Saturday, March 18, 2017

A Year's Worth of Water in 5 Months

Meteorologists and hydrologists call it the water year, the period between October 1 and September 30.  Each year the cumulative precipitation for the water year is totalled starting October 1.

The water year is a natural precipitation measure along the U.S. West Coast because we typically get very little precipitation over the summer and precipitation generally is not significant until October.


Thus, there is a hydrological reset each summer, with the soils dried, the snowpack melted, and the rivers dropping to low early fall levels. So October 1 is a good date to start the new water season.

Now the amazing thing.  Five and one-half months into the 2016-2017 water year, many Northwest stations have ALREADY received their total water year amounts.   You got that right....if there was not another drop of rain or flake of snow for the next 6.5 months, these stations would have their full normal precipitation for the water year.

By late September there is generally very little snowpack left in the Cascades

Want proof?  No problem.   The National Weather Service has some wonderful water year plots, with dark green showing actual accumulated precipitation this water year and light green indicating normal values (shown below).

Here is the plot for Seattle-Tacoma Airport for water year total precipitation and snow.    Amazing... Sea-Tac passed the normal water year amount during the past week...and the show is not over yet. The late winter has been very wet--particularly early February and the last few weeks.  More snow than normal too...mainly from a crazy local snowfall in early February.  Note how the light green curve (normal year) plateaus out after May 1.  And in normal year, the bulk of our precipitation falls between Nov 1 and April 1---our wet season.

Seattle Tacoma Airport Water Year Plot

But let's not stop there.  Let's head to the Washington coast at Hoquiam, where the total amount (63.25 inches) is just behind the normal water year.  They will cross the line this coming week.

 Hoquiam Water Year

But why only look west of the Cascade crest?  Consider Spokane, where the total (17.66 inches) is already well past the normal water year.

Spokane Water Year

Another way to look at the water year situation is in map form.  Here is the percent of average precipitation precipitation for the current water year (again, since October 1) for the entire West (courtesy of the Western Region Climate Center). Virtually the entire West Coast has higher than normal precipitation, with California, Montana, and eastern WA being particularly wet.

Now, a question many of you are asking is whether this anomalous year is an indication of a trend towards wetter winters and thus might be the sign of some cause (like global warming).   To answer, here is a plot of the long-term trend of water year precipitation (from the NOAA/NWS ESRL website).  Bottom line:  no significant trend since 1948.  Thus, what we are experiencing is probably just natural variability.


This plot is consistent with our best climate models, which suggest that global warming will have only a small impact (slight increase) in regional annual precipitation by the end of this century. Climate models project that Northwest will not lose our substantial precipitation as the earth warms, but more of it will fall as rain rather than snow.



15 comments:

  1. Mother nature sure is amazing. We can get an amazing natural variation and suddenly even California has abundant water. And and it can happen with the climate as well. Modern Scientists with all of our technology don't like to admit it, but ultimately with chaotic systems as large as the earth, we don't really know what is going to happen in the future.

    Climate and Weather prediction are the poster children for the phrase under promise and over deliver.

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  2. If my math is right that puts us about 25% above normal. Amazing in its own right, but really highlights how little rain we get from may - Sept.

    On another note Cliff, I was checking my almanac for today and noticed Seattle is getting 12:04 of daylight today. This means this would be the first day this year there is more daylight then nighttime; yet the first day of spring is still 2 days away. How can this be????

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    1. He did that one last year:

      http://cliffmass.blogspot.com/2016/09/today-is-fall-equinox-but-is-day-and.html

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  3. Hi Cliff
    Can you explain why with the record cold and now the record wet this winter, why our snowpack is only hovering around average?

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  4. Hi Cliff
    Can you explaining why with the record cold and now with the record wet this winter, our snowpack is only hovering around average?

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  5. I get the no trend idea. Is an increase in variability still thought to be coming with climate change?

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  6. While this has certainly been a wet water year, down here is Olympia it is not as wet as for the prior water year at this date. I checked totals for several COCORAHS stations. I chose stations with records with readings for at least 360 days. Here are the totals for the beginning of the water year to today--March 18. WA-TH-1 2015-2016: 64.12, 2016-2017: 55.93. WA-TH-28 2015-2016: 52.70, 2016-2017: 47.73. WA-TH-37: 2015-2016: 56.75, 2016-2017: 50.31.

    Reading on my COCORAHS rain gauge also show we received more rain by this date last year than this date this year however since I moved three miles across town in between these dates, I did not include data.

    I thought maybe we had had more wet days than last year at this time but, when I checked a couple days ago, the number of wet days was actually the same. I am certainly feeling it this year; I think it is the cold in addition to the wet which makes it harder to bear.

