Subtropical moisture plume bringing heavy Sierra Nevada precip; widespread active weather this week

Filed in Uncategorized by on December 10, 2016 5,057 Comments

Ongoing atmospheric river event bringing heavy rainfall in Sierra Nevada

Precipitation accumulations could be very heavy in the central/northern Sierra, and significant (but not extreme) elsewhere. (NCEP via tropicatidbits.com)

I’ll cut to the chase in this post, since it’s looking like an active pattern may bring some high-impact weather to much of California over the next 7-10 days. The first of these events is already underway–an abundant plume of subtropical moisture is streaming northeastward over the far eastern Pacific and bringing copious rainfall to the western slopes of the Sierra Nevada. Note that I did specifically say “rainfall” as opposed to snowfall–snow levels are well above 7000 feet in most instances (not surprising, given the warm, low latitude origins of the airmass over California). Heavy rainfall is melting some of the lower elevation snowpack that has developed in the 5000-7000 elevation band–causing heavy runoff into Sierra Nevada streams and rivers. The Truckee River is already expected to crest above flood stage this weekend (and may do so again later this week–see below).

Some heavier rainfall has also been falling in a relatively narrow west-to-east oriented band near the Bay Area, but for the most part significant precipitation and flooding issues have thus far been confined to a relatively small portion of the Sierra Nevada. With this particular atmospheric river event, California’s orograhically-induced rainshadowing is in full effect (keeping Silicon Valley and the Central Valley much drier than the adjacent windward mountain slopes). In the absence of a strong nearby storm system, the extremely moist plume is squeezing out quite a bit of moisture in the hills and mountains but very little closer to sea level. This is a defining characteristic of atmospheric rivers that arrive “unattached” to a parent storm system.

 

Subtropical moisture plume may interact with Arctic airmass later this week

Note the high-amplitude “Omega Block” over the North Pacific, with high pressure near Alaska. (NCEP via tropicaltidbits.com)

Mid-latitude weather is, at its core, driven by “clashes” between airmasses of opposing character. That’s why the American Great Plains constitute the world’s epicenter of severe weather and tornados–there is no other place on Earth where geography allows air as warm and moist as that from the Gulf of Mexico to interact with air as cold and dry as that coming from the Canadian interior. Here in California, the opposite situation is typically the case: it’s actually quite difficult to get either warm/moist OR cold/dry airmasses to come anywhere near us, let alone at the same time. But it’s looking like that may happen to some degree later this week, when a persistent subtropical moisture plume meets a surge of unusually cold Arctic air diving down the West Coast from Canada.

Why is this unusual confluence of Arctic and subtropical airmasses likely to occur over California this week? A pronounced “Omega block” (characterized by a strong high pressure center with satellite low pressure centers to the southwest and southeast) has set up shop near Alaska, which has allowed bitterly cold air to spill out of the Arctic and over the northern North American continent. This airmass has thus far made it as close as the Pacific Northwest, where significant snowfall has blanketed areas from Portland to Vancouver for the first time in several years (even at sea level). At the same time, the southeastern low pressure center associated with the quasi-stationary Omega block has set up shop near Hawaii, which is generating a nearly continuous plume of warm/moist air that is heading in the general direction of California.

The Arctic basin itself remains extraordinarily warm in regions of record low sea ice extent. (NCEP via tropicaltidbits.com)

It’s interesting to note that the Arctic basin itself remains extremely warm relative to average for this time of year–and that the Alaskan blocking pattern is occurring very close to the region of ongoing record low sea ice extent in the Bering Sea. At least in a proximal sense, it does appear that the upcoming unusual weather pattern in California this week is related to the extremely anomalous circulation pattern in the Arctic.

 

High uncertainty for coming week, but strong storms/low snow possible

With such a complex and energetic flow pattern over the northeastern Pacific, it’s very hard to say just how the weather will evolve over the coming week. With a strong “baroclinic” region over California (i.e. a zone of very strong temperature contrasts), it’s possible that deep surface lows could develop fairly rapidly and bring substantial heavy rain (and perhaps strong wind) events to California. It’s also possible, though, that much of the state just sees a steady stream of weak-to-moderate impulses that bring hefty precipitation accumulations in the mountains but modest amounts elsewhere. Current model forecasts have been waffling back and forth between these two possibilities, but I would expect to see at least one significant storm event in California this week. Early, precipitation will remain confined largely to northern and central California, but by later in the week there will be an increasing possibility for rain and wind to reach southern California.

