Weather Blog

October home to quickest weather transition of the year

October home to quickest weather transition of the year
FILE -- Fallen leafs in autumn colors lie on the ground in Munich, southern Germany, Thursday, Oct.10, 2013. (AP Photo/Matthias Schrader)

The leaves aren't the only thing that undergo a stark change in October -- so does our local weather.

October is home to the greatest swing in expected weather from the start of the month to the end of the month. While in the Pacific Northwest, the transition from winter to summer is very, very gradual through the spring, not really peaking until July and August, the transition from summer to winter is much more jarring -- especially in the rain category, where we go from driest to wettest time of year in just about 12-14 weeks.

Here are some statistics to back it up:

October's great temperature swings

When the month begins, the average high on Oct. 1 is 65.3 degrees. On Oct. 31, it's down to 54.8 -- dropping about 0.4 degrees or so per day -- that's about 2 degrees per work week!

Another gauge: The record high on Oct. 1: 89. Record high on Oct. 29? Just 65.

It's really zany in the middle of the month where we are now. October 11th's record high is 83 and the record high a day later on the 12th is just 70 -- that 13-degree change is tied for the greatest spread between consecutive day's record highs with June 9-10 where it goes from 96 to 83. Then just a few days later, the spread is 10 degrees from the 14th (80) to the 16th (70).

For this year, it was 93 degrees on Sept. 11 and just 57 degrees on Oct. 11.

It gets wet in a hurry too

October is also when we really go from dry to wet in a hurry. The average daily rain on Oct. 1: 0.07" By the time we get to Halloween, it's 0.17" per day -- no other month comes close to having that much difference between Day 1 and Day 30/31.

This chart, which shows probability of having a rainy day, clearly shows a big spike in October.



For specific numbers, if we look back over the past 120 years: Number of times it's rained on Oct. 1: 39. Number of times it's rained on Halloween: 65.

So while we joke that if you don't the weather, just wait 20 minutes, if you don't like the climate in October, just wait about 20 days.

October 2013 not really playing along in the rain department

While we would expect October to get increasingly rainy as the month progresses, this year has been an anomaly with heavy, record rains in September giving way to what is essentially a bone-dry October (which I would say 1.44" of rain is by October standards -- much of that rain coming in two convergence zones, not widespread rainy days.) Long range models are currently suggesting the month could finish dry, or at least with several days of dry weather.

But any Seattleite knows we usually pay later for extended sunny periods -- with interest. Makes me wonder if this is akin to Mother Nature "fattening us up" for a big weather event later this autumn, but so far, it's an October that is shaping up to be anything but the official start of the rainy season.