June 22, 2013

The Upcoming Northern Californa Summer Deluge

A highly unusual summer deluge will soon hit northern California.  June is typically quite dry over the Golden State (it isn't called golden for nothing!).  A climatological (1980-2010) precipitation map (see below) tells the story... from the Bay area south and west the monthly totals are .1 to .4 inches and even the northern Sierra Mts. only typically receive about an inch.

But take a look what the models are now consistently forecasting for the 48 h ending
5 AM on Wednesday:   1-5 inches over the northern Sierra (the pink and black colors) and .5 to 1 inches for much of the remainder of northern CA.   We won't escape the rain in the Pacific Northwest, but California will be hit far harder than we.

The origin of this unusual rain is an extraordinary summer atmospheric river, one that is stronger and extends farther west than most of us can remember seeing in decades during this time of the year.  (an atmospheric river is a long, narrow current of high moisture values originating in the tropics or subtropics).  The UW WRF model illustrates the forecast atmospheric river with a plot of the total water vapor in a column for Monday evening.  The dark blues are very high values.  Really quite unusual for this time of the year.

Even today (Saturday) we can see the atmospheric river setting up, as viewed by weather satellites that can sense the amount of water vapor in a vertical column (see image).  You can observe an initial system (green colors of moderate water vapor amounts) that is approaching the NW (yes, we will get some rain tomorrow), but look at the LONG stream of moisture coming off of southeast Asia (highest values are red)!  That atmospheric river is heading for northern California.

The wet pattern on Monday and Tuesday is associated with a highly perturbed weather pattern, with a deep low forming over the eastern Pacific (see graphic for 5 AM Tuesday of the upper level flow pattern).  The flow (which is parallel to the solid lines) will be strong and from the west/southwest...and headed into northern California.

Good news.  The cool, wet pattern should be a memory by next weekend--the models are suggesting a huge high pressure ridge developing over the western U.S. during the second half of the week.


  1. This was explained by Forecaster Weagle at PQR as a strong Madden-Julian Oscillation, perhaps the strongest in 30 years.

    Can you explain the MJO to we dweebs and perhaps refer us to some graphics?

  2. Cliff: Thanks. My ecology students love atmospheric rivers, especially the dynamics as shown on MIMIC; beats the hell out of those 2D drawings of Hadley cells in text books.

  3. This is fascinating, Cliff. Is something unusual happening in the Pacific tropics (Taiwan, etc.) that's driving that huge atmospheric river, or is it a result of Pacific Ocean conditions, or just random? Do you think this is a one-shot deal, or will it happen again this summer? I'm so glad that No. Cal. will get some rain... it will surely help their drinking water and wildfire situations, and their crops. Will it also help locations to the east at all?

  4. This brings to mind a comment several years ago when some councilman in California said they wanted to dig a canal from here to there so they could have some northwest water for gross overconsumption in a desert ( sorry, that last part was my own editorializing). We said, hey, no way! If you want it you have to get it the way we do, with rain! I believe they discovered the Siskiyous shortly thereafter and dropped the plan.
    Happy rain, California!

  5. Clif,
    A friend recently sent me a link to to a blogpost, "The Jet Stream Is Broken" dailykos.com/story/2013/06/26/1218998/-The-Jet-Stream-is-Broken, which contains many links to articles and one of the comments includes a link to a scientific paper on the subject.

    What's your thinking, and the department's, on the subject?


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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