Animation showing one possible progression of storms over California this week. (NCEP via tropicaltidbits.com)

In either scenario, however, heavy to very heavy rainfall is likely in the central Sierra Nevada, and if things pan out as currently projected there could be some significant flood concerns in this region later this week (especially given the response of rivers already to this weekend’s precipitation).

There is a fair bit of model agreement that no matter what happens this week, the active weather phase will slowly start to taper off after next weekend. But as it does so, a very cold airmass may settle further southward across California, perhaps bringing drastically lower snow levels and sub-freezing overnight temperatures. It’s still too early to say just how cold it might get or just how low snow levels may drop, but it is highly likely that heavy Sierra Nevada snow accumulations will occur with late-week precipitation as snow levels plummet following the atmospheric river and the arrival of the Arctic front.

For more frequent updates as this week’s storms develop, follow Weather West on Twitter!

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  • Guest

    Anyone following the GFS?

    • Powerful winter storms of 2016

      yes it trending back too the idea it had a few days a go with the cold weather and low snow if it can keep its trend this time then it will be in line with the EC and canda

  • rob b-Truckee/East Bay

    Bryan posted his latest update on OpenSnow…I think this quote sums it up
    “So if you like pond hockey or a numb face while skiing, you will love what is coming next week. It starts with the initial cold front on New Year’s Eve. ” https://uploads.disquscdn.com/images/439edf469aa782a6969e01ce6da159a5afd45bd053d0fb694873835d469a138d.jpg

    http://opensnow.com/dailysnow/tahoe

    Also this quote for “Powerful Winter Storms”
    “So we will just have to wait and watch the trends over the next few days. We could see anything from a dusting to a couple feet of snow… There is a much better bet on a lot of cold.”

    • AlTahoe

      Somebody on another forum actually mapped the location of the high pressure and cut off low using the current satellite images and found that the GFS is not initializing the features correctly at all. Nobody knows what’s wrong with the GFS but it is starting its runs with the major features in the wrong location even though those features can now be seen on satellite. So with that I am leaning towards the wetter Euro solution for the Sunday Monday storm as it has been locked in on it.

      • Powerful winter storms of 2016

        The 12Z bring back the low snow and colder air it had.

        • AlTahoe

          I hadn’t checked it yet. The analysis the other people ran were for the previous 3 runs that were so out of sync with the euro and Canadian. Seems like the Gfs has some major issues though if it can’t even initialize the current location of high pressure cells correctly.

          • SlashTurn (Santa Barbara)

            Maybe it’s been over emphasizing the strength/placement of downstream blocking…

      • jstrahl

        Is that you, or are you quoting Bryan? If you, how can one get to see the Euro solutions?

        • AlTahoe

          Me. I follow along with other people that have access

        • Bob G (Gustine)

          There are some Euro solutions on the internet. They are limited but still there. Someone posted a link recently to a weather site that posts the Euro.Cant recall that site specifcally now

          • jstrahl

            Thanks, i’ll look. Easy to find the pressure charts, from which one can guess, but such guessing is not quite accurate, as low pressure centers may not be in a good position to tap much moisture.

      • gray whale

        link? sounds like a great read

      • SoCalWXwatcher

        I’m glad the GFS is being replaced by a completely new model soon. It’s been late to the party with regard to some of the recent storms. Euro and GEM have had better consistency by far.

  • RunningSprings6250
    • AlTahoe

      The chance of a 1969 type breakthrough is there with this setup. Or the blocking ridge could migrate over us later in the month and shut off all of the fun. At least we have a chance though!

      • KrisKastForecast

        were you around during 69? or have you just seen 69 storm summarys.

        • AlTahoe

          I didn’t join this world till 20 years later but the summary’s are fascinating.

    • jstrahl

      I like the frame right before that one, shows the purples over the Bay Area too. 🙂

    • SlashTurn (Santa Barbara)

      Purps!

  • click

    Just felt a jolt in Riverside, 4.0 (at the moment) from north of morongo valley
    http://earthquake.usgs.gov/earthquakes/eventpage/ci37772688#executive
    Ian, did you get a good shaking in RS?

    • RunningSprings6250

      Just drove down the hill, we didn’t feel it but have been in the car..

      • scott

        I was thinking of heading up to rimnordic on Friday for some cross country skiing (if the warm doesn’t melt it off by then as it is warm today). It sounds like the snow is really great up there. However, talking to people up there on the phone sounds like the snow play people are like locusts, parking in the middle of roads, blocking driveways, jaywalking everywhere, etc. I’d probably go 330 to the 18, but since you’re a local I thought youd be able to give me more insight. How’s the traffic getting around?

