In the last few days of December, an atmospheric model initiated at the Fleet Numerical Meteorology and Oceanography Center in Monterey advertised the potential for the jet stream to split as a blocking high developed over the Aleutian Islands.
The southern branch of the jet stream was predicted to shift southward near Hawaii and start to steer a significant surge of subtropical moisture from the Pacific Ocean toward California, resulting in a condition referred to as an atmospheric river or Pineapple Express.
A few days later, both the European and NOAA’s Global Forecast System models suggested that the Central Coast would receive between 4 and 10 inches of rainfall from Jan. 4 through the following week and produce the most rain the area has seen since Dec. 2010.
Like clockwork, the first cold front swept in from the Pacific and created persistent southerly winds and continuous rain starting Jan 4. In 24 hours, the PG&E Energy Education Center along Highway 101 in Avila Valley recorded 5 inches of rain, the most precipitation in a 24-hour period since Dec. 18, 2010. Rocky Butte reported 8.72 inches, while Arroyo Grande received 3.47 inches as the uninterrupted supply of subtropical moisture was drawn toward California. Since that Wednesday, nearly every day saw measurable rainfall through Thursday. This deluge has brought a few San Luis Obispo County locations near or above their average annual rainfall for this rain season, which runs from July 1, 2016, to June 30 of this year.
Debby Mix of the Circle 3 Ranch located in the San Simeon Watershed between Vulture Rock and Rocky Butte has meticulously logged 48 inches so far this season. Rocky Butte on Thursday measured in at 40 inches of total rainfall; that’s an inch above its average annual total of 39 inches. Dawn Dunlap at the Walter Ranch above Cambria reported 31 inches. Further South, Cal Poly, home of climatology for San Luis Obispo, has recorded 15.5 inches. Chris Arndt with SLOweather.com in western San Luis Obispo is presently at 16.6 inches, or about 175 percent of normal. The Paso Robles Airport has recorded 7.4 inches so far; their annual total is 12.5 inches. Diablo Canyon Power Plant is currently at 16.3 inches; the average amount of rainfall at this time of the year is 8.5 inches. In the South County, the Lake Lopez ranger station is at 19.2 inches, Arroyo Grande 14 inches and Nipomo 16.7 inches, all above seasonal averages.
Needless to say, this rain has increased reservoir and lake levels. Nacimiento Lake, which sits entirely in San Luis Obispo County, and Lake San Antonio, which is just north of the SLO County line, saw significant increases in their levels. Since Jan. 3, Nacimiento’s percentage of capacity went from 25 percent to 56 percent as of Friday, the highest levels since 2012. Lake San Antonio went from 6 to 14 percent as of Friday. Large watersheds feed both of these lakes, but Nacimiento Lake, as a rule, will fill up about three times faster than Lake San Antonio due to the larger size and proximity of its watershed to the Pacific. However, different amounts of precipitation that fall in our notoriously complex San Luis Obispo County microclimates can play havoc with this rule.
According to data from SLOCountyWater.org, here are the other San Luis Obispo County lake and reservoir percent of capacity figures as of Friday: Lopez Lake is at the 28 percent, Salinas Reservoir in Santa Margarita 31 percent and Whale Rock Reservoir near Cayucos at 45 percent.
According to the U.S. Drought Monitor, the plethora of Pacific storms has removed more than 40 percent of the state from any category of drought. Essentially, all areas north of San Francisco were declared to have no signs of abnormal dryness. Locally, most of San Luis Obispo County went from D4 (Exceptional Drought), the most severe category, to D3 (Extreme Drought).
Looking ahead, the high-pressure ridge responsible for the dry weather this weekend will shift southward in response to a trough of low pressure over the eastern Pacific. Vigorous upper-level winds will draw in another plume of subtropical moisture, the atmospheric river, by Wednesday night. This condition will produce gale-force southerly winds and periods of heavy rain into the following week.
Readers have asked me, “Why all the rain now?” There’s a passage in John Steinbeck’s “East of Eden” that does a pretty good job describing California’s rainfall patterns.
“The water came in a 30-year cycle. There would be five to six wet and wonderful years when there might be 19 to 25 inches of rain, and the land would shout with grass. Then would come six or seven pretty good years of 12 to 16 inches of rain. And then the dry years would come ...”
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