I received a few emails about last week’s subtropical moisture and the sprinkles it brought. We live in a Mediterranean climate, which normally means dry summers and wet winters. In fact, San Luis Obispo County only receives about a third of an inch of rainfall during the July, August and September timeframe — regardless where your location may be along the Central Coast.
The majority of summertime precipitation is caused by the North American monsoon. Sure, it may not be as glamorous as its Asian counterpart. Nevertheless, it can produce large amounts of rain in northern Mexico, much of Texas, New Mexico and southern Arizona. On rare occurrences, it can produce heavy rain in Southern and Central California also.
So, what atmospheric condition causes this subtropical moisture to stream northward into California? Normally, when the desert southwest heats up in summer, it creates a thermal low. This low-pressure zone can change the direction of the jet stream, which steers subtropical moisture northward toward the Central Coast.
This phenomenon is part of a seasonal pattern called the North American monsoon. When this occurs, we often experience higher relative humidity levels and plenty of virga — observable streaks of rain that fall from a cloud but evaporate before reaching the ground.
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If the subtropical moisture source is plentiful — especially when remnants of hurricanes and tropical storms from the eastern Pacific Ocean and the Gulf of Mexico move over our area — periods of rain and thunderstorms can interrupt our Mediterranean climate.
This happened in August 1976, the wettest August on record. Two subtropical weather systems produced nearly 1.5 inches of rain in San Luis Obispo between Aug. 15-20.
Later in September, Hurricane Kathleen developed in the eastern Pacific and took an unusual path northward through Baja California. It crossed the U.S.-Mexico border near El Centro, east of San Diego, as a tropical depression. Kathleen produced gale-force southerly winds and widespread flooding in many parts of the west, especially in California’s Imperial Valley.
The day before this storm reached San Luis Obispo, temperatures reached 99 degrees in San Luis Obispo and 101 degrees in Paso Robles. Subtropical moisture from Kathleen produced 1.72 inches of rain in San Luis Obispo on Sept. 10 and 11.
As the summer progresses, the chance of receiving this type of rainfall becomes greater until it peaks during the month of September. San Luis Obispo County is located on the fringe of the North American monsoon. Rains associated with this weather phenomenon are seldom destructive.
As the atmosphere and oceans continue to warm, long-range climate models are predicting the North American monsoon will become more common in our state in late summer. In fact, recent studies have already identified a shift in seasonality of the monsoon to September/early October with increased amounts of rain late in the monsoon season.
Of course, the increased amount of rain is welcome; however, lightning accompanying these events can spark wildfires. PG&E is serious about working with Cal Fire and other state and local agencies to do our part to reduce that risk — including helping educate our customers about fire safety, operating our own equipment responsibly and being prepared to respond. To learn more about safety, please visit http://www.pgecurrents.com/