    • BigBearHiking

      I felt it pretty well up in Big Bear

  • Bob G (Gustine)

    The CFSv2 is acting the opposite of 2014 and 2015. Pretty much all year it has had a very dry bias. It starts each month with really dry predictions. The map had California colored brown for the first 2/3 of the month. The runs started flipping last week and trending wetter. Lets see how this plays out. Not sure what the CFS is seeing to change its original bias towards a very dry January but hopefully this late forecast comes to pass.

    http://www.cpc.ncep.noaa.gov/products/people/mchen/CFSv2FCST/monthly/images/CFSv2.NaPrec.20161228.201701.gif

    • Aloha12

      Hard to imagine anything more worthless than the CFSv2 monthly precip forecast.

      • Patrick from Stockton

        Isn’t this what WW went by to post that blog update back in early Oct. with the dry and warm Dec – March outlook? If not, I wonder what led him to believe that when so far, it couldn’t be further from reality.

        • Dan the Weatherman

          I believe he was looking at sea surface temperature (SST) patterns and the particular setup of SSTs at the time didn’t favor a wet winter (a weak La Nina type configuration with somewhat cooler then normal SSTs in the eastern tropical Pacific).

          • Bob G (Gustine)

            Yes, that coupled with warmer than normal SSTs in the Northern Pacific favored ridging

        • Upslope

          My God – you don’t actually think that’s all he relied upon, do you? Any monkey with access can read what the model output says. I’m kind of shocked you would bother to read this blog if that’s how little you think of his rigor and prognostication.

          • Patrick from Stockton

            I really have no idea what he relied upon, that’s why I asked the question. And don’t get so upset, it’s not good for your blood pressure. I read the blog because I’m interested in the weather like everyone else on here, not just for the WW updates. I find others on here actually more informative when they comment and analyse the short term outlooks, something that it seems like science has an actual handle on, vs the long term outlooks which are basically a crap shoot in the dark. No one knows what’s ultimately going to happen months from now. And I don’t care how much instrumentation and science they rely upon. It’s proven time and time again to be unreliable.

          • Upslope

            And what a doozy of a question it was. Your second guessing will start to get interesting when you put yourself out there with science-based outlooks that can be subsequently verified. Until then, you’re just another guy excited to see his screen name on the internet while taking pot shots at the guy who is kind enough to provide you with the platform. Stay classy, Stockton.

          • Bob G (Gustine)

            Where do get that he was taking pot shots at Daniel? He simply asked a question about how Daniel came up with his information. I certainly don’t think Daniel would see that as an attack on his credibility. I would venture that Daniel would probably respond to him in thoughtful way and explain how he arrived at his conclusions.

          • SlashTurn (Santa Barbara)

            Its all garbage in/garbage out at the moment. Either by mechanical algorithms or synapses/neurons…no one or thing is immune to the failure/

        • Sokafriend

          I believe it is about the evolving patterns and unusual conditions developing as related to climate change.

          • hermit crab

            Yes I admit the snowpack is important…

        • Bob G (Gustine)

          In the past he has referred to the CFSv2 and often uses the NMME which no longer forecasts a dry J-M period but neutral. The NMME seems more stable.

      • Bob G (Gustine)

        It has been better when looking at end of month trends, the last several days of the month.

      • Charlie B

        I need to get a pet bird. If I had a pet bird I would need a birdcage for my pet bird. If I had a birdcage for my pet bird I would need liner to line the birdcage for my pet bird. In that way it would be easier to clean out the birdcage for my pet bird. I have plenty of liner, which is why I need a pet bird for my pet bird’s birdcage.

        • Aloha12

          Thing is, you might end up liking the bird – could add life value. The CFSv2 adds nothing, but causes needless despair and gives false hope.

  • Crouching Dallas

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    ??| •.•) The GEM is superior to the GFS
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  • Tom
    • Patrick from Stockton

      Interesting. Looks like SoCal is favored for more precip in that period than NorCal, which is a switch from how it’s been.

      • Bob G (Gustine)

        Probably due to the storm track undercutting the ridge in the north pacific

        • Tom

          http://www.cpc.ncep.noaa.gov/products/predictions/610day/fxus06.html

          I won’t quote any of this, but the discussion is interesting..regarding how they came to the forecasts. Well worth checking out.

