Mauka Showers

In early February, the main Hawaiian Islands were hit by an extreme rain and wind event that produced flash flooding, numerous power outages, and multiple reports of structural damage. The rainfall eased but windy conditions persisted through the Lunar New Year (February 17) before finally easing to where it made sense to rake all the leaves that blew off my now mostly bolo-head bamboo tree.

The break in active weather conditions didn’t last long. On February 20th, a strong, negatively tilted upper tropospheric trough started to move into the area and influenced weather conditions over the island chain. Negatively tilted troughs are aligned northwest to southeast and have the potential to produce very intense rainfall and thunderstorms. The orientation of the trough provides upper level wind conditions that enhance upward air motion and instability. A National Oceanic and Atmospheric Administration (NOAA) informational page titled, “Basic Wave Patterns” provides a nice summary of features such as negatively tilted troughs, among others, so check it out if you want to find out more.

The increasing instability on February 20th produced heavy rainfall over the windward slopes from Kauaʻi to Maui. On Kauaʻi, the rainfall produced a large increase in water levels in Hanalei River but did not overflow onto Kūhiō Highway. A subsequent burst of rainfall after midnight on February 21 produced a more substantial flood wave that did close the highway for several hours. The hydrograph below shows water level data from Hanalei River. The two spikes on the right side of the graph go above the level expected to produce flooding on Kūhiō Highway. The first spike was just barely above the line and dropped off quickly. The second spike was higher and more persistent.

Hydrograph of water level data from the U.S. Geological Survey’s gage at Hanalei River covering the period from February 15 – 22, 2026. The image is from NOAA’s National Water Prediction Service (NWPS).

Atmospheric instability increased as the upper tropospheric trough approached the main Hawaiian Islands from the southwest.  The map below shows winds and geopotential heights at 200 millibars (mb, at around 40,000 ft) as of 2 AM HST on February 21st. The red dashed line shows the northwest to southeast negatively tilted trough axis. A GOES-18 infrared (IR) image loop (below the 200 mb map) from the morning of February 21st shows clouds associated with the trough shifting northeastward across the state. The bright green and yellow colors are the highest and coldest cloud tops from embedded thunderstorms.

Geopotential heights and winds at 200 millibars from the GFS model’s 00-hr panel valid at 2 AM HST, February 21, 2026. The figure was downloaded from the NCEP Model Analyses and Guidance website. I added a red oval to identify the main Hawaiian Islands and the red dashed line to show the position of the low pressure trough at this level in the atmosphere.

NOAA/NESDIS GOES-18 color enhanced infrared (IR) image loop covering the tropical central and east Pacific on the morning of February 21, 2026. In this enhancement curve, blue, green, and yellow colors show the coldest (and highest) cloud tops. Lighter gray shades are low clouds, and the darker gray areas are the ocean or land areas. Annotations in yellow are my own.

Zooming in closer to the main Hawaiian Islands, the loop below shows visible satellite images combined with GOES Lightning Mapper (GLM) data to indicate the locations of thunderstorm activity. Note the brief flash of orange over the Kāneʻohe area of Oʻahu, which shows a peak in lightning activity associated with the area of the most intense rainfall.

NOAA/NESDIS visible image loop centered on the main Hawaiian Islands with Geostationary Lightning Mapper (GLM) data overlaid. The loop covers the period from 8:11 AM to 10:36 AM HST, February 21, 2026. For more information on the GLM data, please click here.

Rainfall intensities peaked over Oʻahu during the early to mid-morning hours of February 21. The Multi-Radar, Multi-Sensor (MRMS) loop of Seamless Hybrid Scan Reflectivity (SHSR) data shows intense rainfall over the Hoʻomaluhia and Haʻikū Valley regions of Kāneʻohe, and along the upper slopes of the Koʻolau Range northwest of Kāneʻohe. Note how the area of 50 to 55+ dBZ reflectivity remains nearly stationary over the Hoʻomaluhia and Haʻikū Valley region as intense rainfall cells repeatedly form and move over the same area.

Seamless Hybrid Scan Reflectivity loop from 5 AM to 9 AM HST, Saturday, February 21, 2026 in 10-minute time steps from the NOAA National Severe Storms Lab’s Multi-Radar/Multi-Sensor (MRMS) system website. The images from the website were cropped for visual clarity.

One of the many useful features included with MRMS is the Average Recurrence Interval (ARI) display. MRMS converts radar estimates of rainfall for various durations into ARI estimates every 10 minutes. The loop below shows ARI values for the 6-hour rainfall accumulation period in 30-minute time steps. The Hoʻomaluhia and Haʻikū Valley areas had ARI values above the 100-year return period, meaning the 6-hour rainfall estimates have an annual probability of occurrence at less than 1%. This does NOT mean you can’t have this level of rainfall for another 100+ years. It is just an indication of how rare this amount of 6-hour rainfall is over this particular area.

MRMS Average Recurrence Interval (ARI) data loop covering the period from 5 AM to 11 AM HST, February 21, 2026, in 30-minute time steps. The data are from the NOAA National Severe Storms Lab’s Multi-Radar/Multi-Sensor (MRMS) system website. The images from the website were cropped for visual clarity.

