Unsettled May pattern to bring showers (and some thunderstorms) to California

Filed in Uncategorized by on May 4, 2017 1,823 Comments

A warm spring so far; wet north, dry south

The last 60 days have been warmer than average across virtually all of California, especially in the south. (WRCC)

The last 60 days have been wetter than average in the north, and much drier than average in the south–especially south of Los Angeles. (WRCC)

The first half of spring has been rather warm across most of California, with temperatures ranging from slightly above average in the north to well above average in the south. The winter storm parade continued across Northern California right through most of April, but late-season precipitation has been rather meager indeed south of the Bay Area. Los Angeles and San Diego have both seen less than 25% of average precipitation over the past two months, though seasonal precipitation remains above average even across Southern California due to the heavy precipitation experienced during the heart of winter.

The season’s first real heatwave occurred this past week across much of California, and in some-cases brought record-breaking high temperatures for early May. It should be no surprise that melting of the prodigious Sierra Nevada snowpack is now well underway, and several rivers in snow-fed watersheds are currently near or above flood stage. Since temperatures are likely to cool substantially in the coming days, serious snowmelt flooding is unlikely in the near term. There is still a tremendous amount of water stored in the snowpack at high elevations, however, so a particularly intense or prolonged heatwave later in May or in early June could still pose a risk of more widespread snowmelt flooding.


Striking, high-amplitude atmospheric pattern developing across N. Hemisphere

An unusually “wavy” atmospheric pattern is currently evolving over the entire Northern Hemisphere, with significant implications for California weather over the next couple of weeks. The development of a pronounced and remarkably well-defined “wavenumber 6” pattern in the mid-latitudes has been a source of substantial interest in the meteorological community in recent days, and for good reason: this kind of atmospheric arrangement is strongly associated with slow-moving and sometimes extreme weather conditions as high-energy “kinks” in the jet stream become “stuck in place” across certain regions. For those of you wondering what the heck “wavenumber 6” even means, the explanation is actually rather straightforward: it literally refers to the number of complete atmospheric “ridge-trough” cycles across a particular latitude band. In other words, we know that a “wavenumber 6” pattern is in place just by counting the number of blue troughs (or red ridges) in the adjacent figure.

High wavenumber atmospheric patterns (n=6-8) have recently been of particular interest in the meteorological community due to their propensity to cause extreme weather events. “Wavy” atmospheric patterns tend to displace cold, dry polar airmasses toward the equator and warm, moist tropical airmasses toward the poles–and the strong thermal contrasts this creates are conducive to the development of alternating zones of heatwaves, cold outbreaks, and flooding rains (and that’s exactly what is expected to occur in the coming days across the Northern Hemisphere). Once in place, these patterns tend to remain “quasi-stationary”–exhibiting a high degree of persistence and moving very slowly (if at all). Recent evidence suggests that the frequency of high-wavenumber patterns has increased during the Northern Hemisphere warm season, which may be linked to rapid Arctic warming and the subsequent loss of sea ice. The Potsdam Institute has created an excellent, broadly accessible video on this topic for those interested in learning more.

Animation of persistent “wavenumber 6” pattern expected to develop over the Northern Hemisphere. (NCEP via tropicaltidbits.com)


Unsettled conditions across much of California; showers (& thunderstorms) likely

Cold mid-level temperatures will generate sufficient instability for thunderstorms across much of California this weekend. (NCEP via tropicaltidbits.com)

What are the California implications of the wavy, sluggish atmospheric pattern discussed above? Well, it appears that a fairly persistent trough will develop near the West Coast over the weekend, potentially persisting in some form for the better part of the next two weeks. This will bring much cooler temperatures to all of California, and occasional precipitation to much of the state at times. The initial event may be the most interesting, as a rather deep and cold low pressure system “cut off” from the main jet stream will develop directly over the California coast on Saturday. While this system will be rather moisture-starved, it will be associated with a fair bit of atmospheric instability by California standards due to some impressively cold mid-level temperatures for early May.

As a result, widespread mountain precipitation is expected statewide–including significant Sierra Nevada snowfall and perhaps even some snow accumulation in the Southern California mountains. Showers and even some significant thunderstorm activity are also pretty likely at lower elevations, especially across Southern California where the coldest air aloft will combine with slightly more moisture and favorable mesoscale dynamics. Some thunderstorms will also be possible across the Central Valley over the weekend. Just about the only spot that may miss out on the convective action entirely may be the Bay Area, where only isolated activity is expected. The good news is that the parts of California which have been quite dry in recent weeks–namely, the southernmost third of the state–will likely see fairly widespread (if modest) precipitation over the weekend. Some stronger thunderstorms may also occur, with heavy downpours and hail possible in isolated spots.

Significant precipitation will be possible across the Sierra Nevada and across most of Southern California. (NCEP via tropicaltidbits.com)

As is often the case with cut-off lows, there is some uncertainty regarding how quickly this system will move away from California. While the GFS moves the system out rather quickly, the ECMWF lingers the low pressure area over Southern California through Tuesday–continuing the chance of showers and thunderstorms. At this point, I’d bet on the slower solution…especially given the sluggish large-scale pattern over North America. By the middle of next week, Southern California will likely dry out, but the quasi-stationary West Coast trough may allow more precipitation to creep back across the northern part of the state. In fact, recent model ensemble runs suggest that an above-average likelihood of precipitation and relatively cool temperatures will continue through at least the middle of May.