          • Bob G (Gustine)

            I’ll check it out, thanks

          • Thanks for sharing. I like that they have a wet pattern for most or all of CA in both periods AND they have a high level of confidence in the forecast. Sounds promising to me.

          • jstrahl

            ? The 6-10 seemed to indicate dryer than normal in the north, when i looked a few minutes ago.

          • True, but that does not mean dry, like it would in So Cal. North gets a LOT more rain, so drier is not necessarily all that bad. If it stayed that way for an extended period I would be concerned.

          • jstrahl

            After five years of drought (OK, last year was “normal,” barely so, in spite of the strongest El Nino ever), i get concerned any time i see the brown tones in the Bay Area on the map.

          • Oh, I get it, most of us are gun shy. At least you folks up north had a decent season last year. We were still only 60% of normal (or there about) here. Happy this year is better and hope that we ALL have wet second half of the season.

          • jstrahl

            I’ll, eh,…..”drink” to that.:-)

          • Pfirman

            TODAY’S OFFICIAL 500-HPA BLEND CONSISTS OF 60% OF TODAY’S 6Z GFS ENSEMBLE MEAN
            CENTERED ON DAY 8, AND 40% OF TODAY’S OPERATIONAL 6Z GFS CENTERED ON DAY 8

            Well, looks like they do the same thing witches do when they make a potion. 5 out of 5 confidence though, heh.

      • jstrahl

        Only the 6-10, the 8-14 looks good for both So and Nor.

        • Patrick from Stockton

          yep

    • Cornholemaster (Ventura)

      Cold and rainy. Just the way I like it.

    • RunningSprings6250

      I guess you could say it’s probably going to be a tad cold huh…

    • Black Cat

      Florida lol

  • Patrick from Stockton

    The GFS 18Z coming out now. So far consistent with the 12z. Shows the system on the 31st a bit wetter from the looks of it, especially for SoCal…now if only it will stay this way on subsequent runs

  • Fairweathercactus

    The 18z is rolling off the cart in a bit. Should be interesting to see what it has to say. I am betting the colder more dry pattern.

  • RunningSprings6250
    • rob b-Truckee/East Bay

      BA with OhpenSnow posted on his Twitter update the 18z was trending wetter for Tahoe. Good to see things going towards the wetter end rather than last years seeing big storms 10+ days out only to see them fall apart as they got closer.

      • Bob G (Gustine)

        The NWS mentions in our local weather page the Euro is wetter than the GFS too

      • weather whisperer

        What is Brian twitter feed?

        • Siernev

          [at]TahoeWeather

  • Craig Matthews

    Since winter 1968-69 is being brought up lately (due to the current + evolving pattern sharing some characteristics of that exceptional winter), thought I’d throw in a couple of charts of Jan 20-24, 1969, which generally represents the pattern “set up”/wave-height configurations/orientations over the NPAC across N.A for a good portion of that winter. One chart is a 500mb height pattern anomaly. The other chart is Columnar Precipitable Water, which I found very interesting because it shows how abundant moisture(yellow-orange color) was drawn/funneled from the tropics between an elongated ridge over Mexico and the cyclonic flow of a rather potent sw to ne orientated trough extending from W/Canada to basically the region near Hawaii. This set up was ideal for CA, as polar flow in Northern Stream backed underneath the AK High toward the sw from Canada offshore and phased with a warm and very moist airmass riding along a potent STJ right over the state. This pattern/set- up repeated quite often throughout the 1968-69 winter, especially between December and February. This pattern is quite rare. And I wonder what the return interval of this pattern is, as I have only found a handful of other winters that share similar characteristics to the 1968-69 winter while digging thru the data between the early 1900s and now. I’m sure there are other winters that had this pattern but I haven’t found them yet. Anyway, hope someone else finds this interesting as well…Hope we get to see this pattern in January..rock on!! https://uploads.disquscdn.com/images/b7eeed0ceea6d9ce360484a9000f0b0b9eb2aab971b0c5b35eb741a3b6d03b2f.gif https://uploads.disquscdn.com/images/43f73e03dab841ca01a0eb1e4392ef9dc0c5257d96ee41e18e8cac3ff99eec41.gif

    • Neat! An easy read to see the AR, too. I pulled up vectors around that time frame and earlier to see the genesis from the tropics.

      • Craig Matthews

        Cool, love to see those too.

    • KrisKastForecast

      Thank you for uploading those, i’d be interested to see the last months 500mb geopotential height anomalies. Do you know where I can find them?

