Tuesday, September 9, 2014

Soggy Times in the Desert Southwest

It's not often the rainfall in the Desert Southwest will make the news twice in less than a month's time, but this is one of those times. Late Sunday night into early Monday morning, September 8 thunderstorms feeding on a plume of moisture associated with what remained of Hurricane Norbert flushed out over the Phoenix metropolitan area. Record rainfall amounts occurred in a span of only 8 hours or so in an area not accustomed to multi-inch rainfalls.

Two clusters of storms merged. One moved from southwest to northeast west of Phoenix, and the second developed southeast of Phoenix and moved northwest, where it merged with the first cluster.

Reflectivity image for 11:47 p.m. MST September 7 (left) and 2:03 a.m. MST September 8 showing the two clusters of storms approaching the Phoenix area.

Water vapor satellite image for 3:45 a.m. MST. The complex of storms over southern Arizona can be readily seen.

Radar reflectivity image at 3:20 a.m. MST September 8.

The rain in the Phoenix area continued through mid-morning before tapering off. Rainfall amounts were unusually heavy and widespread.

Phoenix Sky Harbor Airport set new records for the highest daily rainfall for September 8 and the highest rainfall in a calendar day. The previous records had been in place 75 years or more.

Then, from late morning through early afternoon Monday heavy rain fell in the Tucson area. Tuscon received 1.87 inches of rain, a new record for September 8. The old record was 0.94 inch in 1919.  More information on the Phoenix rain can be found on this NWS Phoenix web page.

CoCoRaHS map for the Tucson area for the 24-hour period ending the morning of September 9.

About the same time the rain was tapering off in the Phoenix area thunderstorms erupted over southern Nevada northeast of Las Vegas in northeast Clark County. More than 4 inches of rain was recorded in some locations. Flooding damaged some homes in the small town of Moapa 50 miles northeast Las Vegas just off Interstate 15, forced the evacuation of an Indian reservation and sent torrents of water across Interstate 15. Much of this rain fell in only a two-hour period, pushing the Virgin River to near-flood stage.

Multi-sensor precipitation estimate for southern Nevada for the 24-hour period ending the morning of September 9.
Source:  NOAA Advanced Hydrologic Prediction Service

Here is a video showing some of the flooding on Interstate 15 on Monday.

The heavy rain story isn't over yet. Heavy rain is falling tonight along a stationary front draped across the Midwest, More than 3 inches of rain has already been reported in Iowa and northern Missouri, and flash flood warnings and watches are in effect for a large portion of the Midwest.

Watches, warnings, and advisories in effect as of 11:45 p.m. CDT September 8.

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