Wednesday, August 20, 2014

Deluge in the Valley of the Sun

Last week exceptional rains occurred in the central and eastern U.S.  This week the weather eye turns to, of all places, the Desert Southwest. For most of the year "rain" and Arizona" are two words rarely heard in the same sentence.  However, the summer monsoon is in full swing, and on Tuesday the Phoenix area received some impressive rain amounts which caused major flooding on some creeks and rivers as well as flooding of many roads, including portions of Interstate 17.

The thunderstorms were set off by an approaching trough over the western U.S. With ample moisture available from the monsoonal flow and southwesterly winds forcing the air up over higher terrain, thunderstorms quickly developed and intensified.

The first round of storms occurred early Tuesday morning near Carefree and Cave Creek, located at the northeast corner of the metro area.
Radar reflectivity for 6:02 a.m. PST Tuesday, August 19. Storms are intensifying west and north of Phoenix.

By 7:00 a.m. very heavy rain was falling northeast of Phoenix over the higher terrain of Maricopa County.

Radar reflectivity for 7:02 a.m. PST Tuesday, August 19.  Heavy rain is falling east of I-17 and north of Phoenix.

The CoCoRaHS observer at Cave Creek 2.9 SSW (AZ-MR-17) reported 0.75 inch of rain in 15 minutes (that's a rate of 3 inches per hour) as did the observer at Carefree 2.1 E (AZ-MR-31). Both stations ended up with about 2 inches of rain. However, the heaviest rain was located north of our CoCoRaHS observers.

Initially floodwaters and debris covered the northbound lanes of Interstate 17, but by 10:00 am Interstate 17 four miles north of New River was totally under water and many cars were stranded, necessitating rescues.

Water covers the northbound lanes of I-17 Tuesday morning.
Credit: ABC15 Facebook page

A second round of heavy storms moved over the same area northeast of Phoenix during the early afternoon, and additional thunderstorms moved through the region though the afternoon and evening. However, most of the rain fell in these first two rounds.

Radar reflectivity at 12:02 p.m. PST on Tuesday, August 19.
A second round of heavy storms is moving through the Phoenix area.

Measured ainfall totals for this event reached 5.51 inches at Tower Mountain north of Phoenix, but there was a wide swath of  4-inch plus rainfall that extended to the southeast from there. The highest amount reported by a CoCoRaHS observer was 2.86 inches at Cave Creek 2.4 N (AZ-MR-163).

24-hour rainfall map for period ending the morning of August 20.
Area outlined in orange encloses rainfall amounts of 4 inches or more.
Base map credit: Flood Control District of Maricopa County

The National Weather Service Office in Phoenix has a web page with more information on this event, including photos of the flooding.

1 comment:

  1. For quite a few of us in the Valley, the rainfall started before our 7:00am reading of our CoCoRaHS data. I'm not sure that this article takes that into account. I only reported 1.14" for my 8/20 report. However, I also reported .88" for my 8/19 report that fell in the hour or so before 7:00am. At one point during that period, I received .5" in 5 minutes (using my weather station data). So, the total rainfall for the storm system was 2.02". I think you'll find much the same data from other CoCoRaHS reports.