A mid level trough combined with an upper level low and enhanced moisture allowed significant flooding to occur over portions of Kauai. Mt Wai'ale'ale recorded nearly eight inches of rainfall in a twenty four hour period.
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The episode as NWS HFO wrote it up.
A mid level trough combined with an upper level low and enhanced moisture allowed significant flooding to occur over portions of Kauai. Mt Wai'ale'ale recorded nearly eight inches of rainfall in a twenty four hour period.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Honolulu, HI wrote its last area forecast discussion before the first event at Jul 20, 4:00 AM HST, 1 hour earlier.
Area Forecast Discussion National Weather Service Honolulu HI 400 AM HST Sun Jul 20 2025 .SYNOPSIS... Weak low pressure far northwest of the islands will move further off to the west as high pressure expands over the Central Pacific. Greater lower to mid layer moisture will thin out as drier air moves in from the east the next few days. Coastal and higher terrain trades will generally remain moderate with locally strong winds through the middle of the week. Showers will become less frequent and focus along windward upslope mauka exposures. && .DISCUSSION... The influence of a passing mid level trough across the western third of the island chain, along with weak upper low north churning near 35N165W, within a lingering moisture rich air resident mass provided enough instability to develop overnight Oahu plume and northern moving rain that trained over the Garden Island. The heaviest rain fell parallel to Kauai's high ridgelines with half an inch to inch hourly rates lifting local streams downstream of Mount Waialeale (Wainiha) to flood stage. The trajectory of the precipitation south of Kauai is now sliding a touch more west and away from Kauai thus lessening the morning threat. Many higher elevation regions within central and northern Kauai picked up about one to two inches since last night with neighboring Oahu receiving a few tenths an inch more rain within the Manoa and Nuuanu Valleys since yesterday evening. A drier air mass moving in from the east will suppress widespread rain with widely scattered light showers sticking to windward exposed upslope mauka through the week. This drier week will likely exacerbate short term fire weather conditions as the state will fall deeper into abnormally dry to moderate drought categories (see Fire Weather section below). Surface high pressure far north northeast of the state will continue to produce a gradient tight enough to drive moderate to locally strong trade winds over the islands for the next few days. Lowering heights slinking away to the northwest, as ridging at all levels takes hold from the south and east, will ensure a more stable environment for the week to come. The dry air has yet to arrive as Hilo's 12Z sounding is showing a saturated layer up to 800 mb (7 k ft) within a very moist air mass of near 1.75 inch pwats which is in the lower 10% of climatological occurrence. Upper ridging will anchor itself over the southern latitudes and will directly influence late July drier island weather. The approach of a shortwave mid level trough from the east at mid week may subtly weaken winds but limited moisture will maintain statewide low rain chances. The drier trend is supported by the both ensembles where the majority of the members keep the diurnal periods of higher QPF either north or south of the island archipelago. Slightly weaker synoptic trade flow may allow local breezes to take over and create their own micro-climates (e.g., afternoon Big Island kona slope sea breeze showers). && .AVIATION... High pressure far north of the islands will maintain moderate to locally breezy trades through the weekend. Lingering instability associated with a weak upper level disturbance north of Kauai and a moist air mass across the state will result in periods of MVFR showers across mainly windward and mauka areas through Sunday, with some showers moving into leeward areas overnight. Isolated thunderstorms are also possible across the waters north and east of Kauai through late tonight. AIRMET Sierra for mountain obscuration remains in effect for windward sections of Kauai and Oahu. With little change expected overnight. && .MARINE... A trough passing westward near Kauai will allow the high pressure to build back into the region and strengthen trade winds into the moderate to locally strong range into the middle of next week. Several pulses of southerly and southwesterly swell will continue to boost surf heights along south and some of the west facing shores this week. Surf along east facing shores will remain small and choppy. Surf along north facing shores will remain nearly flat through the forecast period. Peak high tides associated with the lunar cycle could lead to minor coastal flooding along shorelines and low-lying coastal areas next week Monday through Wednesday during the daily peak tide each afternoon. && .FIRE WEATHER... Due to recent rain, statewide fire weather concerns will remain low to begin the week. Drier conditions this week will begin to awaken a heightened situational awareness to the increased wildfire threat. KDBI values exceeding 600 with afternoon humidities falling into the lower to mid 50s under lighter interior winds should hold the near to medium term fire threat at bay. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST Monday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && DISCUSSION...Blood AVIATION...Bohlin MARINE...Bohlin FIRE WEATHER...Blood
The South Kauai radar (PHKI), 46 km away, was scanning. The volume nearest the first event began at 14:59 UTC.
1 flash flood. Local standard time, as filed.
The Kauai Emergency Management Agency reported that a large amount of water inundated Kuhio Highway at the Hanalei Bridge. Flooding was also reported near the mouth of Wailua River.
After NWS Directive 10-1605, the rules offices file Storm Data by.
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