Weather Watch

SLO’s above-average rain season is practically over. But what about thunderstorms?

The combination of three sevens (777) on slot machines stereotypically pays the biggest cash prizes; and this January, February and March averaged around 7 inches a month at Cal Poly’s Irrigation Training & Research Center rain gauge on the north side of campus.

This triple-seven combination drove the station’s rainfall season totals above average for the entire year. Thank goodness it did, for April turned nearly dry.

This April saw the least amount of monthly rain since September 2018. Despite a parched April, this rain season that runs from July 1, 2018, through June 30, 2019, has been better than average for most Central Coast locations.

Rocky Butte has reported nearly 50 inches of rain; it typically receives 40. Cal Poly is over 27 inches; the annual average for that station is 22. The Paso Robles Airport is now over 14 inches but usually gets 12.5. Much like the Paso Robles Municipal Airport, the Santa Maria Public Airport receives about 13 inches each rain season but so far has seen nearly 15.

As of May 4, SLOWeather.com in western San Luis Obispo is at 124 percent of average. Often, April is a transition month — not quite summer and not quite winter. You may have stormy weather one day followed by record-breaking heat the next. This April was anything but with mild temperatures and mostly dry skies.

So how dry was this April?

Typically, Cal Poly receives 1 1/2 inches of rain but recorded less than a quarter of an inch last month. The Santa Maria airport averages about 1 inch for April but has only received a little over a quarter of an inch. Drier yet, the Paso Robles airport has only seen about a 10th of an inch.

The wettest April on record occurred in 1880 when 8.78 inches of precipitation was recorded in San Luis Obispo, followed by 1967 when nearly 7 inches of rain hit the ground. As recently as 2017, over 2 inches of rain was recorded at Cal Poly. Historically, April usually marks the end of the wet months along the Central Coast.

You see, the month of May averages less than half an inch of rain at nearly all San Luis Obispo and Santa Barbara county locations. However, like so many other things in nature, there are notable exceptions, and here is why.

On May 22, 2006, a cold front stalled over the Central Coast and produced 2.76 inches of rain at the Diablo Canyon Ocean Lab. This total was the highest amount of precipitation for a signal day in May since records have been kept at Diablo Canyon. General climate summary precipitation indicates that this rain event was highest one-day total for the month of May.

If you follow the weather forecast, you’ll often hear or read the term upper-level low-pressure system or upper-level trough. Characteristically, most of the energy in these upper-level systems exists roughly between 10,000 feet up to the top of the troposphere, which extends upward to about 33,000 feet, depending on your latitude and the atmospheric conditions.

The word “trough” is a metaphor, like a horse trough, used to describe a line of low pressure that stretches from one location to another. Like a cold front at the earth’s surface, these upper-level troughs can extend for hundreds of miles across the sky and move in similar patterns.

An upper-level low, also known as a cold-core low, is like a surface cyclone. However, like an upper-level trough, most of its energy is located farther upward in the atmosphere. In other words, these storms are stronger aloft than at Earth’s surface. As daylight hours become longer and longer in the month of May, energy from the sun heats 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 air above, which can produce thunderstorms. Thunderstorms are notorious in their ability to create gusty winds, periods of heavy rain, hail and even tornado activity.

An upper-level low-pressure system is forecast to bring rain showers, a chance of thunderstorms and well below-average seasonal temperatures to the Central Coast on Sunday into Monday morning; please stay tuned to The Tribune for updated information about this late-season weather event.

‘Slow for the Cone Zone’

Rain after a dry period releases oil, grease and rubber dust that has accumulated on roadways, creating slippery conditions. Driving too fast is the No. 1 cause of traffic accidents on wet days. Tragically over the years, first responders, such as CHP officers, firefighters, Caltrans highway workers and PG&E line crews have been hit by vehicles. So please, “Slow for the Cone Zone.” One of the most significant hazards for workers is motorists who do not exercise caution while driving through highway work zones.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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