Chasing Central California
by Ken Nakamura
Storm Track, November 30, 1987
© Copyright 1987 Ken Nakamura
Many people think that the central valley of California never gets storms of significance, as it's a desert region with less than 15 inches of annual rainfall. However, nearly every year we get spectacular thunderstorms with hail up to golfball-size, 60 mph winds, and bursts of rain near 1 inch in 15 minutes. Occasionally, a funnel or tornado occurs within a 50 mile radius of Fresno.
The central valley of California is about 75 miles wide and aligned southeast to northwest. The plain is surrounded by mountains. To the west, there is a coastal range between 2000 and 5000 feet high (ASL). To the south, mountains rise 6000 to 8000 feet. The Sierra mountains border the east side of the valley and rise to over 12000 feet. On the northwest end of the valley is the Sacramento Valley.
Most of the chase days here occur in the spring months between February and May when strong winter type storms approach from the west and pass through the valley during the late afternoon. A few chase days occur in the fall and winter months. Few chase days occur in the summer months as a large high pressure system blocks storms system approaching from the west.
The typical chase day begins with a steady rain. I look at the satellite photographs for general cloud cover over most of the state followed by an abrupt clearing along the coast (to the west). I particularly look for an area of convective clouds around an offshore low pressure center. Other important features are: l) a strong jet stream, 2) colder than average 500mb temperatures, 3) warmer than average surface temperatures, and 4) forecasted high temperatures between 65 and 75 degrees Fahrenheit.
If most or all these conditions are met, all I do is wait. Hopefully, the sky begins to clear from the west by 10 a.m. taking the rain with it. By noon, I like to see the sky mostly clear with scattered small cumulus, and thunderstorms beginning to form over the coastal range. Usually, the surface wind at this point becomes calm and temperatures reach a maximum. The air becomes oppressive, a good sign. Soon, a solid squall line forms and sweeps through the area with strong winds, intense lightning, and heavy rains. As the leading edge of the shelf cloud passes overhead, it becomes dead calm, and the underside of the shelf cloud becomes a sight to remember. Dark green, purple, gray, and yellow mingle and boil with turbulent mammatus like clouds. Then the cold downdraft follows, frequently dropping temperatures into the 40's. Sometimes, it snows! The visibility can drop to a few feet with hail or snow piling six inches deep. Strong winds can topple trees and push cars and trucks off the road.
The best storms are multi-cellular which develop explosively west through south of Fresno. Within thirty minutes, a line of medium cumulus with tops to 15000 feet, become full fledged thunderstorms up to 50000 feet. The anvils generally stream toward the northeast, but the line moves to the east-southeast. Regeneration occurs on the southwest end and occasionally cloud rotation develops. Masses of scud are drawn into the cloud base. This southern storm can produce wall clouds, ragged gray funnels, and tornadoes (see below).