Many factors in Tahoe’s dry summer
September 20, 2005
Prediction: Mild autumn ahead
Much of the discussion around town this summer has concerned the relative dryness so far this season and with that the absence of our summer thunderstorms.
Many people have asked why it has been so dry this summer and what the upcoming fall season, which begins Thursday, will bring.
To answer the first question, consider first a few facts concerning the precipitation South Lake Tahoe typically experiences during the summer months. June, July and August are frequently the driest months for our location, averaging approximately 1.4 inches total.
That is 4.5 percent of the average annual precipitation, 30.68 inches. From June 1 through Aug. 22, 1.11 inches of precipitation had fallen, of which 0.54 inches fell in June, a trace fell in July and 0.57 inches in August through Aug. 22.
Typically, eight days receive measurable precipitation from June through August. To date, seven days have had measurable precipitation with five days being in June. The unusual aspect of the summer has been the lack of thunderstorms, with so far only two days reporting thunder for all three months. The average for June through August is 15 days and 19 for the entire year.
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The absence of thunderstorms this summer results from a few factors. The first is the position of the four corners high, which is simply a big bubble of warm air that develops at around 18,000 feet over the Desert Southwest during the summer months.
So far this year, that high has been suppressed farther south and east along with being oriented on a northeast to southwest axis. That means the prevailing wind flow over our region has been from the southwest, which, during the summer months is a very dry flow for us.
The second factor is the position and strength of the Gulf of Alaska low, also centered at near 18,000 feet. This is a pool of cool to cold air aloft. That has been stronger and located farther south off the coast of Western Canada and Southeast Alaska for much of the summer, also keeping our flow from the southwest. The combination of the previous two results in the last factor, the location of the summer “thermal trough,” which is simply an area of low pressure at ground level where winds converge and the air rises.
When the thermal trough is located along the Sierra, it allows the air to rise and thunderstorms to develop. When the thermal trough shifts east into Nevada, the west winds blow and thereby “kill” any clouds that try to develop into thunderstorms.
For the most part, the thermal trough has remained in central Nevada this summer, and therefore the consistent west winds each day help keep it dry. One interesting note is that the same weather patterns that keep our region dry in the summer months also serve to keep our area unsettled during the fall, winter and spring months. Patterns that keep our area dry during the fall, winter and spring months keep us stormy during the summer months.
As for the outlook for the upcoming fall season that begins Thursday, the official outlook from the Climate Prediction Center calls for equal chances of temperatures being either above average, average or below average with the same for precipitation. This is for the rest of September through November. More specifically for our region, my feeling is that the Tahoe area should enjoy a milder and perhaps slightly drier than average autumn season. In my next column, I will address what kind of a winter the Eastern Sierra including the Lake Tahoe Basin may experience.
– Simon Smith is a co-op observer for the National Weather Service in Reno and lives in South Lake Tahoe.
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