[californiadisasters] The Monsoon Gets Going in Early July 2011

The Monsoon Gets Going in Early July

Upper level high pressure shifted into the four-corners region in early July. This pattern, also known as the monsoon or seasonal wind shift aloft, allowed subtropical moisture to
move into southern California. The moisture was anomalously high due to the remnants of Tropical Storm Arlene. Arlene, the first Atlantic tropical cyclone in 2011, made landfall
south of Brownsville Texas in late June and moisture spread out across Mexico.

This moisture arrived across Southern California on July 3 and 4. Showers occurred with the leading moisture surge, but mainly stayed well east of our region. With hot air already in place across inland areas, the increased moisture led to instability on July 4.

Deep easterly flow (as opposed to the normal westerly flow) focused a few thunderstorms west of the San Diego and Riverside Mountains by early afternoon. Soon afterward a line
of thunderstorms formed on the natural convergence of low-level air often found near Lake Elsinore. Residents reported strong wind gusts, cloud to ground lightning, small hail
and a period of heavy rain. Alex Tardy, Warning Coordination Meteorologist, visited the area and found signs of heavy runoff on streets and small broken branches. Additional
thunderstorms formed over the San Bernardino Mountains before the last storms of the day hit the high deserts west of Hesperia.

On Tuesday July 5 the atmosphere was ripe for strong slow moving storms since moisture increased further and the storm steering wind flow decreased. Thunderstorms fired up in the mountains from San Diego County to San Bernardino leaving locally heavy rainfall and brief fire starts from lightning. Similar to Monday, Tuesday had isolated instances of very heavy rain, localized runoff and small hail from thunderstorms. Strong storms also occurred near the Mexico border and moved into extreme eastern San Diego County. There were several helpful weather spotter reports from the mountains.

The peak of this monsoon event turned out to be on Wednesday July 6. This day consisted of the deepest moisture (precipitable water values around two inches on satellite and weather balloons) and very light southerly wind flow aloft. This was also the day that a strong thunderstorm induced dust storm (Haboob) hit Phoenix early in the morning and was seen on the satellite imagery moving across San Bernardino and Riverside County deserts Wednesday afternoon. Separate from this large circulation (named Mesoscale Convective Vortex), strong thunderstorms quickly formed across the San Diego County mountains and spread northward across most of the Riverside and San Bernardino Mountains. Intense rainfall resulted in flash flooding (defined as life and property threatening) in the towns of Phelan and Warner Springs, with less intense flooding in Big Bear City. A spotter report of two inches was recorded in Big Bear City and about 1.5 inches in Phelan. Highways 18 and 138 were closed for almost two hours as one to two feet of water stranded vehicles at intersections. Flooding occurred in poor drainage areas around Big Bear, and small creeks in Warner Springs left their banks and impacted highway 79. Thunderstorms again developed near Perris and slowly moved northward across producing numerous lightning strikes and wind gusts over 40 mph. The lightning and wind resulted in numerous power outages across the Inland Empire. Three hours of rainfall occurred in the San Gorgonio Wilderness where Doppler Radar estimated nearly four inches. The runoff from this intense rainfall traveled down the Whitewater River, reaching Windy Point near Palm Springs several hours later as a wall of mud and debris in the river channel. These type of floods can be particularly dangerous since they can occur away from the heavy rainfall and long after rainfall has ended, making a surprise rise of water and debris. Strong thunderstorms also developed and moved into Apple Valley and across Barstow during the early evening.

The weather pattern changed little on Thursday July 7, with a shift to weak westerly wind flow aloft and only slightly less moisture. Strong thunderstorms formed near the crest of the mountains (light wind flow allows low level merging of air from daily upslope to focus storms), and in particular this day saw persistent thunderstorms with frequent lightning and heavy rain on Onyx Peak. Localized significant runoff occurred.

On Friday July 8 strong thunderstorms occurred over the eastern San Bernardino Mountains. Friday marked the fourth consecutive day of heavy rain in Big Bear City with a total of four inches. The last group of thunderstorms occurred Saturday afternoon and early evening on July 9 across the high deserts near I-40 and south to Onyx Peak.

The peak water vapor observed in the atmosphere was on the order of the 99th percentile and only shy of the record by less than 0.15 inches (record is 2.15 inches for Miramar upper air weather balloon site). Meteorologists use water vapor as a standard measurement of how much water is in the entire column of the atmosphere (not just ground level) if it were to be condensed out.

Source: http://www.wrh.noaa.gov/sgx/newsletter/current-newsletter.pdf

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