The Wettest March in Seattle History and Stillaguamish Precipitation

The rain is mainly over now for the remainder of the month here in western WA, so I can analyze the precipitation totals with some confidence.

Seattle has had the wettest March in its historical record.

As of 5 PM last night, there was 9.44 inches in the gauge at Seattle-Tacoma Airport, more than TWICE the normal March monthly total of 3.72 inches.   This is the all time monthly record for March at this location, for a record going back into the late 19th century (at the airport until 1948 and downtown Seattle before that)

At the National Weather Service office in north Seattle (Sand Point) they also had their monthly record for March (9.22 inches), for a shorter observing period (going back to 1986).

My lawn has turned into a carpet of moss and my deck is now green. 

The plot of precipitation versus normal (yellow line) for 2014 shows the story (upper plot below).  Seattle was quite dry from mid-January to mid-February, but then the pluvial gods took over for the next month.

Many of the stations around Puget Sound are having a top-ten precipitation total for March, but the amounts appears less unusual than Seattle at places like Olympia and Bellingham.

Many of the heavier precipitation events over the region were associated with atmospheric rivers, plumes of moisture appearing to project northeastward out of the subtropics.  The image below shows vertical moisture totals and was provided by Sheldon Kusselson of NOAA/NESDIS.  You see the fingers of high moisture values (generally green) heading right towards us?  When that moisture interacts with our mountains, heavy precipitation results.

Naturally, there is substantial interest in the precipitation that fell on the mountains around the landslide area near Oso, Washington.   As I mentioned in a previous blog, perhaps the most relevant location is the Finney Creek RAWS site (see map, oval shows the landslide, symbols the observing location) in the terran about 7 miles to the NNE of the landsldie

Here is the precipitation at Finney Creek since the beginning of the year.  After a dry stretch it started to rain again around Feb 12th, with a big ramp up the first two weeks of March.

Here is the hourly rainfall and cumulative total in March.  About 22 inches this month, and some periods of intense rainfall (hourly totals exceeding .3 inches an hour). 

There is not a long record at this site, but 2007 was almost as wet (20.04 inches),

Deep-seated landslides, such as the Oso slide on March 22rd, respond more to longer-period rainfall rather than heavy rainfall over hours or a day that result in many shallow slope failures.  Why the slope near Oso failed during this particular period is something that will undoubtedly be studied by geomorphologists more knowledgeable than I am in such matters, such as UW's Professor David Montgomery.