Weather Blog

Seattle: Now officially even wetter than it was before

Seattle: Now officially even wetter than it was before

Just like the census sends everyone back to the drawing board every 10 years, weather statistics are rewritten at the change of the decade too.

When you watch the weather on TV or take a peek at Seattle's climate data online, you might want to compare today's high/low/rain with the "normal" data for the date.

Those records are calculated by taking a 30 year period of average highs, lows, rain, and whatnot, and then applying some statistical smoothing to the data to get a nice gradual curve that, of course, bottoms out in the winter and reaches its peak in the summer (for temperature, that is. It's the reverse for rain around here.)

NOAA uses the past 30 years to get a good sample size, but updates it every 10 years to adjust for climate trends. So for the past 10 years, Seattle's normals had been based on the period from 1970 to 2000. But Friday, NOAA released the new numbers they have calculated for the 1980-2010 period and will begin using on August 1.

In the continental United States, every state’s annual maximum and minimum temperature increased on average. “The climate of the 2000s is about 1.5 degree F warmer than the 1970s, so we would expect the updated 30-year normals to be warmer,” said Thomas R. Karl, L.H.D., National Climate Data Center director. On average, most cities across the U.S. reported about an 0.5 degree increase in temperature.

Here are some of the interesting trends for Seattle:


It's now officially wetter around here.

The annual average rainfall for Seattle will be listed at 37.49" -- a rise of 0.42" from the previous year's 37.07" normal. The largest change is that February and the summer in general has become drier, but November has been much wetter. Our wettest month now an average of 6.57" -- over a half inch more than the 5.90" it was previously, although that could be weighted by the 15.63" of rain in 2006 that obliterated the records.

Here is a chart showing how the monthly rainfall records have evolved over the past decades:

It's getting a little warmer around here

This spring notwithstanding -- whose numbers will have to wait until 2020 to be calculated into Seattle's normals anyway -- Seattle is overall getting a little warmer.

The highest "normal" high temperature remains at 77 degrees (just barely missed climbing to 78 -- the top of the curve is 77.4 degrees from July 30 to August 2) but with rounding, Seattle's normal high will now be listed at 77 degrees from July 19 through August 16. Currently, the 77 degree normal is only listed between July 29 and August 4.

On the flip-side, the coldest normal high of 45 remains, but now only stretches between December 14 through December 30. Currently, the 45 degree normal high stretches from December 13 to January 14.

Here is the chart showing the new 1980-2010 normals:

And here is the chart showing the previous 1970-2000 normals to compare. (Sorry this is aligned 90 degrees off. Had to use two separate sources to compare the data.)

So in summary, the summers are getting a little warmer and drier, but we pay for it with a rainier winter. Sounds like an even trade :)