California anxiously awaits rainy season

For the second consecutive year, the beginning of the California Water Year has been excessively dry.

The lack of early fall precipitation is a primary factor contributing to the extreme wildfire fire conditions that continue into November.

When will the seasonal storm systems finally reach the Bay Area? Like many weather forecast questions, jet stream wind patterns hold the answer.

A jet stream is a band of strong winds in the upper levels of the troposphere, the lowest layer of earth’s atmosphere. The fastest winds in the jet are concentrated 6-10 miles above the surface of the earth.

The two distinct jet streams in the Northern Hemisphere are known as polar and subtropical. Although the subtropical jet can sometimes affect California weather, our day-to-day weather conditions are usually associated with the polar jet stream pattern.

Due to geography, the earth’s rotation and differential surface heating patterns, polar jet winds follow wave-like trajectories as they circle the globe. A jet stream ridge/trough couplet can extend for thousands of miles.

As a general rule, jet stream ridges are areas of downward air currents, high pressure at the earth’s surface and dry weather. Jet stream troughs correspond to lower surface pressures, upward air currents and wetter weather.

Although the winds inside the jet stream can exceed several hundred miles per hour, the wave pattern itself typically progresses from west to east very slowly. The corresponding winter weather pattern under a moving polar jet stream wave would be periods of wet and dry weather.

There are times, however, when the jet stream waves remain stationary over parts of the globe. This type of ridge is known as a blocking ridge.

For the last several weeks, a stationary jet stream ridge has been parked over the West Coast – blocking potential storm systems from the Pacific from reaching us. As a result, Bay Area rainfall totals for October and the first part of November are near zero.

The bad news is that wildland fire dangers will persist until the rains come. The good news is that a dry beginning to our rainy season does not mean that the entire winter season will see below normal rainfall.

Winter rainfall statistics have been kept at San Francisco since the mid-1800s. For the 30 driest years as of Nov. 1, more than half ended up with greater than 90 percent of seasonal normal rain by time the water year ended.

Last winter’s local rainfall statistics are a good example of the fact that a dry beginning to the wet season does not predict a dry winter. Even though barely a drop of rain fell before Thanksgiving last year, above normal rainfall was reported for nearly all the remaining wet season months, and water year totals were about 110 percent of normal.

The entire state is waiting for the winter rains to end the fire season. Weather forecasters are keeping a close eye on the polar jet stream. As soon as the blocking West Coast ridge weakens and begins to move, the rainy relief will be on its way.

Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to
clayton_909@yahoo.com

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