Series of storms bearing down on SLO County, and it could last through Christmas
In a classic la Nina pattern, the storm track has been centered over the Pacific Northwest during much of the meteorological fall (September through November).
Consequently, a series of low-pressure systems and associated cold fronts/atmospheric rivers produced record-breaking precipitation throughout the Pacific Northwest. The Seattle-Tacoma Airport reported 19 inches of rain this meteorological fall, its wettest on record. Not to be outdone, Abbotsford, British Columbia, had the most with 33 inches of the wet stuff, smashing the previous records and causing extensive flooding.
On the other hand, the predominant weather feature off the California coastline this November was the Eastern Pacific High. In summer, the clockwise circulation around this high brings cooler air from the Gulf of Alaska, often producing a persistent marine layer along the West Coast that reduces the need for air conditioners.
Like summer, this fall has seen the Eastern Pacific High anchored off the California coastline, keeping the southern branch of the polar jet, the upper-level winds that bring in storms, well to the north.
Combined with the Eastern Pacific High, transitory high-pressure systems over the Great Basin — the area between the Sierra Nevada range to the west and the Rocky Mountains to the east — created persistent Santa Lucia northeasterly offshore winds.
These winds produced clear and dry skies with cold mornings and warm afternoons. In fact, many daily high-temperature records were broken during the final week of November.
Typically, the San Luis Obispo County Airport records 1.4 inches of rain in November; however, last month only saw 16 hundredths of an inch of rain. The Santa Maria Airport reported 11 hundredths or 10 percent of the historical November average. The Paso Robles Airport was parched in November with a meager one-hundredth of an inch.
The November temperatures were much warmer than average; the mean temperature at the San Luis Obispo Airport was 63.2, the second highest ever seen. The Santa Maria Airport saw a mean of 60 degrees or 3 degrees above the historical averages. The Paso Robles Airport was 5 degrees above normal, with a mean of 57.9 degrees. It’s interesting to note that the warmest November on record at these airports occurred in 1949.
This December, a significant shift in the weather pattern has taken place with the storm door swinging wide open. So, what does that mean?
The Eastern Pacific High acts like a gigantic roadblock, pushing the storm track further northward. This week, the Eastern Pacific High will shift 2,500 miles to the northwest of California, allowing the upper-level winds to move southward and take direct aim at California. In other words, the door is open to tempests.
These upper-level winds will carry a series of storms and associated cold fronts from the Gulf of Alaska into California on Monday into Tuesday, and perhaps longer.
This condition is forecast to produce moderate gale-force to fresh gale-force (32 to 46 mph with gusts to 55 mph), southerly winds, moderate to heavy rain, and significant mountain snowfall above 4,500 feet on Monday into Tuesday. Between 2 and 4 inches of rain is expected with higher amounts along the coastal mountains.
A colder trailing weather system will bring more rain and mountain snow on Thursday or possibly Friday.
Believe it or not, this weather pattern was predicted nearly two weeks ago by the new and improved version of the National Oceanic and Atmospheric Administration’s Global Forecasting (GFS) model, also known as the American model in the weather forecasting community.
This leads to the question, why did the Eastern Pacific High shift well to the northwest and opened the storm door?
Currently, nobody knows for sure. Perhaps the Arctic vortex split into three sections, helping to move the Eastern Pacific High well off the California coastline. This condition may continue to help steer storms into California.
Another low-pressure system is forecast to produce gusty winds, chilly temperatures, and low-elevation snow on Christmas Eve. However, there are no guarantees when trying to predict that far into the future.
Monday and Tuesday’s storm is expected to be the strongest of the season so far.
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John Lindsey is a meteorologist for PG&E.