In addition to the Kāneʻohe area, heavy rainfall over the upper slopes of the central Koʻolaus  produced large amounts of flow in streams draining down the leeward side of the range. Flow was especially high in North Fork Kaukōnāhua Stream and South Fork Kaukōnāhua Stream. These streams flowed into Wahiawā Reservoir (Lake Wilson), which quickly filled up. The hydrograph below shows water levels from February 17th through the morning of February 22nd. Once the water level reached 80 ft, water overflowed into the spillway of Wahiawā Dam.  The reservoir level reached 83.58 ft before slowly decreasing. When flow over the spillway is excessive, the lower reaches of Kaukōnāhua Stream can overflow and inundate places such as Otake Camp in Waialua. Flooding in Otake Camp is not extremely unusual, with significant inundations occurring in December 1999 and December 2008. There may have been more occurrences between December 2008 and this recent event, but I don’t recall any others at this time and my informal records don’t have indications of any others. There was an evacuation of Otake Camp in October 2005, but I don’t know if any property inundation occurred.

Hydrograph of water level data from the USGS gage at Wahiawā Reservoir covering the period from February 17 – 22, 2026. The image is from NOAA’s National Water Prediction Service (NWPS). Water levels over 80 ft will flow over the Wilson Dam spillway.

A look at the statewide rainfall picture for February 20-21, 2026 using MRMS Pass 2 radar + rain gage data (map below) shows that the Koʻolau Range on Oʻahu received the highest amounts, by far. A map zoomed in to just Oʻahu shows the localized extreme amounts near Kāneʻohe. The amplified rainfall over the Koʻolau Range, along with much lower amounts over the open ocean seem to suggest that this was mainly a terrain-driven (orographic) event boosted substantially by the instability from the upper tropospheric trough. This is similar to the Super Bowl Sunday event I covered in my February 13 post.

MRMS 2-day accumulated precipitation over the state covering the period from February 20th through February 21st. The data in this graphic were downloaded from the MRMS repository on the Registry of Open Data on AWS and processed using QGIS software.

Same as above, but zoomed in to Oʻahu.

Similar to my posts about other recent heavy rain events, I’ve included the top 5 event totals from each county in the table below. The rainfall totals were pulled from the Hawaiʻi Rainfall Summary (RRAHFO) text product issued by HFO and archived by Iowa State University. All of the amounts are midnight-to-midnight totals. Locator maps for each of the gages are included below the table. Since the totals come directly from the RRAHFO product, they have not been examined for errors.

Station	Feb 20	Feb 21	2-day total
Kauaʻi
Waiʻaleʻale	5.33	1.48	6.81
Wailua Exp Farm	0.25	4.79	5.04
N. Wailua Ditch	2.14	2.79	4.93
Hanalei	1.49	2.81	4.30
Līhuʻe Variety Stn	0.06	4.11	4.17
Oʻahu
Luluku	0.45	25.10	25.55
Moanalua RG	7.00	17.51	24.51
Poamoho RG No. 1	3.70	16.32	20.02
Heʻeia NERR	0.25	11.25	11.50
St. Stephens	1.87	8.74	10.61
Maui
Hāmoa	0.23	6.70	6.93
Puʻu Kukui	5.02	0.79	5.81
W. Wailuaiki	4.06	1.43	5.49
Hāna Airport	0.11	4.52	4.63
Makapulapai	0.10	2.21	2.31
Big Is
Pāhoa	2.03	1.62	3.65
Honoliʻi Stream	1.00	1.25	2.25
IPIF	1.08	1.03	2.11
Kealakomo	0.67	1.31	1.98
Kulaʻimano	0.66	1.14	1.80

Maps of locations for the top 5 rainfall totals on Kauaʻi, Oahu, Maui County, and the Big Island. Station location data are from the HCDP.

I also took a closer look at the Luluku and Moanalua Rain Gage (RG) totals since these sites are located in the areas that registered high MRMS ARI values. The MRMS ARI values are based on radar-only rainfall estimates and do not include bias adjustments from any rain gage data. As such, it is better to look at the rain gage data for more accuracy. The table below shows the peak rainfall amounts (in inches) for different durations over the February 20th through 21st event. Return periods (in years) are in the rows below the rainfall amounts. Totals at the Luluku gage for the 1-hour to 24-hour durations are extremely rare for this location, with the 3-hour to 24-hour totals having a probability of occurrence of less than 0.1% in any given year (>1000 year return period). The Moanalua RG totals were also impressive, though the return periods were not as high as Luluku’s. The Luluku totals are from the 15-minute data available on the HADS website. Moanalua RG data are from the USGS’ website. Return periods for each site are from NOAA Atlas 14.

Station	15-min	1-hr	3-hr	6-hr	12-hr	24-hr
Luluku	1.79	5.38	13.01	17.07	21.72	25.10
	10-25 yr	200-500 yrs	>1000 yrs	>1000 yrs	>1000 yrs	>1000 yrs
Moanalua RG	1.53	4.76	12.49	13.44	16.37	22.22
	2-5 yrs	25-50 yrs	500-1000 yrs	200 yrs	100-200 yrs	200-500 yrs

With two extreme events in February, I think it’s safe to say that rainfall has picked up quite a bit after a slow start to the 2025-2026 Hawaiian Islands wet season. There’s still March and April to go, so we’ll have to stay tuned to what Mother Nature has in store for us. As always, have a plan, execute that plan, and stay safe!