covering the northern portion of the island (below).

GOES-18 Infrared image at 0901 UTC, with a plot of the initial Flash Flood Warning (red polygon) that was issued for the island of O’ahu at 0857 UTC on 20 March [click to enlarge]

By 2041 UTC, Flash Flood Warnings covered the entire island (below).

GOES-18 Infrared image at 2041 UTC, showing Flash Flood Warnings (red polygons) covering the entire island of O’ahu [click to enlarge]

As of 1749 UTC on 20 March, the highest rainfall amounts on O’ahu included 14.80″ at Waialua (on the north coast), 12.56″ at Shofield Barracks (on Wheeler Army Airfield, METAR identifier PHHI) and 12.21″ at Ka’a’awa (along the northeast coast). A plot of gauge height at the Wahiawa Reservoir Spillway near PHHI (below) showed the rapid rise that occurred after a period of heavy rainfall, peaking past minor flood stage at 85.29 ft (at 1840 UTC).

Plot of gauge height at the Wahiawa Reservoir Spillway [click to enlarge]

This particular episode of heavy rain and flash flooding was driven by the presence of a Kona low southwest and west of Hawai’i, as seen in surface analyses from the Ocean Prediction Center (below).

GOES-18 Infrared images with an overlay of surface analyses, from 0000 UTC on 19 March to 0000 UTC on 21 March; Hawai’i is located in the lower left portion of the images [click to enlarge]

Southwest flow to the east of the Kona low was helping to transport rich tropical moisture across Hawai’i — as depicted by the MIMIC Total Precipitable Water product (below).

MIMIC Total Precipitable Water images, from 0000 UTC on 19 March to 2300 UTC on 20 March

The tropical moisture being advected across Hawai’i was reflected in the Total Precipitable Water (PW) value of 2.01″ sampled at Lihue (the closest rawinsonde site to O’ahu) at 0000 UTC on 20 March (below).

Plot of rawinsonde data from Lihue, Hawai’i at 0000 UTC on 20 March [click to enlarge]

According to a plot of the climatology of Total Precipitable Water values for all Lihue soundings at 0000 UTC on 20 March (below), the 2.01″ sampled by the Lihue sounding seen above exceeded the previous record daily maximum of 1.85″.

Climatology of Total Precipitable Water values for all Lihue, Hawai’i soundings at 0000 UTC on 20 March [click to enlarge]

Categories: GLM, GOES-18, Heavy rain / flooding, Lightning, Microwave, RealEarth