Arizona is bracing for two rounds of heavy rain in three days, starting Friday, October 30, and forecasters are warning that the danger is higher than the rainfall totals alone suggest: the storms will fall on ground still saturated from last week's destructive flooding, so water could race quickly through washes, creeks and low-water crossings from metro Phoenix to the Mogollon Rim.
Source: National Weather Service via Weatherboy
The National Weather Service (NWS) office in Phoenix said early Thursday that wet weather is on its way with two weather events from Friday through Sunday, both of which could produce heavy rainfall. Flood watches are already in place for Friday, and the Phoenix office said a more expansive flood watch will likely be needed for the second, stronger system expected Saturday night into Sunday.
Round one: Friday storms over Phoenix and central Arizona
The Friday flood watch covers the Phoenix metro area, rural Maricopa County and western Pinal County from Friday morning through Friday afternoon. A separate watch for Superior, Globe and Miami, the Tonto Basin, San Carlos, the Mazatzal and Pinal-Superstition mountains and southeast Gila County runs from Friday afternoon through Friday evening. The watch says any strong thunderstorm could produce rainfall rates as high as 1 to 2 inches per hour, with localized totals of 1 to 2 inches expected.
Farther north, the NWS office in Flagstaff issued a flood watch from 10 a.m. to 10 p.m. MST Friday for Yavapai County and northern Gila County, including Oak Creek and Sycamore canyons, citing excessive rainfall over saturated soils and waterways that flooded as recently as last week. Forecasters there expect general totals of half an inch to an inch, with up to 2 inches possible in thunderstorms, and warn that flash flooding is possible in creeks, dry washes, swimming holes and burned areas.
The federal Weather Prediction Center placed parts of central and southern Arizona in a slight risk of excessive rainfall - level 2 of 4 - for Friday. The center said moisture in the atmosphere over the southern half of the state would rise to 2 to 3 standard deviations above normal, and high-resolution models showed a 30% to 60% chance that more than 2 inches of rain would fall within six hours in some areas. Given the sensitive terrain and moist soils, the center said the setup will likely produce scattered instances of flash flooding. Showers and storms could develop around the Phoenix metro as early as 4 a.m. Friday and become more widespread by early afternoon, and a few could turn severe or train repeatedly over the same areas.
Round two: an atmospheric river arrives Saturday night
After a brief lull on Saturday, a stronger system is expected Saturday night into Sunday. The Phoenix weather service office described it as one of the stronger atmospheric rivers that can affect the Desert Southwest - a long plume of moisture streaming in from the Pacific - and said it is shaping up to be a classic El Nino season event of the kind that would normally occur from late December through February. Moisture levels could reach record or near-record values for this time of year, at 300% to 350% of normal.
Rain rates on Sunday are expected to be modest, but forecasters said the long duration will make up for it: totals are forecast to average 1 to 1.5 inches in Phoenix and 2 to 3 inches over higher terrain north and east of the city, with the National Blend of Models showing a 25% chance of more than 2 inches in Phoenix and as much as 3 to 4 inches in the high terrain. The Flagstaff office forecast 1 to 3 inches from early Sunday through early Monday, with a cold front late Sunday that could also bring severe weather and lower snow levels. Even at the average forecast totals, the Phoenix office said, most of Sunday's rain will run off because it is falling on already saturated soils, washes could fill or overflow, and larger creeks and rivers could flood Sunday into Monday.
The same system is part of a wider Halloween-weekend weather pattern across the Southwest and Southern California. Bloomberg reported that coastal flood warnings and advisories were in effect through Friday around San Diego and Los Angeles, with northwest winds pushing tides nearly 2 feet above normal, helped by the full moon and Kelvin waves from the strengthening El Nino, and flood watches were also issued for central Arizona including Phoenix through Friday. Los Angeles County could receive 0.5 to 1.5 inches of rain through the weekend, with the greatest chance downtown on Saturday night, while The Weather Channel said the areas of greatest concern include the north and east sides of the Phoenix metro, the Four Corners and San Diego County's foothills and high country, with a first round of rain also bringing local flash flooding risk to western Texas including Lubbock and Midland.
Source: Insurance Journal (Bloomberg)
Why the soaked ground matters: last week's flooding
The new rain follows severe storms in the week of October 19 that flooded communities across northern and eastern Arizona. The Hopi Tribe declared a state of emergency on October 22 after flash flooding damaged property across the reservation; the Moenkopi Wash gauge reached 33.92 feet, nearly 4.9 feet above major flood stage, and at least eight families were displaced in Moenkopi near Tuba City. On the White Mountain Apache Reservation, about 50 people were evacuated along Cibecue Creek as the creek rose to bridge level, and on the Navajo Nation crews rescued 30 people from flooded homes in Dennehotso. In the eastern Arizona mining town of Clifton, floodwaters and rockslides damaged homes, vehicles and infrastructure, and Greenlee County Sheriff Eric Ellison said Ward Canyon Road was washed out again about two hours after crews reopened it.
The weather service urges residents to monitor forecasts and be ready to act if flash flood warnings are issued, and warns that low-water crossings may be flooded: 'Turn around, don't drown when encountering flooded roads. Most flood deaths occur in vehicles.' The NWS also told parents in Southern California to have backup plans ready for Halloween trick-or-treating, and The Weather Channel's Jonathan Erdman said hikers with weekend plans in Arizona parks should consider cancelling because burn-scar ground is especially prone to fast-moving debris flows.
What happens next
Friday's watches run through Friday evening, and forecasters expect a more expansive flood watch for the Saturday-night-to-Sunday atmospheric river. Creeks and rivers could keep rising Sunday into Monday even after the rain eases, and high pressure is expected to bring warmer, drier weather from Tuesday onward, the Flagstaff office said. ABC15 in Phoenix forecast 1 to 2 inches of rain in the Valley from Friday through Sunday, with 2 to 3 inches or more in parts of the high country. For drivers - including the many Punjabi truckers who cross Arizona on the I-10 through Phoenix and the I-40 through Flagstaff - the practical risk points are flooded low-water crossings, washes and desert roads rather than the interstates themselves, and the weather service advice is to check the latest NWS Phoenix and Flagstaff updates before setting out and never drive through flooded roads.