Heavy Rain, Hawaii
- When
- September 27, 5:57 PM HST to 8:15 PM HST
- Where
- Hawaii County, 4.53 mi E of Holualoa to 4.42 mi ESE of Honaunau National Pk
- Reported by
- Official NWS Observations
- Coordinates
- 19.615, -155.881
Heavy Rain in Hawaii, September 27, 2021
Convection that developed over a portion of leeward Big Island was boosted by an upper low just north of the islands. The rain caused ponding on roadways, and small stream and drainage ditch flooding.
Where it happened
Every event with a place to put it. Tap one for its entry below.
- Tornadoes
- Tropical storms and surge
- Floods
- Wind
- Hail
- Winter weather
- Heat and drought
- Cold
- Fire, smoke and dust
- Marine and coastal
- Other
The toll
- Deaths
- 0
- Injuries
- 0
- Property damage
- None recorded
- Crop damage
- None recorded
- Events
- 1
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
What happened
The episode as NWS HFO wrote it up.
Convection that developed over a portion of leeward Big Island was boosted by an upper low just north of the islands. The rain caused ponding on roadways, and small stream and drainage ditch flooding. No serious injuries or property damage were reported.
What forecasters said that day
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 Sep 27, 3:58 PM HST, 1 hour 59 minutes earlier.
Area forecast discussion, Sep 27, 3:58 PM HST, 1 hour 59 minutes before
Area Forecast Discussion National Weather Service Honolulu HI 358 PM HST Mon Sep 27 2021 .SYNOPSIS... Moderate to breezy trade winds are expected through Saturday. Showers will favor windward and mountain areas with some brief rainfall reaching leeward locations. An increase in showers is possible late in the week. && .DISCUSSION... High pressure directly north of the state will maintain breezy trade winds into Tuesday. Afternoon satellite and radar imagery shows some showery low clouds moving along with the trades upstream of the smaller islands. This will bring an increase in clouds and showers to windward areas late this afternoon and evening. The high pressure will move slowly east over the next few days which will allow our winds to decrease to moderate speeds Wednesday. A second high pressure will build Thursday bringing winds back up to breezy levels through Friday. Both the GFS and ECMWF global models show an area of enhanced moisture reaching the windward Big Island Thursday night bringing increased windward showers into the weekend. This moisture may also enhance windward showers over the smaller islands Friday night and into the weekend. Trade winds are forecast to drop to gentle/moderate levels by Sunday as a front to the north pushes a high pressure ridge closer to the islands. && .AVIATION... Surface high pressure north of the islands will produce breezy to locally strong, east-northeast trade winds through Tuesday. AIRMET Tango for low level turbulence over and downwind of terrain on all islands will remain posted through at least tonight. Moisture immediately upstream is relatively sparse, but is enough to deliver a few showers to windward coasts and slopes, producing isolated MVFR VIS/CIG in SHRA at times. VFR conditions will prevail over most leeward areas, with the exception of some afternoon clouds on the leeward slopes of the Big Island that may produce isolated MVFR conditions into this evening. && .MARINE... High pressure far north of the state will begin to weaken Tuesday and gradually shift eastward. This will cause the pressure gradient to weaken over the state and will result in the trade winds gradually weakening Tuesday into Wednesday. Winds are expected to drop below Small Craft Advisory (SCA) thresholds during this time. A new high will build north of the state and will bring the return of breezy trade winds towards the end of the week. Trade winds will likely decrease over the weekend as a front passes north of the state. A small reinforcing north-northwest swell is expected to fill in late tonight into Tuesday followed by a small short-period north swell towards the middle of the week. Another small north- northwest swell is possible towards the end of the week. Surf along south facing shores will gradually decline over the next few days with only background swells expected throughout this week. A small Tasman swell from the southwest is possible Thursday into Friday. Surf along east facing shores will remain relatively small throughout this week due to the lack of strong trade winds. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && DISCUSSION...Morrison AVIATION...TS MARINE...Kino
The radar
The national radar mosaic for the moment the first event began.
Event by event
1 heavy rain. Local standard time, as filed.
What these are
- Heavy Rain
- Rainfall totals unusual for the place and season that did not produce a flood.
After NWS Directive 10-1605, the rules offices file Storm Data by.
The same day elsewhere
More
Every storm in Hawaii in 2021Every state in 2021This day in weather historyWarnings in Hawaii, September 2021
Sources
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 976284.