Weather Blog

Seattle's 'natural umbrella' comes through

Seattle's 'natural umbrella' comes through
(Updated this story to add in the rain totals from Tuesday night and where we stand for August rain.)

Northwesterners are known for not really needing an umbrella despite the 160-some-odd days of rain year. That's because the rain is usually pretty light, but this time, it might just be because nature is providing one for us.

As a weather system blew in Tuesday night, it brought about 1-2 inches of rain to the Olympic Mountains, but only a small fraction of that to the Puget Sound area.

Why?  It's thanks to the Olympic Rain Shadow.

Sequim residents know all about this -- this is the feature that makes them the driest spot in Western Washington at just 18" of rain a year -- even drier than Flagstaff, Arizona. Typically, our storms come in from the southwest. As that air ramps up the southwestern side of the Olympics, the air is forced up, where it cools and condenses -- in essence, the mountains wring out the atmosphere there.  That's the foundation for the Olympic Rain Forests, which get about 220" of rain a year -- the wettest spot in the lower 48 states.

But once that air crosses the summit, it then sinks down the northeastern side of the Olympics. Sinking air dries as it does so, leaving a dry hole over Sequim and parts nearby.

You can see a good chart of the rain shadow in action here. This is an annual rainfall map of Washington, with yellows and oranges less and blues and purples more.



Note the much lower rainfall totals on the Northeastern Olympic Peninsula.

But also note that it's bit lower around the Puget Sound area than, say Olympia. Seattle gets 37" of rain a year, while Olympia is closer to 50".

That's because sometimes we "borrow" the rain shadow. When a storm comes from the west or northwest, that sinking air slot moves east/southeast over the Puget Sound area.

And that was the case Tuesday night. Here is a rainfall map for the storm:



Note the lower totals around Seattle (Sea-Tac), Tacoma and Bremerton, who were shadowed by the Olympics.  The way the wind was oriented put the shadow's edge right around the Rainier Valley because spots in the city of Seattle (Sand Point: 0.36"; Bellevue 0.39") had more.

A few other notes: (Updated Wednesday morning)

The Everett area gets more of the rain shadow effect than Seattle -- on days when the shadow is in effect, Everett frequently gets much less rain than the Emerald City. But! That is counter-balanced by Everett being home to the Puget Sound Convergence Zone. Enough to where Everett gets about the same amount of rain as Seattle on average per year, but each city will accumulate their rain in different clumps.

Also, as of midday Wednesday, Seattle had 2.87" of rain in August -- good for 5th wettest. With showers in the forecast for Thursday and Friday, we might eke into 4th by the end of the month, but there doesn't appear to be enough heavy rain to knock us into top 3 territory before the end of the month.

Top 6 Wettest Augusts:

1) 4.59"  1968
2) 4.58"  1975
3) 3.59"  1977
4) 3.00"  2004
5) 2.87"  2008
6) 2.71"  1976

Correction: I originally stated that the wettest August was 1967 and said it had the very strange distinction of not only being the wettest August on record, but also the hottest with an average high temperature of 83 degrees.  Oops! My eyes deceive me. In reading the chart, I went off a line. 1968, not 1967 is the wettest August. '67 was the hottest August, though.