Tuesday, March 8, 2016

Seattle's Rainiest Winter Ever

Monday, March 7, 2016
1:43 pm

The Snohomish River topping its banks on December 9, 2015.
Credit: Brie Hawkins

I'll be the first to admit that weather forecasters are often wrong. After all, weather forecasting is the only job where you can be wrong 90 percent of the time and keep your job. Still, this winter has flummoxed many seasonal weather forecasters on the West Coast, and although it may not cost them their jobs, it is certainly bringing to light our ineptitude with regards to seasonal forecasting. Forecast models were adamant that the Pacific Northwest would see drier-than-normal conditions throughout the winter, and that the Southwest would be wetter than normal, bringing desperately-needed rainfall to drought-stricken California. Instead, the Pacific Northwest had one of their wettest winters on record, and the southwest was actually drier than normal. Although farmers in our region aren't complaining, I'm sure that many in California are absolutely furious at meteorologists. I know I would be!

Credit: Western Regional Climate Center

In fact, since our October 1st, which is the beginning of our "water year," some places in the Pacific Northwest have received more than 20 inches above average of their precipitation for this period. In fact, many areas on the Olympic Peninsula have picked up over 100 inches of rain since then!

Meanwhile, Los Angeles and other areas in Southern California are running several inches below average. Talk about no rest for the weary!

Credit: National Weather Service Advanced Hydrologic Prediction Service
Credit: Western Regional Climate Center

So, what happened?

El Ninos generally give wetter-than-normal conditions to California after the New Year (there is little correlation before January 1st), and this is due to a large area of low pressure setting up in the Eastern Pacific, allowing the jet stream to sag southward into California. We had a wet October and November and a downright stormy December, and while an El Nino pattern did develop after the New Year, there was also a persistent ridge of high pressure over Southern California, pushing the jet stream back north into our region for extended periods of time and giving us relatively warm and wet southwesterly flow in the process. In the graphic below, the negative anomalies correspond to areas of lower pressure and vise versa.

Credit: NOAA Earth Systems Research Laboratory

"Astronomical winter" is defined as the period from the winter solstice to the vernal (spring) equinox, which is approximately from December 21st to March 20th, with the dates occasionally changing due to one calendar year not being exactly equal to the period of the Earth's orbit around the sun. However, most people would agree that Hanukkah feels more winter-ish than St. Patrick's Day, so we've come up with the term "meteorological winter" to define winter based on what the weather is like outside.

Meteorological winter is officially defined as December 1st to February 28 for the entire country. We had the wettest meteorological winter on record by far here in Seattle, with 24.54 inches of rain from December to the 28th of February (and 24.63 inches if you count February 29th), besting the record of 22.77 inches set in 1998-1999.

Cliff Mass, one of my atmospheric sciences professors at the University of Washington, defines our "meteorological winter" as the period from October 1st to March 1st, and while this is a fair bit broader than the previous definition, it makes sense, considering that these are the primary months that our storms come through. For this reason, October 1st is said to mark the beginning of the "water year," and boy oh boy did we have a lot of water this year. From October 1st to March 1st, we had 38.62 inches of rain in Sea-Tac, breaking the previous record of 38.19 (also set in 1998-1999). That's more rain than Sea-Tac usually receives in an entire year! Seattle has nearly 40 inches as of Tuesday afternoon, and heavy rains and flooding are predicted Wednesday night, with unsettled weather the rest of the week.

My personal definition of meteorological winter is from Veteran's Day to Valentine's Day, because it's nearly impossible to get significant lowland snowstorms here outside of those dates. Compiling statistics for my own "meteorological winter" definition would be extremely time-consuming and not very productive, but I'm willing to bet that 2015-2016 was also the wettest year for the Veteran's Day to Valentine's Day time frame.

An interesting thing to note was that although this winter was very wet, we didn't have a ton of record-breaking rainfall events. Instead, we simply remained in a rainy pattern for the entire winter and had relatively few dry spells.

Credit: University of Washington Atmospheric Sciences

Even though we've been wet, we've also been warmer than normal (though not nearly as warm as we were last year). As such, our snowpack is right around normal. But you can bet your bonnet that our streams and reservoirs are running high and that our soils are saturated. Salmon will have a much easier time getting up the rivers to spawn this autumn, and hopefully we will have less fire danger this summer due to increased soil and crop moisture.

In the meantime, long range forecasts are predicting us to be cooler and wetter for next winter as we head into a La Nina pattern! However, after this winter, I'm not sure whether I should ever believe the Climate Prediction Center's outlooks again.

Cliff Mass at the 2016 Pacific Northwest Weather Workshop talking about how bad our seasonal precipitation forecasts were this year. Credit: Logan Johnson

Thanks for reading!

No comments:

Post a Comment