April 05, 2018

One of the Greatest West Coast Atmospheric Rivers on Record

The more I look at the strong atmospheric river that will strike California tomorrow, the more amazing it becomes.   In some, but not all, respects it is one of the more extreme atmospheric rivers on record.

Today's water vapor satellite imagery shows a huge plume of moisture stretching from the tropics towards California (see below)

And the values of vertically integrated water vapor (total water vapor in a vertical column of air) forecast for tomorrow (Friday) at 2 PM are immense, with the blue colors indicating values of 40 mm (1.6 inches) or more.  That means if one condensed out all the moisture in a vertical column, 1.6 inches of rain would result.  That is a very large amount.

This intense plume of water vapor will slowly move down the California coast, reaching southern CA on Saturday morning (below is the forecast map for 8 AM Saturday).

The latest forecasts promise major rain (and considering the warmth of this atmospheric river, that is what it will be, even over high terrain).  For the 72-h ending 5 AM Sunday, the total amounts reach 5-10 inches over the coastal mountains and Sierra Nevada of central and northern CA--and we also do "well" in the precipitation arena.

But for me, the most extraordinary part of this event is how unusual it will be, so let me demonstrate that to you.  Here is a plot of the forecast integrated water amount (also known as Total Precipitable Water) for San Francisco from the National Weather Service ensemble system (many forecasts using the GFS model).  The amounts get to 1.65 inches tomorrow and Saturday--and this is a confident forecast (all the ensemble members are going for it).  The black line is the ensemble average...a very good forecast to use.

We can get a feel for how unusual this is by looking at the climatology of these precipitable water values using the nearby radiosonde (balloon-launched weather observations) at Oakland, across the bay from San Francisco (see below).  The averages for each day of the year are shown in black and the daily records are indicated by the upper red lines that gyrate up and down.  The forecast value for tomorrow is shown by the black dashed line.

This is amazing: the forecast value tomorrow is higher than ANY DAY for the entire record at Oakland (about 70 years) from the period October 1 through July 1.  So tomorrow will not only beat the daily record, but the record for ANY day during the wet season of year in California.

There have been some higher values during the summer, but that reflects more moisture in the warmer summer atmosphere...moisture that doesn't result in heavy precipitation, mainly because the incoming flow is weak that time of the year.  To prove this to you, here are the extreme precipitation events at Yosemite National Park for each day of the year.  Extreme precipitation is wimpy during the dry summer.

 So to repeat...this is probably the strongest atmospheric river, in terms of water vapor content, to have ever hit central California when it really counted (during the non-summer portion of the year).    Just extraordinary.

But as strong as this atmospheric river is in terms of moisture content, it will not be the wettest on record (in terms of accumulated precipitation).  Why?  Because the incoming flow has been much stronger in some past events.   And what really counts in terms of producing precipitation on the California mountains is both moisture content AND the rate at which air is forced to rise on the terrain.  Stronger incoming winds promote more rising, upward motion, and thus precipitation.

What about the strong oceanic cyclone approaching the Northwest on Saturday?

  It is still coming.

Here is the latest sea level pressure forecast for 2 PM Saturday.  A 980 hPa low off our coast with a very large pressure gradient south of it.  Very strong winds offshore and along the WA and Oregon coastlines.

The low will slowly drift northwest of Seattle and the latest UW wind gust forecasts over Seattle for 5 PM Saturday shows 50-60 mph gusts over portions of Seattle, with 40 mph+ gusts over much of the city.  That will produce some power outages, with some mitigation by the fact that we are at the end of the winter, with a number of storms already clearing the weak stuff.  And the trees are not leafed out yet.

Anyway, the entire West Coast will be in for very active weather the next 72 hour.
Announcement:  The Northwest Weather Workshop is on April 27-28

The NW Weather Workshop is the big annual meeting for those interested in Northwest meteorology.  This year we will have a major session on the meteorology of NW wildfires and others on other aspects of our regional weather.  The gathering takes place at the NOAA facility in Seattle.  To view the agenda and to register, go to the meeting website.  The workshop is open to everyone, but registration is required.


  1. Cliff I wouldn’t be surprised if there were several reports of funnel clouds, waterspouts, or weak tornados this weekend. While this isn’t a severe weather setup, just the amount of moisture, instability, and wind shear. And given we’re getting a little later in the spring, it could happen.

    And the oragraphic enhancement of precipitation into our Casacdes behind the front looks impressive. Feet of snow above that 850mb line. Finally looking for a solid bet of precipitation here on the leeward side of the Cascades with impressive upslope 850mb winds prior to the front. Hoping for .5 inches Friday night. Our excitement standards are a lot lower over here lol.

  2. Didn't we have a similar, off the charts, storm last year. Not to mention numerous unprecedented weather events the last few years?

  3. We had another unusual storm last April...but those are nothing compared to the atmospheric river...

  4. This is all fine and dandy, but when are we going to finally warmer (and dry) up? 😆

  5. But ... but ... wait! California is in a permanent drought. The New York Times said so, and therefore it must be true! Now: How long will it be until All The Usual Suspects blame this on global warming and tell us that California will face permanent flooding?


  6. You want a good April rain storm, April 4, 1991: 2.5 - 4 inches in 24 hours in the Seattle area.

  7. I think Placeholder needs to learn a bit more about what climate instability means, and that weather is neither binary nor zero sum. Unfortunately in my area we've learned that being in a drought doesn't negate surprise 'pop-up' storms which wreak havoc but don't alleviate the underlying drought conditions.

