: Cliff Mass Weather Blog This blog provides updated forecasts and comments on current weather or other topics Monday , August 12, 2013 Heavy Thunderstorm Rain Closes State Route 20 The eastern slopes of the Cascades are susceptible to very heavy thunderstorm rains during the summer and on Saturday night it happened . again On Friday through Sunday , the region was covered by a moist , relatively unstable air mass that brought strong thunderstorm activity and lots of lightning . nbsp The proximate cause was an upper level low that was slowly moving up the coast , resulting in southeasterly flow in the lower atmosphere see 500 hPa map , roughly 18,000 ft , for Saturday at 5 PM below As the moist air approached the eastern slopes Washington Cascades it was forced to rise and instability was