Weather Watch

The jet stream has tilted into SLO like the Leaning Tower of Pisa — thus all this rain

We are already nearly halfway into 2019, but to me, it only seems like a few months since New Year’s Day.

Perhaps one reason, as we get older, the years seem to go by faster. One theory to explain this phenomenon is as follows: When you are 10 years old, one year represents 10 percent of your lifespan; if you’re fortunate enough to reach 100, one year only represents 1 percent of your time on this planet.

In other words, it’s all about your perception of time.

However, there may be another reason this year seems to be going by so fast, and here is why. This past May resembled the weather patterns you usually experience during a wet Central Coast late-rain season, with a trough of low pressure over California and a strong high-pressure ridge over Alaska pulling and pushing a series of upper-level low-pressure systems into our state.

You see, we haven’t seen any hot days in the coastal regions of San Luis Obispo or northern Santa Barbara counties so far in 2019, just partly to mostly skies with near-record-breaking amounts of May rain. Despite the long days of late spring as we head toward the summer solstice on June 21, the longest day of the year, it still seems that we haven’t left winter.

This May, the Paso Robles Airport reported an average high of 75.7 degrees, the normal high temperature for May is 80.7. Due to the increased amount of cloud cover that acts as a blanket, the overnight lows were a little warmer than usual in Paso Robles with an average low of 47.4 degrees versus a typical low of 46.1 degrees. Normally, Paso Robles receives about 0.26 of an inch of rain; this May the airport recorded 0.77 of an inch. The wettest May on record at the Paso Robles Airport was 2.15 inches recorded in 1998.

Typically, the Cal Poly (home of climatology of San Luis Obispo) rain gauge records about 0.43 of an inch of rain in May. Unofficially, this May it was around 2 inches. That was the most rain in May since 1998, which saw 3.41 inches at Cal Poly. The May rainfall record since 1870 for San Luis Obispo was 4.22 inches set in 1906.

As you might expect, San Luis Obispo also experienced slightly cooler daytime highs but warmer overnight lows. The Santa Maria Airport reported 0.91 of an inch of rain this May; the usual amount is 0.31 of an inch. The highest amount of May rain at the airport occurred in 1952, which saw 1.74 inches.

Interesting to note, the average high at the airport is 68.5 degrees, but this May it was 69.1 degrees. The overnight lows on average were 3.4 degrees warmer than the May normal of 47.5 degrees.

So what are these upper-level systems that occur during the longer days of April and May?

Typically, most of the energy in these upper-level systems exists roughly between 10,000 feet 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. However, 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. 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.

As daylight hours become longer and longer, 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 thunderstorms.

Rest assured, warmer days are on the way as high pressure builds over the West Coast this month.

The massive snowpack means the rivers are full of fast-moving cold water. For PG&;E water safety tips, please visit www.pge.com/en_US/safety/hydropower-and-safety/safety-tips/safety-tips.page.

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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