San Luis Obispo has gone nearly half a year without any significant rain, and it shows. The wild grasses turned to a golden hue months ago and now crackle and nearly pop with dryness as you walk over them. As this parched summer comes to an end with the autumnal equinox Sept. 22, many people have asked what will this year’s rain season bring.
Overall, the long-range weather predictions are notoriously inaccurate. Most weather models will give you predictions out no more than 10 days; however, what if you want guidance for months out? You can turn to the National Oceanographic and Atmospheric Administration, with its predictions about the El Niño/La Niña-Southern Oscillation. You can also turn to historical rain data.
On average over the past 50 years, we receive 24.4 inches of rain per season. According to this historical data, San Luis Obispo receives below 24.4 inches 60 percent of the time; only 40 percent of the time do we actually exceed this amount. With all things being equal, your chances of accurate long-range forecasting are greater if you predict below-average rainfall.
One of the key influences on our weather is El Niño and its sister, La Niña, which are triggered by changing conditions in the Pacific Ocean. NOAA recently predicted the current neutral condition — El Nothing — will transition to a weak or moderate El Niño later this year. This condition produces warmer surface seawater temperatures in the central and eastern equatorial Pacific.
Jan Null, a former National Weather Service lead forecaster and PG&E meteorologist, is recognized as an expert on La Niñas and El Niños and their relationship to California’s weather.
According to Null’s studies (see his website at http://ggweather.com/enso.htm), weak and moderate El Niños generally don’t produce any reliable seasonal rainfall predictions. However, strong El Niños, usually produce above-normal rainfall — typically about 140 percent of normal on the Central Coast as the storm track is shifted southward. On the other hand, weak, moderate or strong La Niña periods usually produce below-normal rainfall — typically about 87 percent of normal on the Central Coast — and even less in Southern California as the storm track is shifted northward.
However, 2010-11’s La Niña cycle serves as a reminder that not all such events are created equal. Most locations throughout San Luis Obispo County recorded above-normal rainfall that year. Diablo Canyon nuclear power plant had 28 inches, or 118 percent of normal.
In the 2011-12 winter, La Niña combined with another ocean water temperature cycle, the Pacific Decadal Oscillation, or PDO, to contribute to the below-average rain amounts along the Central Coast.
The PDO is found primarily in the North Pacific. The phases of the PDO are called warm phases or cool phases. Unlike El Niño and La Niña, the PDO stays in one phase for a much longer period.
Historically, the PDO seems to increase or decrease the El Niño and La Niña effect. Unfortunately, the cool phase of the PDO continues to lurk in the northern Pacific Ocean, which means it could cancel out the El Niño.
In other words, we’ll probably see below-average rainfall this year. But remember, even the driest of years will usually experience significant storms and the rain they bring. Remember, these are long-range forecasts and should be taken with a grain of salt. Only time will tell.
However, another wildcard in this equation is climate change. This past July was the hottest month ever recorded in the United States, even hotter than in the Dust Bowl of the 1930s. Long-range climate models continue to predict that droughts will become more prolonged and paradoxically storm systems more intense.
You see, as the atmosphere continues to warm, its capacity to hold greater amounts of water vapor will continue to increase. Water vapor is one of the primary mechanisms in storm development. As it condenses on microscopic particles as precipitation, it releases tremendous amounts of latent heat that help to intensify storm systems. Long-range climate models continue to predict that droughts will become more prolonged, and storm systems more intense. Overall, climate change will probably make it more difficult to predict the long-range weather.
This morning’s coastal low clouds will quickly clear from the beaches and coastal valleys, leaving behind mostly clear and sunny skies.
An approaching upper-level trough of low pressure will produce strong to gale-force (25- to
38-mph) afternoon northwesterly winds along the beaches and cooler afternoon maximum temperatures along the beaches, coastal valleys and the North County.
Creston reached 102 degrees Saturday.
Today’s high temperatures will be in the low 70s along the westerly facing beaches (Pismo Beach, Grover Beach and Cambria).
The southwesterly facing beaches (Cayucos, Avila Beach and Shell Beach) will reach the mid-70s, while the northwesterly facing areas (Morro Bay, Los Osos, Montaña De Oro and the Nipomo Mesa) will see high temperatures range from the low to mid-70s.
The North County will reach the mid-90s, while the coastal valleys will reach the high 70s.
Decreasing winds and a deepening marine layer will develop Monday into Wednesday, as this trough of low pressure parks itself along the California coast. This condition will produce night and morning coastal clouds, fog and areas of drizzle and cooler temperatures throughout the county.
Temperatures will warm across all areas Thursday into Friday as the high pressure builds across the state.
Today’s surf report
Strong to gale-force (25- to 38-mph) northwesterly winds will generate 4- to 6-foot northwesterly (320-degree deep-water) seas (with a 4- to 7-second period) today through Monday.
A 970-millibar storm developed in the northern Gulf of Alaska on Wednesday. Longer period swell from this storm will continue to arrive along our coastline through Monday at 4 to 5 feet (with a 12- to 14-second period).
A 4- to 5-foot northwesterly (310-degree deep-water) swell (with an 8- to 13-second period) is forecast along our coastline Tuesday and Wednesday, decreasing to 2 to 4 feet (with an 8- to 11-second period) Thursday through Saturday.
Arriving from the Southern Hemisphere
Not much Southern Hemisphere swell activity is expected through Wednesday. A 1- to 2-foot Southern Hemisphere (205-degree deep-water) swell (with a 16- to 18-second period) will arrive along our coastline Thursday, followed by a higher-energy swell Sept. 18.
Seawater temperatures will range between 53 and 55 degrees through Monday.
Seawater temperatures will rise to 55 and 57 degrees on Tuesday, increasing to 56 to 58 degrees Wednesday through Friday.• • •
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.