Thursday, July 23, 2015

Record Rain in Southern California Just a Drop in the Bucket

At the start of this week there was some unusual weather in southern California - heavy rain. A low pressure system off the coast that used to be Hurricane Dolores and the southwestern monsoon flow combined to produce showers and thunderstorms from western Arizona west through southern California.

Visible satellite image of the southwestern U.S. at 4:00 p.m. PDT July 19. Green arrows show direction of winds.

Dolores attained hurricane status on July 13 off the central Mexican coast, then moved north northeast parallel to Baja California. It was downgraded to a tropical storm on Friday, July 17. By Sunday, July 19 it had weakened even further and was a post-tropical cyclone.  The circulation was well-defined, and the low pumped moisture from the Pacific into northern Mexico and southern California. This augmented the moist flow of air associated with the southwestern monsoon, and the result was record-breaking July rainfall for many locations in southern California. San Diego has received 1.71 inches of rain in July so far which breaks the old 150 year-old record of 1.29 inches set in 1865.

Some rainfall records in southern California. From NWS San Diego.

One of our CoCoRaHS observers in San Bernadino County received 3.75 inches of rain over two days (by far the highest amount reported so far that I've seen), with a total for the month of 3.83 inches. Seven observers have measured more than 3.00 inches this month, and another several dozen observers have measured an inch of more. Not bad for an area where the normal July rainfall is just a few hundredths of an inch.  Downtown Los Angeles has received 0.38 inches of rain this month. The normal is 0.01 inch.

24-hour CoCoRaHS totals the morning of July 20.

Total rainfall for the period from ~7:00 a.m. PDT on July 18 to &:00 a.m. PDT on July 20 in southern California.

As you might expect, that amount of rain caused problems. Many roads were flooded and washed out. Power was knocked out in some communities. A bridge on Interstate 10 between Coachella, CA and the Arizona state line washed out on the eastbound side and was compromised on the westbound side by flash flooding. The I-10 eastbound lanes are closed indefinitely until the bridge is repaired. The westbound lanes were opened again on Tuesday (July 21). About 27,000 vehicles normal travel this section of I-10 every day.

Firefighters stabilize a pickup truck that drove into washed out I-10 bridge. The driver was rescued.
Photo credit: CalFire Riverside

Map showing location of damaged bridge on I-10 and section closed (yellow).

On Sunday (July 19) the baseball game between the San Diego Padres and the Colorado Rockies in San Diego was first in a rain delay (6th time in history) and then eventually called off. It was only the second time in Petco Park history (since 2004) that a game has been rained out. The last rain-out occurred in 2006.

The rain did little to relieve drought conditions. The rain, while significant, is just a very small drop in a very large bucket. With evaporation rates on the order of 0.20 to 0.25 inch per day what didn't run off will soon be back in the atmosphere. The water balance chart from CoCoRaHS station CA-RV-21 in Riverside, CA shows just how little dent the recent rain made in the water deficit since May 1.

Drought conditions over the western U.S. and particularly California remain unchanged. They are likely to remain unchanged until the wet season begins in the winter, and then only if precipitation occurs regularly and is well above normal.

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