    What is interest to me is how long this rain will keep up. Will be have an abrupt end early like last year or a more normal continuation of the rain until July.

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  7. Cliff,
    As I understand it, the AGW models predict an increase in winter-time rainfall and a decrease in summer-time rainfall for the PNW. An analysis of annual rainfall only, could mask any change.

    Annual rainfall variability is large. The predicted change for the PNW is small and is expected later this century. I would be surprised to see a definitive trend during the early stages of climate change in the Seattle.

    Also, I noticed you used SEATAC data only and that is a short data set (1948 - present). Drawing conclusions from a single data point is risky business. I'm surprised you would do that.

    Climatologists analyzing the Midwest have noticed a significant increase in the frequency of excessive rainfalls and blizzards. I haven't read of a similar analysis being done for the PNW.

    Enjoy the Sunshine!


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  8. @Reader,

    Yes, last year at SeaTac as well, we had more precipitation at this point in the water year, a little less than 4 inches more than this year.

    In fact, if you look at the past five years of water year precipitation totals, we've managed to squeeze in nearly one additional year of rain at SeaTac, 211.6" (Oct 1, 2012—Mar 17, 2017) vs. 175.8" (normal). And this year isn't complete.

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  9. The tragedy in all of this is CA's severe neglect of building new dams to harvest water in years in which it comes as a definitive surplus. While they blocked any and all proposed dams over the past three decades, excessive amounts of money have been spent on boondoggles of the highest magnitude, and the Bullet Train is just one of many recent examples. Not to mention they've neglected to even maintain many of the dams they already have - when you spend taxpayer dollars on fairy tale projects instead of bolstering your state's critical infrastructure, eventually it will come back to bite you in the posterior.

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  10. Mark...other NW stations have the same trends. We get very little rainfall during summer and the slight declines are overwhelmed by increased precipitation in winter. There have been a number of studies of intensity changes in rainfall over the NW...in fact, I have done some of them...cliff

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  11. That's awesome news. Now when is that wonderful, healing sun gonna show up and give those that need it some relief? ;)

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  12. Unknown asked...Why does the date for spring "appear" to start earlier than the calendar says? The answer= "Basic" explanation is that the curvature of the Earth and the atmosphere surrounding it bends the sunlight like a magnifying glass. BOTH AT THE BEGINNING OF SPRING, AND START OF FALL, the light "appears" to be visible above the horizon, when in fact, it is actually below. It's effects of bending the light are more significant the further away from the equator you are
    .

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  13. The seemingly random dry spots in the percent of average precipitation map continue to intrigue me. For local examples, there's a small one east of Hoquiam, near Oakville, showing about 90% of normal, surrounded by 110% of average. A similar spot hovers near Port Angeles and the Elwha River. I could see the latter one in particular being a transient rain shadow effect due to the southerly tendency of the jet stream this year, place Port Angeles slightly in the lee of the Olympics.

    There's much more radical dry spots elsewhere in the west. A particularly dramatic one is in the vicinity of Tonopah, NV. They're at ~50% of normal precipitation. That means in the last 5-1/2 months, they've had roughly as much rain as Seattle had this weekend alone, yet a mere 100 miles west is an extended zone of over 200% precipitation. According to NOAA, Tahoe City is normally 3-4 inches behind Seattle for precipitation at this point in the water year. Instead they're over a foot and a half ahead of Seattle. Incredible!

    Because these are relative to the local averages, this is more than a mere rflcetion of the fact that Tonopah is in the middle of a desert (with a large solar array nearby specifically for the dry weather), while the region to the west are the relatively wet Sierra Nevadas. That classic rain shadow is not remarkable.

    What's fascinating to me is that the Sierra's have been drenched by storm after storm this past fall and winter, yet this maps suggests nearly all of those atmospheric rivers Cliff has been pointing out over and over again this year has climbed over the Sierra Nevada's only to swing around Tonopah.

    In particular, it looks like the rivers have tended strongly to restrict themselves to a narrow corridor between the dry zones in central Nevada and SE Oregon.

    Cliff, if this piques your interest, I would enjoy a future post with your thoughts on the possible causes of these dry spots.

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  14. iamlucky13 and others. You have to take terrain into effect when looking at hot spots of any perameter. For example, SEATAC is not the ideal location for a weather station that purports to provide Seattle weather. The airport is in direct line with Snoqualmie Pass so winter chinooks (katabatibc winds) are directed like a nozzle at the spot. SSEATAC is also its own little heat island especially with all that tarmac. An examination of the Sierra Nevada range might tell you something about the weather in Nevada.

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