Weather Watch

Two confirmed tornadoes have happened in SLO

The past few weeks have seen a marked increase in the number of tornadoes east of the Rocky Mountains. Around 245 tornadoes were reported last week, with North Carolina probably being the hardest hit.

April and May historically have the greatest occurrence of these violent storms. Tornadoes nearly always develop from thunderstorms, especially from a class of thunderstorms known as supercells.

This type of thunderstorm can be seen over the Central Plains when warm and moist air moving northward in the lower to mid-levels of the atmosphere from the Gulf of Mexico runs into cold air moving southward from Canada.

When this occurs, the warm and less dense air is forced up by the advancing mass of colder and denser air.

As this air rises thousands of feet into the sky, it cools and releases tremendous amounts of latent heat. This condition keeps the air rising inside the cloud, triggering thunderstorms.

These convective storms can contain areas of organized rotation a few miles up in the atmosphere. If the conditions are right, these thunderstorms can spin out tornadoes.

Even though a greater number of tornadoes occur east of the Rocky Mountains during the spring, California is not completely immune from these violently rotating columns of air.

There have been two occurrences of confirmed tornadoes in San Luis Obispo.

The first happened April 7, 1926, when a Pacific storm came in from the west and produced lightning.

The lightning struck large oil tanks along Tank Farm Road. Altogether, more than 5 million gallons of oil burned over five days.

It was reported that burning oil made it all the way 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 true tornadoes.

One of the fire tornados traveled 1,000 yards, picked up a house and carried it 150 feet, killing the two occupants inside.

The other confirmed tornado that hit San Luis Obispo occurred on the morning of May 5, 1998. At the time, I was living in a neighborhood near Cal Poly where it touched down.

About 5:40 a.m., the rain became heavy and the wind caused my windows to vibrate. I thought it was a train at first.

My anemometer — a device to measure wind force — was fluctuating from 60 to 70 mph, and the power lines around my home began to arc.

Tree branches were breaking, and then I saw debris rotating in a counterclockwise direction — clear evidence that a tornado was occurring.

My anemometer reached 86 mph! I called 911 to report the activity. To say that she sounded a bit skeptical would be an understatement.

But later, the National Weather Service indeed confirmed a low-level tornado happened in the area.

This week’s forecast

A 1,009-millibar low-pressure system will pass to the northeast of San Luis Obispo County this Easter morning and will give a few areas of drizzle and light rain showers.

Most areas of the Central Coast will not have any measurable precipitation.

Increasing northwesterly winds, variably cloudy skies and slightly warmer temperatures are expected this afternoon into Monday.

Today’s high temperatures will mostly be in the low to mid-60s, increasing to the mid-60s along the coastline to the low 70s in the North County.

The Eastern Pacific High will gradually move eastward toward California, while a ridge of high pressure builds over the Western United States.

This condition will produce clear and warmer weather Tuesday, with many areas reaching the mid-70s.

Gusty northeasterly (offshore) winds Wednesday morning will help to make Wednesday the warmest day of the week, with temperatures reaching the low to mid-80s throughout San Luis Obispo County.

Moderate gale- to freshgale (32- to 46-mph) northwesterly winds are forecast to develop along our coastline Thursday and Friday. These onshore winds will produce cooler temperatures along the beaches and in our coastal valleys, and night and morning marine low clouds.

However, the North County will continue to see afternoon temperatures reaching the low 80s.

Dry weather and mild temperatures are forecast for next weekend.

Surf and sea report

The Eastern Pacific High, currently about 1,500 miles northeast of Hawaii, will slowly move eastward toward the coast of California through Friday. As this area of high pressure moves closer, the northwesterly winds and seas will progressively increase over the next five days.

Today’s 2- to 3-foot northwesterly (300-degree deep-water) sea and swell (with a 5- to 9-second period) will continue at this height and period through Monday morning.

Fresh- to strong- (19- to 31-mph) northwesterly winds will produce a 3- to 4-foot (300-degree deep-water) sea and swell (with a 4- to 10-second period) Monday afternoon through Tuesday morning.

The northwesterly winds will further build to strong to gale-fresh (25- to 38-mph) levels along the Northern and Central California coastline Tuesday.

These winds will generate 6- to 8-foot (310-degree deep-water) sea and swell (with a 5- to 11-second period) Tuesday afternoon through Wednesday.

The northwesterly winds will further build to moderate gale- to fresh-gale (32- to 46-mph) levels along the Central Coast on Thursday and Friday.

These winds will generate 7- to 9-foot northwesterly (310-degree deep-water) sea and swell (with a 7- to 11-second period) during this timeframe.

Arriving from the southern hemisphere:

Today’s 2- to 4-foot southern hemisphere (205-degree deep-water) swell (with a 16- to 18-second period) will remain at this height but with a gradually shorter period through Monday.

Increasing amounts of upwelling along the coast will produce cooler seawater temperatures this week.

Intake seawater temperatures will range between 49 and 52 degrees through Tuesday, decreasing to 48 and 51 degrees Wednesday through Friday.

Conservation tip

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

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