Weather Watch

Spring in SLO isn’t always sunny and warm. This year, we’re in a loop of instability

NOAA’s GOES-West weather satellite in geostationary orbit on Saturday showed a massive 992 millibar winter-like storm centered about 1,000 miles to the west-northwest of the Central Coast and associated cold front traveling quickly southeastward toward San Luis Obispo and Santa Barbara counties.

On the east side of the cold front, altocumulus and altostratus clouds completely covered the entire Pacific off the California coastline in continuous shades of gray and white. On the west side of the cold front, chilly and unstable air produced enormous amounts of white puffy cumulonimbus clouds, spinning counterclockwise toward the center of the tempests, like small beads of Styrofoam going down a kitchen sink.

The altocumulus and altostratus clouds will create the steady rain that will reach us Saturday night through Sunday morning as the cold front moves southeastward through our region. Cumulonimbus clouds behind this front will produce heavy convective rain showers and isolated thunderstorms Sunday afternoon and night.

Over the last few days, I received emails and phone calls asking, “What are convective rain showers, and why do they seem to happen more often in spring.”

Here’s why.

On a clear day, especially during the longer days of spring and summer, most of the sun’s energy is radiated through the atmosphere and absorbed by the land. Like sitting on a bench that has been warmed by the sun, the ground warms the air in direct contact with it by a process called conduction.

Conduction is the transfer of heat within one substance — in this case, the ground — to another — the air — by molecular action. As the air in contact with the Earth’s surface warms, the amount of motion or vibration of the gases of the atmosphere — nitrogen, oxygen and argon atoms along with molecules of carbon dioxide and water vapor — increases.

That, in turn, produces a more considerable amount of space between the atoms and molecules, making the air less dense, which causes it to rise like a hot-air balloon. That rising of the air is convection. In the science of meteorology, there are two types of convection: dry and moist. As you might guess, dry convection doesn’t produce any clouds. However, moist convection does.

The differences between these two are determined by the air’s relative humidity, temperature and strength of the updrafts. If a thermal can rise high enough, the air will cool and eventually reach its dewpoint temperature and form clouds. The dewpoint is the temperature to which air must be cooled for it to become saturated.

At that point, the air can no longer hold all its water vapor, and some of it condenses into visible clouds, rain or snow. Condensation is a warming process, which is why it feels warmer right before it rains or snows. That process can sustain the convection by warming the air further and making it rise even higher.

This convection circulation can severely destabilize the atmosphere as the relatively warm air slams into the cold air above, forming a positive feedback loop that can push clouds into the stratosphere, producing cumulonimbus incus or “anvil” clouds. Anvil clouds can develop into supercells, resulting in such severe storm phenomena as lightning and thunder, hail and even tornadoes.

Meteorologist often refers to those clouds as convective. The nature of convective rain showers is often intense but usually short-lived. Historically along the Central Coast, the most severe thunderstorms have occurred in spring. The only verifiable tornadoes have happened in the months of April and May.

Thunderstorm safety tip

If you hear thunder, that means lightning is close enough to strike you. There is no location outside that is safe when thunderstorms are in the area. When you hear thunder, immediately move indoors or inside a vehicle with its windows up. Stay in safe shelter at least 30 minutes after you hear the last sound of thunder.

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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