February 03, 2009

A dry winter?

Several of you have suggested that this winter has seemed unusually dry. Is this true? It turns out that the answer is a bit complicated.

If you look at the precipitation since October 1 (known as the beginning of the water year), we have been only a little bit drier than normal. For example at Seattle we have "enjoyed" 18.19 inches since that date, while normal is 20.32 inches. Thus, we have been 2.13 inches below normal, which is really no big deal...and well within the range of normal variation. So is this an usually dry year overall? Not really by this measure.

But there is something else. Have we had an unusual number of dry days? The answer to this question is different. Look at the attached plot of precipitation this fall and winter--blue is normal and red is this year. At the end (now), the blue is above the red by about two inches..consistent with the numbers I give you above. But the really interesting part is how we got the wet stuff. Most of it occurred in relatively short, very intense periods...one in early November and the other in early January. Much of the fall and winter was characterized by extended dry periods... like the one we are in now. The only period with longer term continuous precipitation was during the snow of mid to late December...and snow just doesn't seem as wet as rain does it? I suspect that if we compared the number of dry days this winter with the normal situation, we would find that this year so far was highly unusual regarding the large number of dry days. Perhaps one of you would like to track those numbers down. Perhaps it will make you feel better that you missed out that trip to Hawaii.


  1. Cliff, this year`s rainfall pattern is very similar to a typical Califonia coastal station: many dry days but rainfall in big increments.This kind of pattern seems to occasionally occur during La Nina phases. I believe the winter of 2000-2001--if I am correct, also a La Nina year-- also had a lot of days with no precip.
    One thing you neglected to mention was the huge variability in January`s precipitation totals, due to the rain shadow effect during the month`s major rain event.Stations from the Downtown Seattle area northward, on the average had only about 60% of Sea-Tac`s total. ( I`m located 10 miles due north of Sea-Tac and was more than two inches drier than them--the greast disparity I`ve seen in my 35 years at this location.)So there is a valid argument by some of us to say that it is indeed a drier winter, too.

  2. Great analysis Cliff -- the graph really proves your point and our suspicions.

    It also seems like the rain shadow has been far more prevalent over Seattle and points northward this year (more so than I can remember in the past). This may be exaggerated in my memory because the only time I ever pay attention to the shadow is when it prevents us from getting snow.

    I swear that the University of Washington has been in the absolute dead center of all our notorious rain/snow shadows this year. Could they have established a school in a more benign (relatively) weather environment? If only we could relocate the atmospheric sciences building to the King-Snohomish county line!

  3. The reason it all appears drier is because of the position of the jet stream. Look at the periods of zonal flow and northwest flow when the precipitation comes. We have had very little southwesterly flow, yet alone hardly any southerly flow. It's been out of NW or due W. That makes a HUGE difference in precip totals. As a result, the rainshadow ends up engulfing a much more expansive region.

  4. You may be right that we have had more dry days this year than we usually do, but we have to be careful not to infer that from the graphs shown. The blue graph of a "normal year" shows a continuous accumulation of precipitation. But that is an average over many years and averages are always smoother than individual years. The "normal year" shouldn't be confused with a "typical year".

  5. Pretty nice day here today with high of 55 and low of 29. Though with weak cold front just off shore, there was thin high clouds drifting by at times.

  6. We like to hike in the woods, mostly at McCormick Park or Banner Forest, and we've noticed that there have been many more days when the ground in the forest is relatively dry this winter than last. On the other hand there was more damage to trees and shrubs from snow accumulations and wind than last year, almost all from January's heavy snow and subsequent windy days, requiring much more maintenance work to reopen and groom trails. And there were more days when low portions of the trails were flooded. All these are consistent with an unusual number of dry days interspersed with a few big events.

    - Pete

  7. January certainly was drier than average as my average precip is 5.40" for that month, yet I only picked up 2.49" worth Jan.

  8. Doesn't make me feel one bit better that I missed that trip to Hawaii. Why would I ever want to go to Hawaii in the winter anyway? I'd rather go someplace like the UP of Michigan. So much nicer this time of year!!

    Please don't assume everyone is the same.

