Could last weekend’s storm and another this week bring wettest April since 2011?

The meteorologist at the National Weather Service Los Angeles/Oxnard office nicknamed it the “Endless Upper-Level Low” and for a good reason.

Last Sunday, April 5, this 543 dm upper-level low-pressure system dropped southward from the Pacific Northwest and produced gusty southerly winds and persistent rain throughout the day and night. The upper-level low took a liking to the Central Coast and didn’t leave our area until Friday when it finally traveled southeastward to San Diego.

From Sunday through Thursday, it gave us well above average amounts of rain for April. Paso Robles typically receives 0.68 of an inch during April, while San Luis Obispo records 1.11 inches, and Santa Maria averages 0.98 of an inch. Cal Poly (home of climatology for San Luis Obispo) recorded 2.45 inches, while Santa Maria reported 1.78 inches and Paso Robles 1.46 inches from this one system.

These amounts have double or even triple April’s averages. They may go up even further, as yet another upper-level low-pressure system was forecast to reach the Central Coast by Friday and hang around till next Sunday, generating more precipitation. In fact, this could be the wettest April since 2011.

This upper-level low has brought seasonal rainfall totals to 66 percent of normal, according to SLOWeather.com, in western San Luis Obispo. Santa Maria has recorded 10.7 inches or 70 percent of average, while Paso Robles is reporting 72 percent of normal.

This one storm brought Lake Nacimiento from 51 percent of capacity to 55 percent, raising its level 4 feet in just five days. Here are the other percent of capacity for our lakes and reservoirs: Lake San Antonio 38 percent, Whale Rock Reservoir 84 percent, Santa Margarita Lake 87 percent, Lopez Lake 52 percent, Cachuma Reservoir 79 percent and Gibraltar Reservoir 100 percent.

The lack of moisture in February drove most of California from the D0 (Abnormally Dry) to D1 (Moderate Drought) category by early March, according to the weekly United States Drought Monitor. Much of San Luis Obispo County and all of Santa Barbara and Ventura counties were listed as D1 (Moderate Drought) level.

The well-above-average rainfall of March and April has dropped the drought level from D0 (Abnormally Dry) category throughout the Central Coast to “None” in the southern half of San Luis Obispo County and nearly all of Santa Barbara County.

These late-season rains could also help to produce a beautiful display of wildflowers later this month.

Like a surface cyclone

So, what is an upper-level low?

If you follow the weather forecast, you’ll often hear or read the term upper-level low-pressure system or upper-level trough. Typically, most of the energy in these upper-level systems exist 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.

Meteorologists determine the thickness of the atmosphere by analyzing 500-millibar upper-level charts measured in decameters (dm). In other words, this chart will tell you how low or high the pressure is 3 or 4 miles above the Earth’s surface. The lower the 500-millibar line, the colder or stormy it could be. The higher the 500-millibar line, the thicker the atmosphere, and the warmer it will be. In summer, anything above 580 dm will usually give hot temperatures; if near 600 dm, record-breaking. Along the Central Coast, readings below 565 dm in April, can produce unsettled weather.

The word trough is a metaphor, like a horse trough to water equine, 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 up in the atmosphere. In other words, these storms are stronger aloft than at the Earth’s surface. That’s one reason we didn’t see any significant swell events along our coastline, despite all the rain.

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 below zero. So often, these systems will separate from the jet stream, which causes them to move unpredictable directions. As 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.

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

As daylight hours become longer and longer in April, energy from the sun heats the 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 severe weather like thunderstorms.

Dig this warning

With all Californians, except essential personnel, being required to stay-at-home and work-from-home until further notice due to the COVID-19 outbreak, PG&E reminds the public of the importance of calling 811 before starting projects that require digging or excavation.

April is designated as National Safe Digging Month across the country.