The winds have been constantly blowing out of the south and it’s been raining off and on since Wednesday. In fact, Rocky Butte near San Simeon has recorded more than 5 inches of rainfall. Cal Poly’s home of record for climatology had recorded nearly 2 inches of the wet stuff as of Saturday.
Not only has it been drizzling, sprinkling or raining since Wednesday, but air temperatures have been mild with daytime highs in the 60s with overnight lows in the 50s. Along our rugged coastline, these persistent southerly winds have created a northerly flowing ocean current and warmer seawater temperatures. Medium to high westerly swells and short period southerly seas have also persisted along the coastline.
The culprit is a large 964-millibar storm that set up shop in the Gulf of Alaska. This intense midlatitude cyclone was more than 1,000 miles across! If you have looked at the surface charts over the last few days, the isobar lines resemble tree rings stretched across much of northeastern Pacific, a large storm indeed!
This parent storm has spun out a series of smaller-scale low-pressure systems that have tapped into abundant subtropical moisture from the southwestern Pacific. The storms have produced remarkable rainfall totals north of San Luis Obispo County. As of Saturday, more than 8 inches of rain had been recorded at Mining Ridge along the Big Sur coastline.
This wet weather pattern couldn’t be more different than what we experienced last December. If you remember, December 2011 went nearly dry. Only 0.18 of an inch of rain was recorded at Cal Poly.
Normally, San Luis Obispo receives 3.79 inches of rain during December. Paso Robles was even drier,
only recording 0.07 inches of rain. Historically, Paso Robles receives 2.02 inches of rainfall.
This December, the Climate Prediction Center in Maryland is predicting above-normal temperatures and below average rainfall for San Luis Obispo County. So far, the county has started out on good footing with these recent rains. Actually, by tonight, a few Central Coast locations could reach near-normal levels of rainfall for the entire month of December.
This leads to a question: Where does all this water go? If the rainfall is light, say less than a tenth of an inch, most of it evaporates back into the atmosphere, from which it can return another day.
Heavier amounts of rain will soak into the ground, where it’s absorbed by plants and then transported back into the atmosphere by transpiration. An amazing amount of water transpires from the leaves of plants. A single large oak tree in our county can move 100 gallons or more of water into the air in just one day during the summer. An acre of corn gives off about 3,000 to 4,000 gallons of water each day.
During heavier storms, a lot of the water is absorbed by the ground and goes through a process called infiltration. This filtered water recharges aquifers, which can later be pumped from wells and used for industrial, agricultural and residential use. This water also seeps into creeks and streams and provides a base flow. However, impervious surfaces such as paved highways, parking lots and rooftops, even during light rain events, can produce storm runoff which can carry oils and other pollutants into creeks, lakes and eventually the ocean.
If the ground is saturated after a series of storms, much of the new rain runs off, adding to the flow of streams.
In hydrology, there is a concept called “time of concentration.” Basically, it is the time needed for a raindrop to fall on the highest point in the watershed and make its way to the watershed outlet.
Jill Ogren, a registered civil engineer with Hydraulic Planning of San Luis Obispo County Public Works Department, calculated that a raindrop falling in the San Luis Creek headwaters at the top of Cuesta Grade would take about 3.6 hours to reach Avila Beach. The time of concentration in Arroyo Grande Creek for a raindrop to go from the farthest point in the Lopez Watershed through the Lopez Reservoir and then down the creek to the ocean would be approximately 10.5 hours. This information is vital in determining the time for a creek or stream to reach its peak flow and aids in flood preparation efforts in the county.
As of last week, Lopez Lake was at 76 percent of capacity, while Margarita Lake was at 58 percent of capacity. Lake Nacimiento and San Antonio were much lower, at 34 and 48 percent of capacity, respectively. It will be interesting to see how much these numbers go up due to this weekend’s rain.
According to the United States Geological Survey, an average of some 70 percent of the annual precipitation returns to the atmosphere by evaporation from land and water surfaces and by transpiration from vegetation. The remaining 30 percent eventually reaches a stream, lake or ocean.
The westerly swell peaked Saturday at 13 feet with a 14-second period.
A 9- to 11-foot west-southwesterly (260-degree deep-water) sea and swell (with a 5- to 13-second period) is forecast along our coastline today, decreasing and shifting out of the west-northwest (280-
degree deep-water) to 7 to 9 feet (with an 8- to 12-second period) Monday.
This west-northwesterly swell will further lower to 3 to 5 feet (with a 9- to 11-second period) Tuesday and will remain at this level through Wednesday.
Strong to gale-force (24- to 38-mph) northwesterly winds will generate a 4- to 6-foot northwesterly (300-degree deep-water) sea and swell (with a 5- to 11-second period) Wednesday afternoon through Saturday.
The last in a series of Gulf of Alaska storm systems will move westward toward the Great Basin. The associated cold front will reach far northern San Luis Obispo County later this morning. This cold front will pass over San Luis Obispo between 2 and 6 p.m., with moderate to heavy rain and strong to gale-force (24- to 38-mph) coastal southwesterly winds. Rain will continue through tonight, ending by Monday morning.
This storm will tap into subtropical moisture from the southwestern Pacific, and rainfall amounts will be impressive, with 1 and 3 inches across the low elevation locations and 3 to 5 inches of precipitation in the mountains. Snow levels will be high and generally above 7,000 feet.
In Northern California, the threat of flooding exists due to the several days of rainfall that have saturated the ground. The Russian and Napa rivers are at flood stage with an additional 3 and 5 feet of rise forecasted to occur with today’s storm, which is expected to result in flooding for the communities of Guerneville and Saint Helena.
A weak ridge of high pressure is then expected to build over California on Monday with mostly clear skies and night and morning gentle offshore winds. Little change is expected Tuesday.
A weak front will produce increasing clouds Wednesday but rain is not expected. A ridge of high pressure will then begin to build over the region Thursday, bringing fair and dry conditions through next weekend.
Gentle to moderate (8- to 18-mph) southeasterly winds will continue through this morning.
These fresh to strong (24- to 31-mph) southwesterly winds will develop late this morning and will peak at strong to gale force (24- to 38-mph) levels this afternoon, decreasing tonight.
A pattern of gentle northeasterly (offshore) winds developing during the night and morning hours, shifting and increasing to moderate to fresh (13- to 24-mph) levels during the afternoon hours, will start Monday and will continue through Tuesday. Strong to gale-force (24- to 38-mph) afternoon northwesterly winds are forecast for Wednesday through next Saturday.
Preparing for power outages
• Have battery-operated flashlights and radios with fresh batteries ready.
To report a power outage, call 800-743-5002.
If you have any questions or comments about weather or this column, I would love to hear from you. You can also subscribe to my daily weather forecast by emailing me at PGEweather@pge.com.
John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and longtime local meteorologist.