Finally: for those who are interested, I will be giving a public talk (and Q&A session) on California weather and climate in Ojai on May 20. Details are available here.

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  • Jim

    Amazing mammatus sunset in New Mexico…..

  • Chris

    At 3:00pm today:
    San Jose 82
    Morgan Hill 96

    At 9:00pm
    San Jose 66
    Morgan Hill 58

    Typical daily summer phenomenon. Great sleeping weather by evening and great swimming weather midday.
    Love our amazingly unique micro-climates!

    • AlTahoe

      Growing up we had a two story house with no A.C. in Morgan Hill. I remember going to sleep with the upstairs being 100F at 10:00pm and with a box fan in the window. By morning the fog would roll in and it would be in the mid 50’s and you would need all of your blankets. The second my twin brother and I moved out my dad put in central A.C.. something something builds character. Lol

      • weathergeek100

        It’s interesting how some inland valleys experience this and others don’t. Obviously, distance from the ocean plays a factor but how mountain ranges and valleys are oriented can make all the difference between a sea breeze funneling through the valley and carrying fog in or the valley heat remaining through the night.

        I say this because I used to have family living in Morgan Hill and when I visited them, I would experience the exact same phenomenon you mentioned. However, I also lived in Walnut Creek and rarely experienced this. The marine layer had a tough time getting into Walnut Creek at the end of a heat wave and when it did, it would take its sweet time. Yet, we were only 15 mins away from Berkeley where the golden gate drenches that city in fog constantly. My conclusion to this was that southerly surges of moisture tend to end Bay Area heat waves very dramatically if you’re able to get that southerly breeze. Morgan Hill probably gets that from Monterey bay. In Walnut Creek, despite being not far from the coast, you have mostly hills and land to the south so you don’t get to feel that air conditioner so suddenly.

        The Bay Area microclimates are beyond fascinating.

        • inclinejj

          I’m 4 miles from the beach in Pacifica. While the beach can be fogged in it can be in the high 70’s or 80’s at my house. Old timers said when Westlake subdivisions were built in Daly City the Fog patterns changed.

        • Henry

          A southerly surge can reach Los Gatos if the marine layer is above the pass in the Santa Cruz Mountains along highway 17, which is 1800 feet above sea level. Occasionally in the summer Los Gatos will cool down before San Jose does. But when there is no southerly surge, or if there is only a weak one or the marine layer is shallow, Los Gatos is typically several degrees warmer than San Jose.

      • When did you bug out? I moved my office to MH in 2010 and moved family here almost two years ago.

  • redlands

    Was 100 again Redlands, Ca – Southern Ca again — the 2nd 100 and above day for 2017

    • Pfirman

      Guessing you will have many more according to the hobgoblin of consistency.

      • Dan the Weatherman

        I am sure he will experience quite a few during the heart of summer, but late May is a little early for any consistent 100 degree weather for that area.

        • redlands

          I remember when I was still in High School in May of 1984 — it got up to 112 in Redlands — that’s hot !!!!

  • matthew

    On my trip down to Reno this afternoon I saw 87 on the car thermometer. Dropped down to 76 by the time I made it back to Gods country in the Shire. Will probably top 80 tomorrow.

  • K?ros

    While the Pfeiffer Canyon Bridge failure on the north end of Big Sur got the press, the real problem lies to the south. Slide at Mud Creek (near Gorda) went off the other day, still is apparently quite active, and it’ll be a very very long time before anyone drives up Highway 1.

    • Nathan

      tunnel time.

  • When I was a kid I remember the heat up then the cool downs in a three to five day span happened often in late spring and summer in San Jose. Seems a little like the pattern again for greater SF Bay Area


  • tomocean

    Couple of shots from 30,000 over Lake Tahoe on a flight to Las Vegas yesterday. Snow pack is still quite impressive. https://uploads.disquscdn.com/images/10d354835d937564e0c7f3bd9252ebe8a06287844ab7c38939c98425ba16b593.jpg https://uploads.disquscdn.com/images/68c2f929fb96d2a8a5068a50aa2483707fc9900f18fa605186fdd0a502e168db.jpg

    It was cloud-free until Nevada, then there was a pretty thick cover. Think this wasn’t far from Tonopah.

  • Chris

    Trivia question:
    When was the last time we had below average SST off the CA coast???

  • Showing SWE difference between May 23, 2011 and May 23, 2017. A few places have more SWE than 2011 now. I’m curious to see these comparisons as far as high elevations as melt season progresses through June. Also graph showing difference in SWE comparing both years which to me suggests there is quite a bit more SWE at high elevations this year to be close to 2011 in Central and Southern Sierra.

  • In response to recent site issues (including downtime, irritating advertisements, and etc.), I’m going to try something new. I’ve set up a Patreon account that will allow blog readers to make voluntary contributions toward the substantial and rapidly growing costs of site hosting/maintenance. Don’t worry; Weather West is not moving toward a subscription model: all content will always remain freely accessible to all. But if even a small subset of readers are able/willing to subsidize a better site experience for everyone, I might be able to substantially reduce advertisements on the site. Thanks, everyone, for your continued support!


    • gray whale


      Heading over to donate. Now I don’t have to feel guilty for using an ad blocker 🙂

  • rob b-Truckee/East Bay

    Note really directly weather related but it looks like former site sponsor Bloomsky just soft launched an Amazon Echo skill for their devices. Seems to work well but is missing the ability to change between devices. But you can ask temp, humidity and wind speed if there’s a storm connected to the device.