It’s April, but SLO County could still see decent rain and frost

Typically, the month of April brings roughly a third of the normal rainfall of March, while May only collects about one-tenth of March’s averages. Paso Robles typically receives 0.68 of an inch in April, while San Luis Obispo records 1.11 inches and Santa Maria averages 0.98 of an inch.

But in April 2020, we may get a whole lot more. A pair of cold fronts will move southward down the California coastline and reach the Central Coast on Sunday.

The first cold front will move through San Luis Obispo and Santa Barbara counties Sunday morning with fresh to strong (19 to 31 mph) southerly winds and moderate to heavy rain. The second front will follow Sunday evening into Monday morning with moderate gale-force to fresh gale-force (32 to 46 mph) southerly winds and even more torrential rain.

Total rainfall on Sunday into Monday is expected to range between 1.50 and 2.25 inches. An upper-level trough will follow Tuesday into Wednesday with convective rain showers and a chance of thunderstorms with hail. Overall, this storm system could easily double the usual amount of rainfall we typically receive along the Central Coast during April.

Even though we’re in spring, some of the most severe storms I’ve seen have occurred during April, and here’s why.

The longer days and increasing amounts of sunlight heat the earth’s surface, which in turn warms the surface air and causes it to rise into the atmosphere. This convection circulation can severely destabilize the atmosphere as the relatively warm air slams into the cold and moist air above, which can produce violent tempest.

Here are some examples of late-season storms:

On April 2, 2014, in the middle of the night, the Doppler weather radar at Vandenberg Air Force Base indicated a thick red line of severe weather over Cayucos moving southeast through the Chorro Valley toward San Luis Obispo.

Later that morning, I received photographs and emails from numerous residents in Cayucos, documenting 1-inch in diameter hailstones. This band of thunderstorms may have produced a weak tornado at Camp San Luis.

On April 7, 1926, a storm from the Pacific moved through the Los Osos Valley and produced lightning. The lightning struck large oil tanks along Tank Farm Road in San Luis Obispo. Altogether, more than 5 million gallons of oil burned over five days.

Burning oil made it to Avila Beach by way of San Luis Obispo Creek. Intense heat from these fires produced hundreds of fire whirls — many of them showed characteristics of actual tornadoes.

Not to be outdone by severe weather, freezing temperatures can rear their ugly heads at this time of year. The mere mention of Jack Frost in spring can cause many to look for cardboard boxes and old sheets to cover their newly sprouted plants. Vineyards are reporting bud break, and the young shoots are vulnerable to frost damage.

Here’s the scary part: Frost can develop when the air temperature is as high as 38 degrees Fahrenheit. This happens because many things cool faster than the air that surrounds them, such as blades of grass, vines, car windshields, and roofs on buildings. When these objects reach 32 degrees, water vapor in the atmosphere will accumulate as ice crystals through a process called “deposition.”

According to the UC Cooperative Extension, the last frost of the season in Paso Robles historically occurs April 7 — with the last hard frost by March 5. A hard frost happens when the temperature drops below 28 degrees.

Closer to the ocean, San Luis Obispo will usually have its last frost by Feb. 15. These dates hold true at our beaches as well. Historical temperature data from the Santa Maria Public Airport indicates a 27 percent chance the temperature could hit 32 degrees by April 1, falling to a less than 10 percent chance by April 18.

Lompoc drops to a 10 percent chance of a spring freeze by April 3, according to the Western Regional Climate Center. A few days later, in April, most of the Santa Ynez Valley drops below a 10 percent chance of a freeze.

In the far-eastern regions, Cuyama has to wait until April 20 for the last frost of the season.

However, like spinning a roulette wheel, these are only average dates. It’s not unusual for locations such as Cuyama or Windrose Farm near Creston to see frost as late as early May.

To get a sense of the actual probabilities of dropping below the freezing level before you plant your garden, the Western Regional Climate Center has information at wrcc.dri.edu/Climate/west_coop_summaries.php. It provides climate data for communities throughout the United States.

This week’s storms could cause power outages. PG&E is committed to delivering safe and reliable power to each of our customers every day. We continuously work to safeguard our electric system to minimize the risk of service interruptions.

During storms, when outages may occur, our crews and emergency response teams do everything they can to keep the lights on. When storms result in an interruption to your service, we do our best to restore service as quickly and as safely as possible.

Take steps now to stay safe in the event of a power outage:

• Keep a battery-operated flashlight and radio within easy reach. Ensure those items are always accessible and that your batteries are fresh. Listen for updates on storm conditions and power outages.
• Use safer LED candles. Wax candles are not recommended.
• Plan for another way to communicate. Don’t depend on a phone that requires electricity to communicate. Keep a standard handset or mobile phone ready as a backup.
• Store water-filled plastic containers in your freezer. You can use them as blocks of ice to prevent food from spoiling in your refrigerator.