Sunday, May 2, 2010
A Weather Odyssey Across the Cascades
Yesterday morning there was a cool, unstable air mass over western Washington in the wake of a Pacific front. In western Washington, a cool day with lots of clouds, scattered showers, a Puget Sound convergence zone, and rain on the western slopes beckoned. At roughly 9 AM, the above cam shot at the UW showed the dismal scene (top image). But the satellite image showed another option (see below).
East of the Cascades, as the air descended down the terrain their was profound drying of of the air mass , with sunny skies and some warmth , even though the air mass was fairly cool. My favorite place in such situations is the Vantage/Quincy area... a low region near the Columbia River that is typically sunny under westerly flow. A look at the Silica Road WSDOT cam was promising (see image). The WSDOT cams and weather observations are a real treasure trove for travelers, by the way (http://www.wsdot.wa.gov/traffic/weather/)
My family and I were off around 10:30 AM heading across the Cascades on I90. Dry and cloudy until roughly North Bend, and THEN the weather went bad. With a little lift the atmosphere was destabilized, producing growing cumulus and showers, some very heavy. The weather radar showed some of these windward showers (see below).
Moving up towards Snoqualmie Pass (see cam) it was pretty miserable (see image). 43F and pouring. The showers held on across the Pass and only let up around Easton. But then the sky brightened and east of Cle Elum it started to open up.
But as we moved towards Ellensburg, something else was noticeable--gusts were pushing my car around--and that was expected. With deep cool air moving in from the west, a large pressure difference formed across the mountains. With strong downslope flow and warming east of the Cascades, the pressure had dropped over the eastern Cascade slopes (called a pressure trough) and a large pressure difference was accelerating the flow eastward (see pressure and temperature forecast for 2 PM below). The lines are isobars, lines of constant pressure, and you will notice quite a few of the lines across the mountains.
And I knew that such situations produce another, somewhat unwelcome, feature--strong winds, particularly in the Kittitas Valley area near Ellensburg. There is a reason there are lots of wind farms there. There is a weakness in the Cascades...called the Stampede Gap--and when there are large pressure differences across the Cascades, the air is able to accelerate through this gap into eastern WA (see my book for more information on this). The National Weather Service even had a wind warning up.
When we got to Vantage, there were whitecaps on the river, but the sun was glorious. We headed north a bit (getting off I90 at Silica Road) and stopped for a nice lunch at Cave-B winery/inn near the Gorge Amphitheater (http://www.sagecliffe.com/Inn.htm). Really good view of the river from their restaurant and then we hiked down to the Columbia from the inn. Beautiful day and views, but it was windy! Here is a video I made with my camera:
click on the arrow!
The air temperature was nearly 65F, with winds around 20 kts, with higher gusts, particularly near cliffs or promontories in the terrain. The ground was warm, wild flowers were all over, and the for the trip back to the Inn we passed by a nice waterfall. And then we stopped at their winery's tasting room on the way out. Sun, wind, scenery, the fragrance of sage, and wine....not a bad way to spend the day.
But then there was the two-hour drive back into the clouds.
On the return, the winds had clearly strengthened (typical, due to heating east of the Cascades), with justs to 40 mph near Ellensburg, and a wall of clouds was waiting for us west of Thorp, near the eastern slopes of the Cascades. In addition, lots of dust was rising off some fields east of Ellensburg...some farmer was losing quite a bit of top soil from his/her newly plowed fields. Loss of topsoil is a major problem in eastern Washington and duststorms have caused serious, and sometimes fatal, accidents.
PS: A strange situation is happening right now (Sunday). Rain is spreading over the region...but NOT over Seattle--we are being rainshadowed by the Olympics! The reason...the unusual combination of westerly flow aloft and a wet system.
PSS: As noted in the side bar, I will be talking about local windstorms on Tuesday, at 7:30 PM at McHugh's Irish Bar in Seattle. Shows what I am willing to do for a beer!
Posted by Cliff Mass at 7:04 AM