Tuesday, April 7, 2009
The First Push of the Year
One of the key warm season weather features of the Northwest is the onshore or marine "push"---one of my personal favorites. These events occur following a period of above-average temperatures associated with high pressure inland and offshore flow. The high pressure subsequently moves inland and a weather disturbance approaches from the west, causing an influx of cool, often cloudy, air from off the Pacific.
You know what I am talking about...following a warm day, the winds strengthen and turn to the southwest. Leaves russle and wind chimes ring, and the air become perceptibly cooler. You got to love this. It is the natural air conditioning of western Washington, BC, and Oregon.
Well, a weak version is happening right now. We have had high pressure west of us, providing sunny skies, and warm, offshore flow..with the warming aided by descent down the western slopes of the Cascades (image). The offshore flow helps causing warming and pressure falls (warm air is less dense, thus lower pressure) and the establishment of a "thermal" trough over us (image). Then as the high pressure west of us weakens and a Pacific disturbance approaches, the low "jumps" into eastern Washington and cool air surges inland (image). You can see the cooling...yellows and reds (warm) replaced by greens (cooler).
I have a whole section on this feature in the book if you would like to learn more. One way to see the push happening is to look at the difference in pressure between the coast and the western interior...I like to check out the Hoquiam minus Seattle pressure difference (HQM-SEA). When that gets to around 3-4 mb, you know a major push is underway. You can follow the changing pressure gradients at http://www.atmos.washington.edu/marka/pd.cgi
Onshore pushes happen around a dozen times over a typical summer and usually are very well forecast. This is the first of hopefully many.
By the way, I will be giving a public talk on microclimates and horticulture at the Center for Urban Horticulture next week (April 16)....you can see the details to the right if you are interested...this is a luncheon sponsored by Dunn Gardens and the Seattle Horticultural Associaton.
PS: The governor should be commended for advocating a tuition increase large enough to preserve the core strengths of the state universities. To turn away students or throw away years of building of academic strength would be foolish for both the students and the state.
Posted by Cliff Mass at 7:36 PM