Fall's coming: Seattle's average high temp dropping 0.01 per hour

Photo: Puget Sound Clean Air Agency

Those of you eagerly anticipating autumn's arrival can take glee in the following statistic: The average high temperature in Seattle is now dropping at the rate of about 0.01 degrees per hour. It's down to 71.8 degrees on Wednesday; it'll be 70.7 degrees on Sunday, and 65.7 degrees by the time we get to the end of the month.

But don't worry summer fans. Mother Nature is ignoring the data.

September begins a dramatic two-month transition of the weather in Western Washington. While our climb from winter to spring to summer is very gradual, the fall from summer to autumn is akin to walking off a cliff.

Figure that September 1 is on the back edge of the driest times of the year -- odds of rain are about 27 percent on any given day -- while Oct. 31 that chance has zoomed up to 55 percent. Average monthly rainfall just about doubles each month from August through November.

Meanwhile, the average high temperature drops about a quarter degree each day in September, which then boosts to about 0.4 degrees per day in October as our free fall into fall continues.

To compare, in March around the time of the other equinox, the average temperature is rising about half as fast -- about 0.15 degrees per day.

But sometimes September can do a good job of masking the transition with an "Indian Summer" and rattle off some days well into the 70s if not low 80s in mid to late September. But that just makes the drop off even more stark.

Take last year for example. Seattle had a four-day stretch in the upper 70s and low 80s as fall officially began around Sept. 21. Barely a week later, Seattle was in the 50s and low 60s and wouldn't get above 65 again until the spring. In 2003, it was 82 on Sept. 27, 84 on Sept. 28, 65 on Sept. 29, 71 on Oct. 1... and that was the last day over 70.

Record highs tell a similar story: mid-upper 80s somewhat common in late September (latest 90 degree day is Sept. 22, but that's happened twice!); mid 70s-low 80s early October. The record high on Sept. 27 is 89, on Oct. 2 it's 79, on Oct. 12th it's 70, on Oct. 26th it's 65 -- the average high on Oct. 1 is 65.

This summer has potential for a similar abrupt end. Long range models hint that after a brief warm spell this week, we could be in for another extended sunny and warm streak next week into the start of autumn. But history suggests don't be surprised if the following week if once that warm spell ends, we don't get close to that warm again.