Flash Flooding and Flood in Arizona, October 2 to 3, 2018

Name: Flash Flooding and Flood in Arizona, October 2 to 3, 2018
Start: 2018-10-02T03:45
End: 2018-10-03T04:00
Location: Arizona

October 2 to 3, 2018 · NWS TWC · 3 events in 1 county or zone

What was Major Hurricane Rosa crossed the northern Baja peninsula as a tropical depression October 1st. The remnant circulation entered southwest Arizona on October 2nd.

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

Open the live radar over this ground

The toll

Deaths: 0
Injuries: 0
Property damage: $3.8 million
Crop damage: None recorded
Events: 3

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 TWC wrote it up.

What was Major Hurricane Rosa crossed the northern Baja peninsula as a tropical depression October 1st. The remnant circulation entered southwest Arizona on October 2nd. Moisture ahead of these remnants fueled thunderstorms in southeast Arizona on September 30th. Rain associated with the remnant circulation moved northeast across southeast Arizona through October 2nd. The primary period of heavy rain occurred from 8 pm MST October 1st through 8 am MST October 2nd. Rainfall totals averaged two to four inches on the west half of the Tohono O'odham Nation with isolated heavier amounts of five to eight inches. The heavy rain initially resulted in flash flooding, but evolved into a longer term period of flooding which led to evacuations of several villages, road closures, considerable infrastructure damage, and a subsequent disaster declaration.

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 Tucson, AZ wrote its last area forecast discussion before the first event at Oct 2, 3:25 AM MST, 20 minutes earlier.

Area forecast discussion, Oct 2, 3:25 AM MST, 20 minutes before

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The radar

The Phoenix radar (KIWA), 186 km away, was scanning. The volume nearest the first event began at 10:46 UTC.

Open the radar archive at this moment

Event by event

2 flash flood, 1 flood. Local standard time, as filed.

Flash Flood, Pima

When: October 2, 3:45 AM MST to 9:45 AM MST
Where: Pima County, 3.35 mi W of Ali Chuk to 6.13 mi W of Stoa Pitk
Cause: Heavy Rain / Tropical System
Property damage: $250,000
Reported by: Tribal Official
Coordinates: 31.816, -112.607

Widespread heavy rain of 2 to 4 inches with isolated amounts of 5 to 8 inches from 9 pm October 1st to 6 am October 2nd resulted in flash flooding, especially in the Gu Vo and Pisinemo Districts. Indian Route 1 flooded between Pio Oik and Ali Chugk (Menagers) and homes were flooded in Ali Chugk. Highway 86 was impassable between MP 80 and 90. Indian Route 21 became flooded and impassable from just north of Pisinemo southward and flood waters topped the Vamori Wash bridge near Santa Cruz (Kom Vo).

Flash Flood, Pima

When: October 2, 7:45 AM MST to 9:45 AM MST
Where: Pima County, 2.01 mi SW of Sil Nakya to 4.39 mi NNE of Anegam
Cause: Heavy Rain / Tropical System
Property damage: $500,000
Reported by: Tribal Official
Coordinates: 32.197, -111.841

Widespread heavy rain of 2 to 4 inches from 11 pm October 1st to 7 am October 2nd resulted in flash flooding, especially in the Gu Achi and Sif Oidak Districts. Indian Route 15 was flooded in spots from south of Santa Rosa to south of Chuichu and Indian Route 42 became flooded and impassable. Homes were flooded several villages including Anegam, San Luis and Kohatk.

Flood, Pima

When: October 2, 9:45 AM MST to October 3, 4:00 AM MST
Where: Pima County, 3.26 mi W of Ali Chuk to 0.87 mi S of Stan Shuatuk
Cause: Heavy Rain / Tropical System
Property damage: $3.0 million
Reported by: Tribal Official
Coordinates: 31.816, -112.605

Widespread heavy rain of 2 to 4 inches with isolated amounts of 5 to 8 inches fell mainly from the evening of October 1st through the morning hours of October 2nd. Runoff from this rainfall continued through the night of the 2nd, with the peak flows generally during the afternoon. Several roads, including Indian Routes 1, 15, 21, 23, 34, and 45, as well as sections of Highway 86 became, or continued to be, impassable on the Tohono O'odham Nation. Several of these roads sustained significant damage. Homes that were flooded earlier in the morning from flash flooding remained flooded or saw water levels rise. Additional homes were flooded by continued runoff during the afternoon. At least 50 homes were flooded in the following districts: Gu Vo (11); Pisinemo (17); Sif Oidak (20); and Hickiwan (2). Several dozen residents were evacuated from their homes. At least 20 earthen dams and levees were damaged along with the tribal water and utility system infrastructure. Runoff nearly filled Menagers Lake, prompting fears of a dam failure and the evacuation of the village of Ali Chugk.

What these are

Flash Flood: Flooding that begins within six hours of the rain that causes it, or of a dam or levee failure or ice jam release, often with deadly speed.
Flood: Flooding that develops more slowly than a flash flood: rivers leaving their banks, or water standing over land for an extended time.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in Arizona in 2018 Every state in 2018 This day in weather history Warnings in Arizona, October 2018

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 2026-10-01. Each event’s entry at NCEI: 788367, 788368, 788373.