Saturday, April 25, 2009
A Cloudy Day West, But Sun East
Some people saw some fleeting sunshine this morning..but for most of western Washington, marine cloudiness will rule. Take a look at the visible satellite image this morning. Mostly cloudy from the Pacific to the crest of the Cascades--but generally sunny in eastern Washington. The spring is a great time for a trip east of the Cascades...generally sunny there and the temperatures are quite nice. In winter eastern Washington tends to cool and cloudy, while summer is hot. But grapes like the climate, so I won't complain. The only negative perhaps for eastern Washington this weekend will the winds in Ellensburg and vicinity...but they are used to it.
The current radar shows only a few light, showers ... and most of the them should be on the western slopes of the Cascades. There is a suggestion of a very weak convergence zone north of Seattle..which will probably strengthen later today as an upper trough moves through the region (see satellite image). That upper trough was the initiator of the change this morning.
One indicator of the influx of marine air is the difference in pressure between the coast and the western interior..and the Hoquiam-Seattle difference is probably the most valuable for the Seattle area. (see attached shot of the UW pressure difference web page from last night, times are in UTC/GMT...0000 GMT is 5 PM). If you look carefully, you will see that that difference was negative yesterday (higher pressure inland)...which prevents the marine air from moving in. But that changed later in the afternoon as the pressure difference went positive (marine air moving in). You will also notice the difference across the Cascades (Seattle-Yakima) went very positive....thus air from western Washington is being pushed to the east in any gaps (like the one upstream of Ellensburg). During the summer these pressure differences are crucial for short-term forecasting.Sunday should be better..with more sun and temps getting to around 60 in the lowlands.
Posted by Cliff Mass at 8:25 AM