Yes, we need rain, but it’s not time to panic

It’s been an odd winter rainfall season for the Bay Area.

It was nearly bone dry until Thanksgiving. We had a wet late-November through December period, followed by a rather dry January and a record dry February. A stubborn high-pressure jet stream pattern has directed Pacific storms well north of California since mid-January.

In this case, the record dry February means no measureable rainfall for the entire 29-day period. San Francisco, which has rainfall data extending back to 1849, has only experienced one other completely dry February – in 1863. Normal February rainfall is just less than 4½ inches at San Francisco, representing almost 20 percent of the annual expected rainfall.

Winter dry spells more than 10 days in length are common in our corner of the world. But the recent dry period exceeded 35 consecutive days, making it the fourth longest on record. The longest winter dry spell is 43 days, most recently occurring during the winter of 2013-’14.

The dry weather wasn’t limited to the Bay Area. Recent rainfall and snowpack measurements show that not much precipitation has fallen recently in the California mountains.

As of March 1, the Northern and Central Sierra rainfall indices are 51 and 38 percent of normal, respectively. The most recent Sierra snow survey indicates the snow water equivalent in the Northern Sierra is 43 percent of normal, 25 percent lower than the early February survey measurements.

Thanks to a few recent wet winters and improved water management techniques, the latest reservoir storage data is not as discouraging as the rainfall and snowpack data. According to the Department of Water Resources, California’s largest reservoirs are either near or above their historical capacity averages for this time of year.

Several people have asked if we are headed toward another drought. The most recent U.S. Drought Monitor graphics show most of the Bay Area as “abnormally dry,” or drought category D0. Coastal sections of the Bay Area are classified as “moderate drought,” category D1. We are a long way from the worst-case drought scenario, “exceptional drought,” category D4.

We have a few months remaining in the rainy season, so there is time for the stubborn jet stream ridge to break down and let rainstorms reach the Bay Area. As of this writing, the medium range forecast models show a few potential storms on the horizon.

When you read this, the record winter season dry spell should have ended. It is unlikely, however, that we will recover from the rainfall deficit of the last few months before the hot and dry summer season begins.

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