A complex of thunderstorms developed over the northeast corner of Washington and north Idaho by mid-afternoon on August 12th. The thunderstorms developed ahead of a cold front pushing southwest over the continental divide. These thunderstorms moved into the Spokane area after 330 pm, producing winds to 50 mph, dime-sized hail, and over 25 fire starts (from lightning and downed power lines).

The thunderstorms lost much of their strength as they passed south of the Spokane area, however the event was far from over. This thunderstorm cluster, combined with a push of cooler air from the cold front, generated a massive outflow boundary which affected most of eastern Washington later that afternoon.

An outflow boundary is formed as cooler air from the mid-levels of a thunderstorm (or thunderstorms) is mixed down to the ground. This cool and subsequently heavier airmass then spreads out ahead of the thunderstorms and can potentially spawn additional thunderstorms. While the occurrence of outflow boundaries is not itself unusual, the scope and aerial coverage of the one on August 12th was remarkable.

The animation to the right depicts a loop of visible satellite pictures from approximately 415 PM until 700 PM This animation clearly shows a massive outflow boundary spreading radially west and south away from the cluster thunderstorms over the Spokane area and north Idaho. Spotter reports and weather sensors indicated that winds behind this feature were easily blowing up to 40 mph.

This outflow boundary was initially detected clearly by the Spokane Doppler radar. The animation to the right shows this boundary surging southwest from western Spokane and eastern Lincoln counties. The bright red and pale yellow colors contained in this image represent winds moving away from the radar at speeds of 30-40 mph. Also notice the light blue and purple colors over northern Spokane County. These correlate to incoming (or northeast ) winds of 30-40 mph.

This boundary was detected until it passed through the Ritzville area. Although the outflow was still going strong, the radar was beginning to overshoot the feature. The approximate height of the lowest radar beam over the Ritzville area is around 4000 feet.

As the outflow moved into the Ritzville area, it began to pick up an appreciable amount of dust from the open fields to the north..

The first image captured at 520 PM from a nearby webcam shows an ominous wall of dust approaching the area from the north to northeast.

Twelve minutes later, the image showed how thick this dust really was. Nearby visibility sensors indicated that visibilities along I-90 were reduced to a quarter mile or less. 

Images courtesy of WSDOT

The image to the right shows the approximate location of the outflow boundary as of 6pm.

While visible satellite imagery is useful for detecting cloud cover, it typically cannot show areas of blowing dust due to poor reflective qualities.

This wall of dust was different however, perhaps due to its significant magnitude and depth. It also indicated that the wall of dust would soon enter the Moses Lake area.

The following images clearly illustrate the scope of this feature as it eerily pushed into the Moses Lake area. The weather observation equipment located at the Moses Lake Airport clocked winds gusts to 45 mph and visibilities as low as a quarter mile.

The outflow boundary continued to push westward through 700 PM bringing more dust and wind to the Wenatchee and Ellensberg area. It also was able to produce isolated showers and thunderstorms northwest of the Kittitas Valley. The boundary began to weaken during the remainder of the evening as it was unable to pass over the Cascade Range.

Top Image courtesy: David Dorman
Bottom Image courtesy: Chris & Julie Verkerk