Published Tuesday, April 9,  2002 in the San Jose Mercury News


Thunderstorms, tornadoes brew in central states

Special to the Mercury News
As the storm track along the West Coast weakens, the number of rainy days is diminishing in the Bay Area -- and the longer days mean warmer temperatures. But along the Gulf Coast and in the Plains, weather patterns are aligning for the beginning of the thunderstorm and tornado season.

These massive thunderstorms pose a threat to the region with strong downdraft winds, tornadoes, hail, lightning and flooding from copious cloudbursts.

The key meteorological ingredients in the recipe that forms the intense thunderstorms over the central states are the warm, moist surface air streaming north from the Gulf of Mexico and, above this, drier air carried along by southwesterly winds.

The combination creates a very unstable atmosphere in which huge thunderstorms can tower more than 12 miles high.

Even though tornadoes that are sometimes spawned by these massive storms get most of the press, they are not the deadliest thunderstorm hazard. In the past 10 years, there has been an average of 57 tornado deaths per year in the United States, while floods from thunderstorms have accounted for about 70 deaths per year.

Lightning has caused an average of 55 fatalities per year, followed by thunderstorm winds at 31. Hail causes an average of one. The real hazards from hail are crop and property damage, amounting to more than $750 million per year.

To follow the latest thunderstorm activity or to find out more about thunderstorms and tornadoes, check out  on the Web.

Q Why does the Weather Channel, in the local weather update, have a space to list both wind speed and gust speed, but never shows a gust? It seems to me that they used to record them some years ago, and a quick poll among my friends says there has not been a recorded wind gust for a number of years. If the Weather Channel insists that gusts do not exist, they can come to my place. I live about three miles from the airport, and I can show them some gusts, especially this time of year. William Lynch - San Jose

A The omission or at least infrequent reporting of wind gusts may be the result of the new Automated Weather Observing Systems that the National Weather Service and the Federal Aviation Administration have deployed at Bay Area airports.

The old way to determine gusts required a human observer to pick the highest instantaneous wind readings from either a digital or analog display during about a 10-minute period when there were fluctuations of 11 mph or more.

However, the new method samples and averages the wind speed over a five-second period. It records gusts only if the two-minute average wind speed is 10 mph or greater and the gust exceeded it by 6 mph or more.

Consequently, there will be far fewer gusts recorded, especially in the South Bay where winds tend to be lighter than farther north near San Francisco International Airport.

Q I thought the vernal equinox was the time when the sun shines exactly 12 hours on Earth. That happened several days before March 20. How is the equinox determined and why isn't it a 12-hour day? Al Legasa - Los Gatos

A This continues to be one of the questions asked most often of the Weather Corner. The differences have to do with the methodology used to calculate the equinox vs. methods to calculate sunrise and sunset.

The equinox occurs when the geometric center of the sun's disc crosses the equator. On this day, the center of the sun is above the horizon every place on Earth for 12 hours.

But sunrise is calculated from the time the upper portion of the disc first becomes visible. Sunset is timed when the upper edge of the sun's disk slips below the horizon. Therefore, the length of day and night aren't exactly equal on the equinoxes.

In the Northern Hemisphere, the date of equal day and night is several days before the March equinox and several days after the September equinox. Consequently, night and day are equal around March 16 and Sept. 26 at 36 degrees north latitude, near San Jose.

Setting the record straight

The correct amount of water used in the Bay Area in a year is 7,115,000 acre-feet, not 7,115 acre-feet. Consequently, the total number of gallons used is 2.3 TRILLION. And, finally, an acre-foot is 325,851 gallons.

Jan Null, founder of Golden Gate Weather Services is a retired Lead Forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif., 95190. You also can send questions via telephone (510-657-2246), fax (510-315-3015) or e-mail