Midwinter dry spells are not uncommon.
In the January 1900 edition of the “Monthly Weather Review” published by the Weather Bureau (forerunner of National Weather Service) is an article about dry spells.
Author Mr. F.H. Brandenburg says, “Since the distribution of barometric pressure, which brings about dry weather, is generally widespread, therefore these dry spells often prevail simultaneously over extended areas.”
Brandenburg compiled a list of dry spells 20 days or longer throughout each season, including midwinter dry spells. Surely, agriculturalists and others who observed weather patterns across the western United States at that time already knew that midwinter dry spells are not uncommon.
Recently, Jan Null, a former National Weather Service lead forecaster and PG&E meteorologist, posted an interesting story on his website, ggweather.com. According to Null’s studies, over the past 65 rainfall seasons (July 1 through June 30) in San Francisco, there has been a “dry” period in December or January that averaged 20 days. A dry spell, according to Null, is defined as consecutive dry days with no rain, or consecutive days broken by no more than two nonconsecutive intervening days with rain of 0.08 inches or less.
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Using these parameters and based on rainfall data from Diablo Canyon Power Plant, last year’s midwinter dry spell persisted for 51 days. January 2015 was nearly completely dry. The winter of 2014 was even worse with a dry spell of 55 days. In fact, only one-hundredth of an inch of rain was recorded that January.
Historically, very strong El Niño events shorten the length of these midwinter dry periods. For example, the last very strong El Niño event in 1997-98 experienced a 14-day dry spell that started Dec. 19 and continued through New Year’s Day.
This midwinter’s dry spell was 13 days, beginning before Christmas and continuing through Jan. 3. So far, this month and the January of 1998 have followed a similar pattern of rainfall. If you remember, by the end of January 1998, many wondered if all the media attention leading up to that year’s El Niño was justified.
Then February 1998 arrived, and the storm door swung wide open. The mid-latitude westerly winds at the surface and in the upper levels of the atmosphere dramatically increased and brought a series of storms that marched across the Pacific through the Central Coast. Rain gauges throughout the Central Coast recorded rainfall nearly every day through Feb. 24. Overall, 15.6 inches of rain was recorded that month at Diablo Canyon.
In a classic El Niño signature, the southern branch of the polar jet has shifted southward this January and has brought a series of low-pressure systems to the Central Coast. The longer-range models are still indicating that the jet stream will shift farther south this February and March, and with it, the promise of heavier rainfall.
To sum up this week’s Weather Watch column, I couldn’t agree more with Null’s quote at the end of his article: “The bottom line is that an extended period of dry weather is the rule and not the exception in the middle of almost every winter as the atmosphere settles into a two- to three-week period of equilibrium dominated by high pressure.”
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The last major El Niño storm season in 1997-98 created widespread flooding and caused power outages impacting more than 1 million PG&E customers. PG&E has been preparing for storms like those, while urging its customers to be ready for natural disasters. That includes having a family emergency plan and keeping emergency kits for your home, your office and your vehicle. PG&E offers emergency-preparation tips on its website at www.pge.com/en/safety/preparedness/index.page.