  9. But we ARE all the same. I've never met a person in my life that is different than me. Crazy but true!

    I don't remember the Olympic Mtns providing near the rain/snow shadow that it has at times this winter. When the snowy period was beginning I remember it snowing seemingly everywhere but where I was--on the Eastside. I took a look and sure enough there was quite the shadow keeping the snow away from areas such as Seattle, Bellevue, Kirkland, Lynnwood. To the north and south the snow was coming down. I would say where I live it has been a bit drier than normal, certainly the number of wet days have been unusually low. We could use another one of those wet periods about now, the ski season has been pretty dismal for the most part.

  10. Cliff - I have been wondering about this as well. We have had a number of very severe weather events this winter, separated by several rather long unusually pleasant dry periods.

    I will note that last winter after Jan 1 it also seemed we had quite a dry winter. My house was being renovated and we had tarps everyhwere. I watched the weather closely. Jan and Feb were quite benign with no rainfall over 1 in on any one day.

    Anyway, later!

  11. Looking at the average would make you think wetness arrived in an even distribution over the period. This clearly does not happen.

    Does the concentration of precipitation events you refer to fall within the normal range for "this" area, or does it fall near or outside the normal bounds.

    Does the distribution of dry days fall within the range of normal distributions of dry days? I assume there is some way to look at the total count of dry days, as well as the number of consecutive dry days, where the amount of precipitation is less than some amount.

  12. Cliff,

    Kudos for making an interesting blog post during a period of boring weather!

  13. I don't know how relevant this is...

    Here in Abbotsford, BC, we average 73.9 days with measurable precipitation from October 1st to January 31st. This winter we've had 68 days.

    The average precipitation during that time period is 792.9 mm (31.2 in). This winter we've had just 734.2 mm (28.9 in).

    So we're about 2 inches below average here as well, with about 6 more dry days than average.

    Not all that far off normal, but it sure seems dryer.

  14. This winter will definitely be shorter than last winter. By one day. Last year was a leap year!

  15. All..yes the climatological mean precipitation increases in a very steady way that you would never see in a "typical" year. But I do think it is true that we have had an unusual distribution of very short periods of "action" followed by dead periods. Someone could evaluate this quantitatively.

  16. A little off topic from the dry winter you all are talking about, but the latest WRF-GFS model is showing snow chances as soon as next Mon morning.

    So here`s what it looks like.

    1am Monday. It appeasr the central sound region could get hit with quite a few inches of snow for early Monday morning as 925mb temps go a little below 0c with decent NW flow and 850mb temps of about -7 to -8c. Still a little ways out there, but`s nice to look at though.

    4am Monday 24hr totals.

    4pm Tue 3hr totals. A few inches from the central sound and points north with a big ol` snow shadow in parts of the south sound

    4pm Tue 24hr totals. It also looks like 925mb temps go a little below freezing starting early Monday morning and not really rising above 0c till LATE Tue evening when SW winds flood the interior lowlands. However, looking at the 850mb level, SW winds start coming in for mid Tue afternoon, so the change from snow to rain could take place during the early afternoon hours. Still though, something nice to see since it`s the very short term period and with in the 5 day forecast period.

  17. Hey Andy, I think the GFS has been showing that for awhile now. See past posts.

  18. Cliff...just wondering....we didn't get any good windstorms this year.....What does the El Nino, La Nina weather pattern look like it's shaping up to be for next winter....or can you tell this early....and if you can, does it look like it's shaping up to be a mild or wild 2009/2010 winter as far as wind storms go? The snow this year was fun, and the flooding nervewracking, but I love a good windstorm! I'm really getting board with this weather this year!

  19. Cliff, can you tell us why there's a ring around the moon? Thanks !

  20. The ring around the moon is a halo caused by the moon's light being bent (refracted) by a layer of cirrostratus clouds...cliff

  21. Hi, No one has mentioned what forces or conditions have caused the jet stream to stray so far off course. It comes out of the polar regions, heads south parallel to the west of the Washington coast and then bears east and heads inland over California bring the storms and rain which are our usual winter fare.
    Also our weather has not merely been dry, it has been excessively stagnant, de facto we have been experiencing non-weather, just a limp, insipid wash of polluted foggy soup for weeks on end. This has been the ugliest winters I can recall.


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