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On July 19, 2015 there was record rainfall over the Los Angeles and San Diego areas. This is amidst a historic drought and the rain is needed.

This article: http://www.weather.com/storms/hurricane/news/southwest-flood-threat-july-2015-tropical-storm-hurricane-dolores

states that

Widespread showers and thunderstorms across the Desert Southwest are being fueled by moisture from former Hurricane Dolores.

and

Los Angeles and San Diego set all-time rainfall records for the month of July on Saturday.

and

National Weather Service meteorologist called [it] "super historic."

What is the significance of the "super historic" rainfall for July in California (July 19, 2015) in terms of climate change and water reservoirs used for municipalities? In other words, what is the likelihood that climate change could bring more of these types of rain events in the future? Related, what is the relative impact on water reservoirs and how much relief from the drought does such an event provide? Does California have a long-term plan to capture more of this type of flash-flood rainfall in the future?

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  • $\begingroup$ El Nino isn't it ? You are having above normal SSTs along the Pacific coast ? $\endgroup$
    – user1066
    Jul 20, 2015 at 6:15
  • $\begingroup$ @gansub: Even in El Nino years, it's not really normal to get much beyond scattered thunderstorms in much of the Western US. There's also been much more rain than usual in the northern Sierra Nevada & neighboring Great Basin areas. $\endgroup$
    – jamesqf
    Jul 20, 2015 at 23:07
  • $\begingroup$ @jamesqf - is that what the models are concluding ? $\endgroup$
    – user1066
    Jul 21, 2015 at 1:41
  • $\begingroup$ @gansub: No, it's observation from someone who has lived here for 40+ years, and paid fairly close attention to weather patterns. Also from talking to older people, like my neighbor (who'll be 101 in September). $\endgroup$
    – jamesqf
    Jul 21, 2015 at 17:50
  • $\begingroup$ @jamesqf - take a look at how strong this El Nino is (including along the California coast)- nnvl.noaa.gov/MediaDetail2.php?MediaID=1726&MediaTypeID=1 $\endgroup$
    – user1066
    Jul 22, 2015 at 2:28

1 Answer 1

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This event is "super historic" only because of its proximity to the coast. It is fairly common to get flash flooding in the mountains and deserts of Socal at least a few times each summer, due to the Southwest Monsoon. This event was different in that a dying tropical system moving north off of the coast of Mexico interacted with a trough off of the Northern California coast. The combination of these two essentially funneled tropical moisture north directly into the coastal regions of Southern California.

Linking single events to long term global change is virtually impossible to do reliably, so I won't speculate. However one could argue that rising sea surface temperatures could allow tropical systems to stay more organized near Southern California, and increase the probability of events like this. But again, it is never a good idea to try and link a localized extreme event with a global phenomenon.

In terms of drought relief and reservoir storage these rains have virtually zero impact. They impacted the southern third of California, while the majority of water storage is in the central and northern part of the state along the Sierras. The rain came down pretty quick and didn't soak in to any of the soils, so vegetation will still be just as dry as before the rain. California needs heavy snows in the Sierras for any serious drought relief. Although these rains have given many people a false sense of security.

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  • $\begingroup$ @pythonisjam - can you point me the exact height and location of the trough off the Northern California coast? It is not clear to me -earth.nullschool.net/#2015/07/19/0900Z/wind/isobaric/850hPa/…. Is it the one at 26 N ? $\endgroup$
    – user1066
    Jul 21, 2015 at 5:01
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    $\begingroup$ Its more visible if you look at 500hPa on the 18th. Have a look at this link: earth.nullschool.net/#2015/07/18/0600Z/wind/isobaric/500hPa/… the trough is the weak counter-clockwise circulation off the socal coast around 32 N, the tropical storm is the stronger circulation to the south around 20 N $\endgroup$
    – pythonismyjam
    Jul 21, 2015 at 5:32
  • $\begingroup$ thanks. much more clear. Can you add some location details to your answer for somebody wanting to investigate this further ? $\endgroup$
    – user1066
    Jul 21, 2015 at 5:43

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