  8. Ah, thanks for the clarification Cliff. Now if CA would divert some of that water to settling ponds to recharge the aquifers like they do in Arizona (good Sci Am article on this) they might have a shot at getting things back to normal.

  9. I'm late to the conversation, but having looked at the latest satellite images, and having read the forecast discussion, it's seeming less likely that the deep low will approach us enough in the right location to cause significant wind, assuming no changes to the forecast.

  10. How about instead, Joanna B, we talk about how the media can hurt science and damage their own credibility along the way.

    This isn't a "surprise 'pop-up' storm". These have been going on in California long before we got there. So has drought. It is, in fact, their climate. No amount of revisionist history is going to change that.

    The fact - that is quantitative, verifiable fact - is that the last 2 years have objectively negated the "California permanent drought" theme. 6 inches of precip in the Sierras from this storm alone... overflowing reservoirs... multiple posts right here by Cliff saying it is over...

    That article -and the thousands of others like it - hurt science. The concern here should not be directed at the person posting it; it should be directed at the publication that printed it. It needs to stop.

  11. I think Joanna may want to spend a little more time reading Cliff's posts on the subject before casting aspersions.

  12. Why is it the NWS puts up a special weather statement a couple days ago for a potential wind situation on Saturday...then pulls it today without explanation. They do this all the time. You are left wondering if another post is coming or if the forecast has changed. They do this all the time. It would be nice to see a statement that explains “the low is taking a different track and this is what to expect”. Just a thought about communication.

  13. Just to be perfectly clear, the NYT article cited by Placeslipper is about the potential for "permanent water restrictions" which not surprisingly has transmogrified magically in this thread of discussion into "permanent drought".

    Which no matter how deep you mine, is only a term perpetuated by fringe alarmists, not the majority of mainstream media.

    Now back to tossing horshoes

  14. JoannaB, snowso12, Eric Blair

    Instead of demeaning each other on Cliff's site, in addition to Cliff's obvious contributions to the discussion of drought, why not just post what those likely to know about drought in California say about drought in California?


  15. John - that topic has been endlessly discussed here, please refer to prior posts on the subject during the CA drought a few years back. CA is, and has always been, subject to extreme droughts throughout it's geological history. Why you feel the need to post yet another study I have no idea.

  16. Ah, Bruce Kay. It takes a special kind to argue that an article headlined, "California Braces for Unending Drought" isn't about an unending drought in California.

    Bruce, if you haven't yet run for office in Seattle, I suggest that you do so. You will fit right in.

  17. By the way, California has a cyclical drought-drench climate. This has not changed. The recent drought was severe, just as a bunch of others have been since detailed records began to be kept. Their droughts are followed by drench cycles, which the state is now in.

    This is why CA built its extensive water system. In recent years, the pseudo-"environmentalists" who run the state have let that system deteriorate, and have adamantly refused to even maintain it adequately much less expand it to accommodate the state's growing population.

    It's all part of a global warming cult whose scientific basis never existed, and whose "proof" statements are vanishing as the models are re-examined and found to be based on statistical and observational errors. Being "progressives," they cannot admit their errors, and instead double down on specious claims, while becoming angrier and angrier with those who point out that their emporers and priests have no clothes.

  18. Thats the headline Place Slipper. I think by now we all should know that most of the issues discussed here require reading beyond the headlines, which is why I said "Article", not "headline".

    The closest the article comes to corroborating your headline generated judgement is with Governor Brown's quote:

    "“But now we know that drought is becoming a regular occurrence and water conservation must be a part of our everyday life.”

    Note two things which generally summarize the bulk of the article

    1) "But now we know that drought is becoming a regular occurrence". About as true a statement as any forecast can be. Drought is expected to become regular ( not permanent, regular)

    2) "water conservation must be a part of our everyday life". Suggesting the potential for permanent water restrictions, due not just to drought but very many things.

    The article is not at all about "permanent droughts".

    I repeat: As exquisitely triggerable as your dopamine receptors are to mere headlines, only Trump fails to read deeper so I suggest you not follow his lead lest we spiral as he does down rabbit holes of delusion.

  19. Placeholder do you feel that we have no effect on the environment? Or that the environment has no effect on the weather? both?

    I don't understand why you call out "environmentalists" or "progressives" as if they're acting with no reason. Is it not ok to be concerned and "progressive" towards a goal to protect the environment? I assume you feel they are just acting concerned, and really just trying to grab some cash? Which clearly many do. This doesn't make the problems (uncontrolled - pollution, mining, agriculture) any less real.

    I only see a few groups here..
    people concerned with the effects of human activity and trying to change things with very little resources.
    People with too much to loose if the environment was to be protected.
    People who think everything is fine
    People who do not care.

    It seems to me that anyone could easily see that we are headed in a bad direction. I don't understand your opposition.

    I may have misread your post? just seems that you are very upset with these "progressive environmentalists".

  20. Misquoted - "progressives", and "environmentalists"

  21. I put "progressives" and "environmentalists" in quotes because, in my opinion, they are phonies and liars. They can't tell the truth about anything, not even something as simple as a New York Times article about California's "unending drought."

    And then you wonder why most people pay nothing but lip service, except for the voters of Seattle, who are complete suckers.


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

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