From sunny and hot to windy and wet: Here’s the science behind this week’s wild weather

An upper-level low-pressure system is forecast to bring blustery weather with scattered rain showers, a chance of thunderstorms and well below average seasonal temperatures to the Central Coast this weekend. If you follow the weather forecast, you’ll often hear or read the term upper-level low-pressure system or upper-level trough.

So what are these systems?

Typically, most of the energy in these upper-level systems exists roughly between 10,000 feet all the way up to the top of the troposphere, which extends upward to about 33,000 feet, depending on your latitude and the atmospheric conditions. Some days, this level can be higher, other days lower.

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. But 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.

These upper-level lows often contain an isolated pool of cold air at their core with temperatures at our latitude reaching minus 30, 40, even 50 degrees Fahrenheit. So often, these systems will separate from the jet stream, which causes them to move in unpredictable directions. As the famous weatherman Dr. George Fischbeck would say, “A cutoff low is a weatherman’s woe.”

Sometimes at our latitude, the tilt of the cyclone through the atmosphere is severe enough to allow the upper-level low to break away from the surface low, like the Leaning Tower of Pisa.

Numerical forecast models, which meteorologists rely on for guidance in weather forecasting, have a more challenging task to initialize the atmosphere at these altitudes and create an accurate prediction because there is much less atmospheric data available than at Earth’s surface, where marine buoys and weather stations reside.

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 a tempest.

This type of pattern created a small tornado that hit San Luis Obispo on May 5, 1998. At the time, I was living on the corner of Kentucky and Fredericks streets in the neighborhood near Cal Poly, where it touched down. At first, I thought it was a fast-moving train along California Boulevard. However, my anemometer — a device to measure wind force — was fluctuating from 60 to 70 mph. Tree branches were breaking, and then I saw debris rotating in a counterclockwise direction, clear evidence that a tornado was occurring. Later that day, the National Weather Service came out to the site and confirmed that a low-level tornado had indeed occurred.

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This weekend’s blustery and chilly weather is quite the transition from earlier this week, when warmer-than-normal temperatures hit California.

Bakersfield reached 104 degrees Thursday, which broke the previous record of 101 degrees set in 2004. In fact, April’s mean monthly temperature was 1.7 degrees Fahrenheit warmer than average for California.

These warmer-than-normal conditions across California combined with one of the deepest snow packs in the state’s history and record rainfall in many parts of the Sierra Nevada has produced colder and higher river and stream flows this spring — and it will continue into summer months.

California State Parks Division of Boating and Waterways (DBW) and PG&E warn water enthusiasts that river flows are expected to last longer and be greater than they have in several years. Swift water can create treacherous conditions for all recreationists — waders, swimmers, paddlers, boaters, anglers and hikers cooling off at the water’s edge. Enthusiasts are encouraged to take extra precautions when in or near the water.

In California’s high-water years, recreational boating fatalities related to swift water conditions more than doubled when compared with similar conditions during low-water years. According to DBW annual boating accident statistics, 48 California residents lost their lives in swift water conditions during the state’s high-water years of 1993, 1998, 2005, 2010 and 2011 combined. During the state’s drought or low water period of 2012-16, accident statistics confirm that 23 fatalities occurred in swift water conditions.

“With the most snowmelt in decades, we ask those enjoying the outdoors to be careful near mountain streams, rivers and reservoirs. Water flows can fluctuate as the snow melts faster on warmer days, so always be prepared for a change in conditions,” said Ed Halpin, PG&E’s senior vice president of generation and chief nuclear officer.

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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