Text by the Seattle forecast office of the National Weather Service
Depending on which day on the calendar you point to, the weather of 2011 was unseasonably wet, unseasonably dry, unseasonably cool or unseasonably warm. There was an unusual number of low-temperature and precipitation records. At Sea-Tac International Airport, April was the coolest ever. Spring was also one of the wettest on record. And yet we finished the year with less precipitation than normal.
Last year began with one of the strongest La Niñas of the past 60 years in full swing, and 2011 finished with La Niña returning — for only the fifth time in consecutive years since 1950. La Niña years typically bring active winter weather. As if to throw us off, though, there was also relatively tranquil weather last January and again in December.
The year began with a pause in La Niña and relatively mild weather in January. Temperatures were about 1 to 2 degrees above normal. Then La Niña conditions returned in February. Temperatures were between 3 degrees and 5 degrees below normal.
La Niña hung on through the spring, with temperatures well below average and precipitation above normal. Sea-Tac International Airport saw the coldest April ever. And the combined April-May precipitation at the airport was the third-wettest ever.
The wet spring also resulted in one of the latest winter-season flood events at the end of March and in early April. The Snoqualmie River at Carnation recorded major flood stage on April 1, the latest ever.
Many felt we did not have summer, that it was cool and wet. Yet with the exception of the north coast of Washington, June, July, August and September were all below normal for rain. June and July were between about 1 degree to 1.5 degrees below average. August was close to average, and September was quite warm, with average temperatures running between 2 degrees and 3 degrees above normal.
With La Niña returning, temperatures turned cooler in November and December. Precipitation was close to normal for October and November, but there was well-below-normal precipitation in December. In fact, December was of the driest on record. Thanks to the dry December, many locations in Western Washington ended the year with below-normal precipitation.
Highlights and lowlights of 2011
Jan. 13-19: A period of rainfall resulted in flooding and a landslide that affected a segment of the Mountain Loop Highway east of Granite Falls. Flooding on the north fork of the Skykomish River washed away a cabin near Index. The Snohomish River reached major flood stage. Other flooding rivers included the Carbon, Cedar, Chehalis from Grand Mound downstream, Green, Newaukum, Nisqually, Nooksack and Puyallup near Orting, Satsop, Skagit, Skokomish, Stillaguamish, Skookumchuck, Snoqualmie and Tolt.
March 30-April 3: More heavy rainfall produced one of the latest winter season flood scenarios on record on the Snoqualmie, Skykomish, Snohomish, Tolt and Stillaguamish.
Nov. 22-24: A period of heavy rainfall of up to 10 inches in the mountains produced the first flood scenario of the fall/winter season. Rivers that exceeded flood stage included the Stillaguamish.
Feb. 22-23: Snow fell over much of western Washington. Amounts varied from one to 10 inches with Mount Vernon reporting 16 inches.
March 10-17: A period of rainfall helped create a landslide that damaged four homes on a slope near Snohomish.
Given the healthy mountain snowpack during the winter of 2010-11 and the cool, wet spring, the snowpack was slow to melt in 2011. Paradise on Mount Rainier on Aug. 15 still had 44 inches of snow — the highest ever on that date.
Sept. 25-26: Strong southerly winds blew along the coast, the north interior and the Puget Sound region. Winds up to 75 mph were recorded in Whatcom County. Nine thousand households lost power in Snohomish County.
Extreme high pressure
Dec. 1: Sea-Tac Airport recorded the highest atmospheric pressure on record going back to 1948. The barometer hit 1043.4 millibars, or 30.81 inches, of mercury, breaking the previous record set in 1949.
More information about local climate
National Weather Service, Seattle: www.wrh.noaa.gov/sew
Office of the Washington State Climatologist: www.climate.washington.edu
University of Washington Department of Atmospheric Sciences: www.atmos.washington.edu