WEATHER CORNER

BY JAN NULL
Special to the Mercury News

With the first, albeit wimpy, storm of the season behind us, it's time to consider what the upcoming winter may hold for California and the Bay Area.

Unlike the past four years, when El Niņo or La Niņa conditions have existed, we enter this winter with neutral conditions in the tropical Pacific.

Without one of these strong climate features to influence the forecast, most long-range forecasters are indicating a winter with near-normal temperatures and precipitation for California.

However, this does not mean there may be no extreme wet or cold events during the season. Historically, about half of the major flood events in the state have happened during El Niņo-neutral years.

For example, the last major California floods came during New Year's in 1997 -- when there was no El Niņo or La Niņa.

A key element to flooding in California is when a ``pineapple express'' occurs. This is a weather pattern in which the jet stream dips far enough south to bring warm, moisture-laden air from near Hawaii to the West Coast, resulting in copious rainfall for the Golden State.

Recent research has identified something called the Madden-Julian Oscillation as being a factor in these events.

The MJO is a surge of moisture and thunderstorms that migrates periodically over the equatorial waters from the vicinity of Indonesia to near the international date line over 30 to 60 days. When these surges coincide with a dipping jet stream, a pineapple express results for California.

During El Niņo-neutral years, there is a slightly greater incidence of MJOs and consequently a slightly greater risk of a pineapple express.

Q  I was recently in San Diego, and at dusk the locals said it looked like a night when there would be a green flash as the sun set.

Sure enough, at that instant when the sun went below the horizon, I saw a green flash. Was this a trick played on my eyes or do certain conditions create this green flash?  Ed Daly - Cupertino

A  What you are describing is fairly common in some parts of the country. For it to be seen, the atmosphere needs to be fairly clear and still and the observer must have an unobstructed view of the horizon. Consequently, it's often seen over the ocean, but the green flash can be seen over land if the horizon is far enough away.

As the sun sets, sometimes the last bit of light from the disk itself is an emerald green. The same is true of the first bit of light from the rising sun. While it typically lasts only a second or two, on rare occasions it can be longer.

This flash of green is the result of looking at the sun through the great thickness of atmosphere. Water vapor in the atmosphere absorbs the yellow and orange colors of white sunlight, and it scatters the violet light. That leaves red and blue-green light to travel directly toward the observer.

Near the horizon, the sun's light is highly bent, or refracted. It almost appears as though there are two suns, a red one and a blue-green one, partly covering each other. The red one is always closer to the horizon, so when it sets or before it rises, you see only the fleeting edge of the blue-green disk.

Q  My daughter is doing a report on the United States and has been asked to find out what the average rainfall and climate for the United States is.

I can seem to locate it only per city or state, but not for the whole country.

Do you know where I could find this information?   Diane Bartoschs - Westminster

A  This is an interesting question. While it can be answered, no single value for the rainfall is going to be very meaningful. But here are a few considerations.

First, because of the vastness of the United States and its varied geography, a great number of distinct climates across the continent have annual average rainfall varying from less than 5 inches in the deserts of the southwest to more than 150 inches in parts of Alaska and Hawaii.

Second, there is a much higher density of weather-reporting locations from which averages would be computed in the eastern United States, so figures would be more skewed by these areas as opposed to the relatively small numbers that represent the vast drier spaces of the West.

A simple average of about 300 of these weather stations yields a nationwide average of about 37 inches of rain per year.

However, if an area-weighted average were to be done for the entire United States, it would be about 30 inches.