UW scientists explain mid-April snow

Western Washington was hit with record low temperatures and unseasonable lowland snowfall this past weekend.

Maximum and minimum daily temperatures have been less than normal since April 1, and snowfall has been seen on at least two occasions in the past week.

Cold temperatures have not been restricted to Western Washington. On the morning of April 21, the temperature in Great Falls, Mont., dropped to minus 8 degrees F, breaking the previous record low temperature by 19 degrees.

“To have set a record in that fashion this late in the month is really extraordinary,” said Cliff Mass, a UW professor of atmospheric sciences.

Scientists pointed to two main factors influencing the unusual weather: cooler than normal surface ocean waters in the tropical Pacific and the persistence of low pressure along the Pacific coast of North America.

“The whole Northern Hemisphere has been cold,” Mass said. “We’re really seeing the influence of the strongest La Niña since 1998 and a persistent trough of low pressure over the Northwest.”

La Niña is the regime name given to a pattern of cooler than normal sea surface temperatures (SST) across the equatorial Pacific Ocean. Its converse, El Niño, is defined by warmer than normal SSTs, and is well known due to an extreme occurrence which occurred in the winter of 1997.

Both phases of the phenomenon influence weather and climate globally, manifesting various effects dependent on time of year and geographic location.

“Historically, La Niña conditions favor above normal precipitation for [Washington state] with cooler than average winter temperatures around Western Washington,” said Josiah Mault, assistant state climatologist of the Joint Institute for the Study of the Atmosphere and Ocean. “However, with the long-term trend, La Niña merely offsets the warming trend.”

Despite fallen records and lowland snow in late April, this year’s temperatures and precipitation statewide remain close to normal, according to the National Climatic Data Center.

JISAO state climatologist Philip Mote said people’s perceptions of 2008 as unusually cool and wet are somewhat inaccurate.

“March in Seattle was 1.6 degrees F cooler than average, but still only the 18th coolest out of 65,” Mote said. “What was unusual was the number, and certainly lateness, of the events favoring lowland snow.”

The forecast for Western Washington features more of the same in the short-term, but there are signs of milder weather on the horizon.

“It will be unsettled and cool for the next few days,” graduate student Robert Hahn said. “But higher pressure should build into the region early in the weekend, providing for clearing skies and warmer temperatures.”