Weather Watch

Nature does great special effects

dmiddlecamp@thetribunenews.com

Flashes of light in the eastern sky and the distant rumble of thunder, followed by hail and rain showers, may have been your wake up call the morning of Sept. 10.

As the day progressed, potent cumulonimbus clouds with the slate gray bases and tall billowing towers moved westward over San Luis Obispo. The air turned electric and fashioned one of the most intense displays of lightning that I have ever seen.

According to the lightning tracker at SLOweather.com, about 30,000 lightning strikes were recorded within a 50-mile radius of San Luis Obispo on that Saturday. By that evening, lightning strikes reached more than 100 per minute. These lightning strikes kept Cal Fire and other fire departments throughout Central California extremely busy extinguishing vegetation fires.

These thunderstorms were caused by an upper-level low pressure system that moved over the Central Coast and pulled in warm and humid monsoon moisture from the south. The warm and moist air rose rapidly into the sky.

As the air rises, it condenses and forms cumulus clouds that can tower over the earth. This condensation process releases latent heat and, like turning on the burner in a hot-air balloon, it warms the surrounding air, which continues to rise at surprisingly high speeds. At times, it can reach more than 40,000 feet in altitude and actually break into the stratosphere.

Last year, a hailstone weighing nearly 2 pounds plummeted to Earth from a thunderstorm in South Dakota. This monster piece of ice was found by ranch hand Leslie “Les” Scott near a small town of about 100 people named Vivian, on the high plains. The hailstone probably took violent updrafts that exceeded 160 mph to remain suspended in the cloud over the course of five to 10 minutes!

The updrafts and downdrafts found in thunderstorms can create friction between rapidly moving ice particles and rain traveling in opposite directions.

This wipes off electrical charges, which generate an increase in electrical pressure or voltage. The same effect can sometimes be experienced by rubbing your shoes across a wool carpet on a dry day and then touching a door knob. The shock you feel is the electricity discharged to the doorknob.

When electrical pressure or voltage becomes high enough, charges between parts of the cloud or between the cloud and the ground are released as lightning. Lightning occurs at all levels in a thunderstorm. The majority of lightning strikes never hit the ground.

When lightning does hit the ground, it starts with a relatively thin “leader” stroke from the cloud, followed immediately by a heavy return stroke from the ground.

Lightning discharges are incredibly powerful — up to 30 million volts at 100,000 amps. Just one bolt is more than six times hotter than the surface of the sun. But they last only briefly. The sudden and rapid increase in heat causes the air around the lightning bolt to rapidly expand, then collapse, causing the shock waves that we call thunder.

Lightning takes the path of least resistance. It tends to hit the highest places. Never stand under a lone tree in an open field or on top of a mountain during thunderstorms. If you’re backpacking in the mountains and you feel your hair standing on end, get as low to the ground as you can and try to take cover.

On buildings, lightning rods allow electrons to stream off into the air or harmlessly to the ground. Wood structures and trees have high electrical resistance and can be heavily damaged unless grounded.

At PG&E, the safety of our customers and crews in the field is top priority. So, if you see a downed power line — whether it was caused by a lightning storm or something else, assume it is energized and keep yourself and others away.

Call 911 immediately to report the location of the downed line, then call PG&E’s 24-Hour Emergency and Customer Service Line at 800-743-5002.

Today’s forecast A high-pressure ridge will continue to build off the Pacific Ocean and will significantly decrease the depth of the marine layer.

This condition will produce a greater amount of sunshine along beaches and much less marine overcast in the morning.

Today’s temperatures will range from the mid- to high 70s along southwesterly facing beaches (Cayucos, Avila Beach and Shell Beach). Westerly facing beaches (Pismo Beach and Oceano) will be a little cooler, ranging from the high 60s to the low 70s, while northwesterly facing beaches (Cambria, Morro Bay, Los Osos, Montaña De Oro and Nipomo Mesa) will range from the low to the mid-60s.

The coastal valleys, including San Luis Obispo, will reach the high 70s, while the North County will hit the high 80s.

Gradual warming is expected through Wednesday with enjoyable late summer weather on tap for all locations. Only patchy morning low clouds are expected along the coast. Otherwise, clear and sunny skies are forecast.

Slightly cooler temperatures with more extensive low clouds, fog and drizzle along the coast are forecast for Thursday and Friday, followed again by gradual warming over next weekend.

The long-range charts indicate that storm track will remain north of the Central Coast for at least another two weeks, keeping our area dry.

Today’s surf report

Arriving from the Northwest: Today’s 3- to 5-foot northwesterly (290-degree deep-water) sea and swell (with a 7- to 11-second period) will continue at this height and period through Tuesday.

Today’s charts are indicating a 962-millibar storm developing in the Gulf of Alaska this weekend with 50 knot west-northwesterly winds. This storm should produce a 3- to 5-foot northwesterly (290-degree deep-water) swell (with an 11- to 20-second period) Wednesday, increasing to 6 to 8 feet (with a 14- to 16-second period) Thursday. This swell will decrease to 5 to 7 feet (with a 13- to 15-second period) Friday.

Preliminary analysis: The longer-range models and charts are indicating increasing storm activity in the Gulf of Alaska late this week. If this condition develops, increasing swell conditions will develop on the coast by Sept. 30.

Arriving from the Southern Hemisphere: A 1- to 2-foot Southern Hemisphere (190-degree deep-water) swell (with a 16- to 18-second period) will arrive on the coast Tuesday.

Seawater temperatures: Temperatures will range between 56 and 59 degrees through Monday, increasing to 57 and 60 degrees Tuesday and will remain at this level through Friday.

John Lindsey, media relations representative for PG&E and local weather expert, has lived on the Central Coast for more than 25 years. To subscribe to his daily weather forecast or ask him a question, email him at pgeweather@pge.com.

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