Upper level shortwave upper trough brings weak winds to the Bay Area, Gorge and Southern California:
What in the world does this geek speak mean?
by Mike Godsey, mike@iwindsurf.com
Don’t you hate it when the forecast contains geek speak like “An upper level shortwave upper trough will increase the clouds and bring a chance of rain” Of course you don’t have to know what those words mean to use the forecast tables and bullets to get the wind picture. But understanding the basic concept helps you learn what it means at your favorite site when that phenomenon appears. For example savvy East Bay kiters and sailors know that when the forecast contains the phrase “there is a tiny surface eddy just west of the Golden Gate bridge” they should get ready for great Pt. Isabel winds.
Today an upper level short wave is delivering weak winds and scattered light showers to the San Francisco Bay Area, The Gorge and a Catalina Eddy and weak winds to Southern California.
So what is a shortwave anyway? On the average the winds from around ≈ 18,000 ft. feet all the way to the jet stream level go around the world from West to East. However most of the time there is not a solid mass of wind moving from the west. Rather the upper level winds weave a pathway around the earth with north and southward extending loops.
In the N. hemisphere a loop extending northward is called an upper ridge and has higher pressure within the loop.
A loop extending southward is called an upper trough and has lower pressure within the loop.
In the summer on the California coast upper trough tend to deepen the marine layer, bring cooler air and create southerly aloft as they approach us and NW flow as they pass over us. Upper ridges bring warmer weather and can create heat waves at the surface.
A shortwave upper trough is an especially sharp loop embedded in a larger scale upper trough. If the atmosphere is moist, as it is today, a shortwave can create lift causing moist air like you see in the green below to condense and form rain. That is why there is a chance of rain in todays forecast.
Above is a satellite video of today’s upper level short wave. First, to get oriented, study the still imagery on the right. Notice the wind flow indicated by the green arrows.
Now watch the video and watch the shortwave from towards the last few images. Note the sharp loop in the upper winds. This is an especially sharp shortwave so the wind direction is not typical.