Weather Watch

SLO County is expected to get heavy rain Monday and Tuesday — here’s why

SLO County gets a dose of December rain

San Luis Obispo County saw up to a quarter of an inch of rain by early Wednesday afternoon, December 20, 2017. The forecast says more could come next week.
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San Luis Obispo County saw up to a quarter of an inch of rain by early Wednesday afternoon, December 20, 2017. The forecast says more could come next week.

I have seen cold fronts fall apart and seemingly vanish between Ragged Point and Point Sal. Or they can intensify and stall over a particular section of our coastal region, producing copious amounts of rain over one part while leaving other areas dry.

It looks like Monday and Tuesday’s cold front will create moderate to heavy rain throughout the Central Coast, and here’s why.

At our latitude, the jet stream is typically a tubular ribbon of high-speed winds some 18,000 to 40,000 feet up, flowing in wave-like patterns from the west to east for thousands of miles.

The jet stream is usually about 300 miles wide at its core. Its speed averages about 100 mph in winter and 50 mph in summer.

As the wave amplitude of the jet stream increases, the air flow will transform from a wave-like structure to that of a counter-clockwise circulation. That will intensify a low-pressure system off the Pacific Northwest to 998 millibars as it moves toward Northern California along with frontal genesis this Sunday.

A well-defined cold front associated with this low-pressure system will approach the Central Coast on Monday with moderate gale-force to fresh gale-force (32 to 46 mph with gusts over 55 mph) southeasterly winds along the coastline Monday afternoon into early Tuesday morning.

The jet stream (upper-level winds) on Monday will be vigorous and will blow from the southwest, an optimum condition to intensify this front.

Think of a cold front as a wave of energy extending away from the core of the storm — like sweeping your hand from the center of a draining kitchen sink to the edge. Cold fronts along the California coastline usually move in from the northwest, in a southeasterly direction. As cold fronts approach the Central Coast, they often produce increasing southeasterly (prefrontal) winds. Interesting rule of thumb that I’ve noticed over the years: Often, the speed at which the cold front moves down the coastline is about 75 percent of the post-frontal northwesterly wind speed behind it.

Series of storms that hit areas of Santa Barbara County hard on Sunday, Sept. 3, 2017, brings mostly gentle rain to San Luis Obispo. Other areas of SLO County saw downpours.

As the frontal boundary approaches our area Monday, the denser cold air at the leading edge of the front will cut under the less-dense, warmer air from the south, forcing it upward. As the air mass rises in the atmosphere, the reduction in surface pressure enables the air to expand, which results in falling temperatures. As the air cools, it reaches its dew point, producing precipitation. In other words, it’s like wringing out a sponge — or the wet adiabatic lapse rate.

Rain from this phenomenon will start Monday morning and become heavy Monday night into Tuesday morning with the frontal passage. As the cold fronts pass, it will cause a 180-degree wind shift from southeast, then out of the southwest and finally out of the northwest.

Cold fronts in winter can bring frigid air masses and on rare occurrences even snow to coastal regions. Cold and unstable air behind this front will continue to produce rain showers, heavy at times, on Tuesday afternoon into early Wednesday morning.

Total rainfall amounts in San Luis Obispo and northern Santa Barbara counties are expected to range between 2 and 3 inches with higher amounts in the coastal mountains.

Farther south, rainfall amounts will be even greater. Due to the position of the jet stream, a considerable amount of orographic enhancement is expected to develop over the burn areas of Santa Barbara and Ventura counties resulting in heavy rain Monday into Tuesday. In fact, the models are advertising total rainfall amounts of between 3 and 6 inches. This condition could produce heavy debris flows in and around the burn areas. If you live near these areas, please stay alert to the weather forecast and listen to your local emergency managers.

Are you ready for Monday and Tuesday’s storm? Here are tips for preparing for power outages:

▪ Have flashlights with fresh batteries on hand; don’t use wax candles.

▪ Have battery-operated radios with fresh batteries for storm and outage updates.

▪ Freeze water-filled plastic containers to make ice blocks to place in the refrigerator and freezer.

▪ Secure outside items that can be blown into power lines.

▪ Treat all downed power lines as live, keep away and call 911, then PG&E at 800-743- 5002.

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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