December 02, 2010

Climatologically, The Worst is Over

It may seem really strange, but on average the worst of the winter is over and its all uphill from now on. Strange, but true!

Here are the facts. Look at the plot of average rainfall per day at Sea-Tac Airport:

The period of highest average precipitation per year--ranging up to .3 inches per day--is in November, with the last half of November being the wettest. Then in December things improve, with a substantial decline in daily average values (to around .2 inches per day). January has similar daily rainfall, which is followed by the LONG, SLOW decline in late winter, spring, and early summer to a minimum in July.

What about daily extreme precipitation? Check it out below:

There is improvement in the middle to end of December, with worst in late October into the first week of December.

So this is pretty amazing...we ARE THROUGH the worst of the rain, on average. Interestingly, the jet stream--which directs storms towards us and is essentially the source of energy for the storms...weakens in December versus November over the Northwest. To show that, below are the winds at around 18,000 ft (really 500 mb pressure) averaged over November and December 1979-2008. The changes are perhaps subtle, but significant.

If we looked at surface winds, a similar story would be evident..the end of November is ground zero for rainy, stormy weather and the situation improves in December.
And certainly the first week of December this year is going to seem like a walk in the park compared to what we had at the end of November.

But the worst is yet to come for one parameter...snow. As shown below for Sea-Tac, January is the snowiest month in our area. KING-5s Jim Forman better keep his parka handy and the Mayor might insure his staff is ready for action! However, the next several days looks completely benign and strong storms or heavy precipitation in the offering, and Saturday might even have some sun!


  1. Not to be gloomy or anything, but the title of this post would make a wonderful epitaph on any meteorologist's tombstone.

  2. But for jilted rainlovers, the worst is only beginning.

    You only spoke of rain, but what day is climatologically the worse when you factor precip, wind, cold, darkness etc. I'm thinking its sometime in June.

  3. Cliff-

    What occurred to set up those 3 April snow days?



  4. This comment has been removed by a blog administrator.

  5. Yeah, over in the Palouse, they're still shoveling all that "early winter" out of Martin Stadium for the Apple Cup game...

  6. Rainiest month, yes.

    But how about winds? Do they also peak in November, and can we hope that the worst windstorms of the year are, statistically at least, behind us? We sure had a few doozies this November -- it would be nice to think they were perhaps the worst of the winter!

  7. Cliff, looking at the blocking charts for the winter 2010/2011, it appears to be a benign winter until March. Any comments to support this?

  8. I know we've had some big floods in November but I seem to remember more in January & February after we've had massive snow and freezing then we get a "Pineapple Express".... Here's hopeing for lots of snow, wind, and exciting weather...but no Pineapple Express's... Seeing others lose so much due to flooding kind of suck the fun out of a good storm!

  9. Are the patterns for daily precipitation and extreme precipitation the same in all "types" of years? (i.e., La Nina, El Nino, and neutral)

  10. It is encouraging to hear that "statistically" speaking the worst appears to be over but I am curious what the data would convey just looking at La Nina winters?

  11. It is encouraging to hear that "statistically" speaking the worst is over but I am curious what the same data looks like if it based on La Nina winters only? Is there enough data to make it statistically relevant?

  12. It looks like avg precipitation goes up slightly from mid May to early June to "buck the trend"... I wonder if this is this is caused by a few outliers (which if you look at this period on the extreme precip chart, you will notice some heavy amounts).

  13. Cliff ~ The NWS said last week that there could be a "windy" day on Tuesday or Wednesday of this week. In todays afternoon update they said this: "Yet another wave is forecast to rotate through on Wednesday bringing
    more rain. A 990mb surface low tracking through the offshore waters
    and moving inland over Vancouver Island could bring some very windy
    conditions if things pan out as currently forecast." Anything to get excited about?

  14. If the worst is truly over, then color me unimpressed. With the exception of that one week period of cold and snow, November wasn't all that stormy. In fact I'd say it paled in comparison to many past Novembers, with 2006 being the most recent example.

    It's early yet. I have a feeling that climatology is going to take a beating this winter.

  15. I think we've learned Prof Mass' secret...

    Bob ^,,^

  16. Funny you posted this topic the other day. I've been keeping track of the climate data around Seattle over the past five years, and just happened to plot out a chart of cumulative precipitation over this period as a data visualization exercise (an interest of mine):

    I cleaned the chart up a little today, but here is what I've got. I set Dec 01 2005 as the 'zero' point.

    The little blue boxes surround the 'November' data for each year. Indeed, in the normal year line, this is the steepest part of the curve (ever so slightly).

    This holds true for most of the recent years, except for maybe 2007 and 2010.

    It has also been a damp five year period. The big dumps of Nov 2006 and this fall have managed to keep the recent years line well above what the normal line would expect.


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