Rainfall amounts so far this rain season have dropped to about 27 percent of average through mid-February in Hanford and 23 percent in Fresno. A “rain season” is defined as the 12-month period beginning July 1 that continues through June 30 of the subsequent year. A “water year” is defined by hydrologists as the 12-month period that starts Oct. 1 and continues through Sept. 30 the following year.
Not only has it been mostly dry since our last significant rain around Jan. 9, but also warm. In fact, combined day and night temperatures in the previous week have averaged 7 degrees warmer than typical at the Hanford Municipal Airport, and 9 degrees hotter at the Fresno Yosemite International Airport.
The combination of lack of rainfall and well above-normal temperatures has driven most of Fresno, Kings and Tulare counties into a D1 (Moderate Drought) condition, according to the United States Drought Monitor.
So what has occurred to produce this dry and warm weather pattern? Well, it appears that the “Ridiculously Resilient Ridge of High Pressure,” made famous by the previous drought, has redeveloped over California.
The current La Niña condition is wrapping itself around the equator. “The presence of a mild La Niña, the notorious ‘Diva of Drought’ present at the equator, doesn’t bode well for winter rainfall,” said climatologist Bill Patzert, of NASA’s Jet Propulsion Laboratory.
The Climate Prediction Center in their latest El Niño Southern Oscillation (ENSO) report states that Equatorial Sea Surface Temperatures (SST) are below average across the central and eastern Pacific Ocean. Consequently, a La Niña condition will continue through winter 2017-18, with a transition to ENSO-neutral (El-Nothing) during mid-to-late spring, which indicates below-average rainfall and above-normal temperatures for Central and Southern California through March.
So why is this year’s La Niña different from last year’s, which produced record amounts of precipitation in Northern California and above normal rainfall in much of the San Joaquin Valley?
During the 2017 (July 1, 2016, through June 30, 2017) rainfall season, the PDO was positive, and more importantly, there was a tremendous amount of warm water left over from the 2015-16 El Niño in the western tropical Pacific near Malaysia and to a lesser extent in the far eastern Pacific. If you remember, the 2015-16 El Niño event was one of the strongest ever seen; many oceanographers would say it was the warmest. This condition caused the Variable Pacific Jetstream, or the Southern Branch of the Polar Jetstream, to shift southward toward California, which brought one storm after another last year. This year, the reinvents of the Godzilla El Niño have dissipated into the depths of the Pacific.
Another cause that continues to gain interest was put forth several years ago by Dr. Jennifer Francis of Rutgers University. She hypothesizes that Arctic Amplification (the Arctic is warming at a quicker rate than the rest of the earth) will increase the amplitude of jet stream waves. In other words, the upper-level winds are more likely to travel in a more north to south pattern. You see, the polar jet stream often forms the border between the frigid air to the north and the warmer air to the south. As a rule of thumb, the higher the temperature differential between these two air masses the faster the upper-level winds will blow. The faster the jet stream, the more direct route it takes. The slower the jet stream, the more likely it will tend to change direction.
In California, the most advantageous pattern is for the upper-level winds to travel is a direct path across the Pacific to the coast; as the mid-latitude westerly winds increase the higher the likelihood of rain. Lately, the jet stream hasn’t had enough momentum (speed) to break through this ridge of high pressure to allow the mid-latitude westerly winds to carry storms to Central California.
On a positive note, the long-range models continue to advertise the possibility of periods of rain starting around Valentine’s Day and continuing through the end of the month. Of course, March can also produce massive amounts of rain, as evident by the heavy rains of 1995 but we are running out of time in the rain season. We can also hope for a series of Atmospheric Rivers (AR), Pineapple Express or, in the meteorological community, turning on the hose. Last year, ARs came through in a big way. These systems can transport ridiculous amounts of water across the Pacific. In fact, they can carry more freshwater than the Amazon River.