    • AlTahoe

      In the Pacific Northwest Forums somebody posted this regarding the blocking high being shown on the 18z

      “That block was the size of 65-70% of the United States! I’ve never see
      anything even close to that on any model run in my life. This is how you
      get a 1950, 1862, or 1899……… wow”

      • Craig Matthews

        Huge Ridge!!. I saw the Jan 1950 chart on the forums as well.

    • Sokafriend

      Thank you, this is great!

  • kipling

    Well, the large fantasy Mega Storm is still showing on January 8th. This has been on the GFS runs for some time now. I know Daniel has mentioned that beyond 10 days the models are inaccurate to the point of being somewhat worthless. But given the run-to-run consistency of this massive storm, can we at least be hopeful that this could come to fruition?

    • matthew

      Hope and $2 will get you a cup of coffee at starbucks.

      • inclinejj

        Friends don’t let friends do Starbucks!!

        • Charlie B

          The Big Green Monster.

      • DelMarSD

        More like $4.50.

        • jstrahl

          Really? I can get an organic cappuccino at my downtown hangout for less than that.

    • Bob G (Gustine)

      I wouldn’t discount it entirely. This year these storms have materialized

      • Patrick from Stockton

        Yeah, I agree

    • yenlard

      That storm would give some a month’s worth of precipitation in two days if it happened

    • alanstorm
    • jstrahl

      No! Won’t get fooled again.

  • My parents are heading to Portland for a few days and they sent me this picture of Mount Shasta just now.

    https://uploads.disquscdn.com/images/a534ddd65e185f070189cc9e0b3991fa6f12b320dadec0ec6b327924ef557545.jpg

  • Black Cat

    Went up to 73F todays the warmest day since Thanksgiving, which is also the last time it reached 70F+. Its going to get cold here! (Relatively speaking) I’m not used to the 70’s anymore it feels warm!

    https://uploads.disquscdn.com/images/b3189988e6e2e36ad32f1519f34a259f53a657f30bad15378e99f0236e836988.png

    • redlands

      78 in Redlands, Ca— Southern Ca for dec-28-2016 — too warm for me

  • HighWater

    Snow pics 1969: pic 1) looking down at the mill of Union Carbide. Pine Creek canyon outside Bishop.
    https://uploads.disquscdn.com/images/1c99752c278d4163946c53127489a14850a1fc5272851357de54f6fc5b2c9234.jpg pic 2) pine creek canyon https://uploads.disquscdn.com/images/6d64a6b83f38de41668ac6cc1ee90bf8d1dea80ba504590bd05a55d17a43a3ac.jpg
    Pic 3) parents shoveling snow off the roof of Round Valley Elementary School. 10 mi north of Bishop. The White Mtns are in the background https://uploads.disquscdn.com/images/0f00cf1df512c6d583f33a6d329699105b38874f2701c5c628431c32f27e5587.jpg

    • HighWater

      ???

    • Guest

      The thing that gets me about older photos and snow/mountains is the cars they used to drive on pretty harrowing roads. My neighbor has pics of her youth in the 1930’s/40’s in family cars going to Tahoe and Yosemite. I look at those and think I’m a complete coward to have any concern in my 4 wheel drive, snow tire Subaru. (of course it’s other drivers I’m usually worried about).

    • Nathan

      So cool. Thanks for sharing.

    • Charlie B

      Are that the “Mine in the Sky” in the top one?

      • HighWater

        Yes sir

        • Charlie B

          There is a great book out there on that mine. (This is not news to you I know….)

    • redlands

      nice pics

  • HighWater

    https://youtu.be/5BSDrocE6u4

    Mine in the sky Union Carbide Pine Creek

    • Pfirman

      Awesome.

  • Sokafriend
  • Powerful winter storms of 2016

    dont be fooled by the looks of monday and in too wed storm the little storm the GFS is forecasting can some time bring significant snows with them and it looks like the best ch for significant snows will be for foothill area about 500 to 1,000ft area about 1500ft or higher could easy see a foot or more of snow but every thing has too come in this right

    • Patrick from Stockton

      Yep, also that storm on the 9th keeps showing up on all the runs now pretty consistently. I remember back in Oct. a system showed up in the LR that actually happened. Hopefully this one will too.

  • Juggernaut

    Pretty epic 00z! (fingers crossed)

  • hermit crab

    Happy New Year everyone! Hoping for a decent water year. ??

  • Sokafriend

    19 areas of rivers above Flood Stage right now: Critical Stage
    http://www.cnrfc.noaa.gov/mobile/criticalStages.php