December 22, 2017

A Fairly Normal Fall

We have had some warm periods and some cool ones.   Some wet periods and some dry ones. 

But looking at the entire fall, everywhere came out in the wash, leaving us with a fairly typical fall.

As we will see there is an important message in all this:  the weather on a particular day is often not "normal" but that is itself "normal", with mean conditions for a particularly date averaged over above-normal and below-normal gyrations of past years.

Consider temperatures during the past 12 weeks at Seattle-Tacoma Airport (below).  The purple and blue lines show the average maximum and minimum temperatures, respectively.   We had some warmer than normal periods, but almost the same number of below-normal ones.   This is what a normal fall looks like temperature-wise.
Did we approach or exceed some daily temperature records?   Yes, as shown below, with purple dots being the daily high and the blue dots the daily low records for Seattle-Tacoma Airport.  Sea Tac achieved a daily high record in early November and tied some low records earlier in the period. 
Keep in mind that it is normal to break some daily records...we do this all the time. 

Why is it normal?  Because there is a certain amount of random variability in the weather/climate systems and sometimes the requirements for unusual events just come together naturally.

What about precipitation?  Similar points.  Below is the cumulative precipitation at Sea-Tac, with observed shown by purple and "normal" by the blue line.  This fall is ending up slightly wetter than

normal, but we got there through very wet and very dry periods.  Typical.

Let's get a better view of our fall from a spatial viewpoint.   First, the difference of average daily temperature from normal for the past 90 days (9/19 through 12/17).  The entire NW is close to normal (light green or yellow, or different from normal by less than 2F).   Portions of the southwest U.S. are warmer than normal (particularly Arizona and New Mexico).

Precipitation?   Slightly wetter than normal over most of Washington and near normal over Oregon.  California is drier than normal (2-5 inches).

Bottom line: a totally unremarkable fall over the Northwest.


  1. Cliff, I heard your talk this morning. So true about our snow! As an experienced back-country skier I can testify to what a challenge it is finding good snow around here. And sometimes when you think it is clear and cold after a storm you get up there and find you are in an inversion, as I did two weeks ago. The air feels nice but the snow quality is poor. This can also be said of the Pineapple express, which feels wonderfully subtropical at sea level. But this pleasure is tempered for back-country skiers, who knows that the snow is toast afterwards.

    This is why so many NW skiers prefer spring. After a period of mild temps, a warm day in May features corn snow which, though not as aesthetic as powder, is, once it forms, reliable and relatively consistent and easy to ski on. Unlike Cascade Cement.

    1. Storm riding is where US is the only way to go in the PNW

  2. Thanks for the info on the Fall weather. BUT what about the Christmas Eve and Christmas Day
    possibility of SNOW?

    1. As wishy/washy as the models have been and with the difficulty of forecasting snow around here I'm guessing he's waiting til the last minute to update us to give us the best idea possible of what may or may not happen. Even then it won't be a perfect forecast....someone will complain and criticize his efforts. But I have to say...I'm a bit anxious myself to see what he has to say about this! He's usually the most clear and correct of any forecasters in our area.

  3. I don't know what Cliff has to say, but my weather AP is forcasting snow, but no details, I just updated it.

  4. Hey Cliff! What's your best considered opinion about a snow event in the Puget Sound lowlands this week? Been many a year since the last white Chritmas in Seattle! What's the outlook for the ski areas in the next week? We could always use more base, especially at Mission Ridge. Thanks! Skier Jay

  5. Hi Cliff - Could you please clarify what you mean you say "normal" and "average"? Average suggests the mean of data taken over some period, what is the accepted period when calculating an average weather metric. In some cases I have seen a sliding average calculated over the last "N" years, but it seems to me that has the effect of diminishing the effect of a changing climate.


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