If you plot Seattle’s cumulative daily rainfall across 12 months, you will see two hills spaced between a plain. As every Seattleite knows, most of our rain falls between January and June (that first uphill on the graph) and then the clouds disappear (that flat plain) until October comes and we close out our year with more consistent rain (the second hill on that graph of rain).
With yesterday’s rain, we’ve officially hit that second uphill. The rain is falling and it isn’t likely to end anytime soon. Joe Zagrodnik, my favorite atmospheric science PhD candidate at University of Washington, published an interesting post to his blog recently showing how our recently-departed dry season was actually one of the driest on record.
After a particularly rainy April, including 1.7 inches of rain on April 14 alone (I still remember that rainy, rainy week), the sun came out and precipitation ran away. Only 3.49 inches of rain fell at Sea-Tac during the six months between April 1 and the end of September, according to Zagrodnik. The average during that time period was 9.42 inches, also according to Zagrodnik.
If you remove that particularly wet early April and just count the five months between May 1 and the end of September, then we just experienced the driest five-month stretch on record, according to King5. This is when we made that distinct flat plain on our cumulative rainfall graph, as we inched through the summer with almost no rain. This year’s flat plain was especially flat. Record-breakingly flat. Like, as flat as the top of a mesa.
But was Seattle actually as dry as a desert during these months? According to Zagrodnik, we were—and we were even drier than Salt Lake City and Phoenix during this time period.
But now we've abandoned the dry, flat months of summer and are climbing our hill of cumulative rainfall. I asked Zagrodnik how much rain we should expect in the next few weeks now that we have entered our rainy season. This Tweet was his response, which shows that every single day for the rest of the year has better than a 50 percent chance of being rainy.
Here's an annual probability of exceeding various rain thresholds in Seattle. In late October it's a 50/50 chance of measurable rain on any given day and about a 10% chance of a really rainy day like today (>0.50" likely today). @leddder @ScottSKOMO @KSeattleWeather pic.twitter.com/YCYCAFd9Sz
— Joe Zagrodnik (@joejoezz) October 25, 2018
I’m going to go grab my umbrella.