"Pacific Northwest Winter Storm"
23 November 1999

On November 24, 1999, an early winter storm moved onshore in the Pacific Northwest. The storm brought heavy precipitation, while the associated cold front brought strong winds to the coasts of Oregon and Washington.

The north Oregon coastline reported high winds November 24, as Canon Beach reported wind of 55-60 knots with gusts to 69 knots, while Arch Cape reported gusts to 69 knots. The northern Oregon coast reported up to 10 inches of rain, causing flooding and landslides. The Nehalem River rose to 5 feet above flood stage, and the Siletz River rose to 16 feet above flood stage. The governor proclaimed a state of emergency for Lincoln and Tillamook counties, as property damages were estimated at $4.7 million.

Synoptic Overview

Following the passage of a shortwave midday on the 23rd, the flow aloft became westerly and strengthened until 21Z 24 November, and remained strong through 26 November. A strong jet max moved onshore on the Canadian/Washington border at 00Z 25 November, with most of the cloud cover ahead and on the anti-cyclonic side of the jet max.

The Salem, Oregon 00Z 24 November sounding shows saturation at the surface but a very dry layer between 700-500mb. Winds up to 500mb are generally weak and veering. This changes rapidly as the 12Z 24 November sounding shows a rapidly diminishing dry layer, and deep layer of saturation. Winds are increasing in all levels as jet strengthens and jet max approaches. Winds are veering, indicating warm-air advection.

Strong warm-air advection can be seen at 700mb. Compare the 700mb heights and temperatures at 01Z 24 November and 22Z 24 November. Warm-air advection is an important ingredient in precipitation formation in zonal flow in the Northwest.

Much of the precipitation that fell was due to orographic effects. Compare the nowrad image from 1515Z 24 November with the topographic map of Washington here. Later, the precipitation intensified and spread inland in Washington and Oregon by 2020Z 24 November.

For more information on precipitation development in zonal flow over the Pacific Northwest, read the interesting introduction to this lab exercise.

Case Study 26