Small-scale weather features made this week’s lowland snow events especially hard to forecast | KNKX

Small-scale weather features made this week’s lowland snow events especially hard to forecast

Jan 17, 2020

Forecasting technology has come a long way since KNKX weather expert Cliff Mass first got his start in the field. He often marvels at how precisely most events can be predicted, using powerful computers that run ensembles of modeling programs that meteorologists compare before they decide what to tell the public. The aim is helping people prepare, especially for potentially dangerous weather.

But in the Northwest, snow – especially the kind that hit parts of Western Washington this week – is notoriously difficult to forecast. Mass agrees, this past week was a case study in that challenge.

“Some things we got right. For instance, snow in the mountains,” Mass says.

The predicted snow dump in the Olympics and Cascades did, in fact, happen – replenishing the reservoirs and snow pack that water managers rely on.  Mass says those levels went from 30-40 percent of normal a month ago to about 95 percent of normal now.

But the predictions for lowland snow – the kind that can shut down entire school districts and cripple a hilly city like Seattle – left many people who live here wondering: "what happened?" Both of the big snow events this past week seemed to stump the experts.

“Remember the forecast for temperatures a week ago?” Mass asks. “We were probably forecasting 5 to 10 degrees too cold,” he says. “We were going for really extreme cold values and it didn’t happen.”

They also got the timing and placement of precipitation wrong, he says. In the first instance, a notorious Puget Sound convergence zone went much further north than predicted, sparing most of Seattle from snow while Bellingham was pounded.

The second one, from Tuesday night into the early hours of Wednesday morning, described as a “snow band,” also went much further north and east than originally predicted.

Seattle officials held daily briefings, warning of the dangers of snow and urging people to be prepared. In the end, very little snow fell on the Emerald City.

“We were off by about 50-75 miles,” Mass says, referring to forecasts of where the edge of the snow band would land.

“This was a bizarre, very small-scale system,” he says. 

Mass says that’s the key thing that made this week’s forecasts so challenging, especially compared to last February, when the National Weather Service largely got the forecasts right. Mass says the features then were much larger scale, making the ensembles of computer models more accurate.

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.