Picture courtesy of Justin Sharp
The snow was associated with a low pressure system that crossed the southern Oregon coast and headed eastward VERY SLOWLY south of Portland. To illustrate this, below is a short-term forecast map for 10 PM Wednesday, showing sea level pressure (solid lines), lower tropospheric temperatures (shading), and near surface winds (barbs). With the low south of Portland, cold air moves into the northern Willamette Valley from the north and through the Columbia Gorge. Furthermore, there was a zone of confluent flow just north of the low (called a deformation zone), which produces a band of upward motion and precipitation. It was a combination of cold and upward motion that produced the snow.
A radar image at 5:30 AM Wed. morning shows the modest precipitation with this low and the center of the circulation (curved radar echoes) between Salem and Portland.
Forecast models indicated snow, but nearly enough over Portland. Here is the 24hr snow total forecast ending 4 PM Wednesday for the forecast started at 4 PM Monday. A few inches over Portland, but much more to its west and southeast.
Modest errors in the structure, position, and movement of the low pressure area and the associated deformation zone resulted in this forecast error.
And even the ensemble forecasts (running the models many times) had difficulty with this prediction. For example, the NWS SREF forecast initialized at 1 AM Tuesday for snow accumulation at Portland Airport (which got around 7 inches) showed a lot of uncertainty, with totals ranging from zero to 8 inches and an ensemble average of 1.5 inches.
These ensemble forecasts indicated a possibility of heavy snow, but suggested it was improbable.
Can we do better in such a difficult event? I believe so. But a big issue is that the system came off the Pacific Ocean, where we have less near-surface information. A coastal radar, such as the one recently placed on the Washington coast (the Langley Hill radar), would undoubtedly have helped the short-term forecast (0-12 hr), allowing folks more time to prepare. Improvements in our models and how we use observations (data assimilation) could also have helped.
Folks in Oregon need to tell their political leadership that placing a powerful weather radar on the central Oregon coast is important and requires priority. Quite honestly, it could pay for itself many times over in even one storm like this.
The Oregon Coast Needs One of These!
Northwest Weather Workshop
The Northwest Weather Workshop, the region's main gathering to discuss all aspects of Northwest weather, will take place on March 3-4, 2017 in Seattle at NOAA Sand Point. There will be a special session of communicating forecast uncertainty during the first day. More information on the meeting, as well as registration details, are found at: https://www.atmos.washington.edu/pnww/
If you are interested in giving a talk at the meeting, please send me a title and short abstract by February 1.