Why did it rain? Blame the cutoff low

Special to The TribuneOctober 13, 2012 

Rain drove many to pop open umbrellas in downtown San Luis Obispo on Thursday, including this man walking near the post office parking area.

DAVID MIDDLECAMP — dmiddlecamp@thetribunenews.com

Meteorologist Dr. George Fischbeck was on KABC-TV Channel 7 in Los Angeles during the 1970s and ‘80s. Dr. George wore a bowtie along with thick, black-rimmed glasses and joyfully delivered his forecast with the demeanor of a science school teacher. He loved to show the 500-millibar charts on his broadcast and coined the insightful phrase “A cutoff low is a weatherman’s woe.”

That’s because an upper-level low-pressure system will sometimes break away from the jet stream, becoming a cutoff low. In other words, the jet stream shifts to a higher latitude. Left behind is a circulating low-pressure system that’s cut off from the jet stream.

The jet stream is the main mechanism that drives storms from the Pacific Ocean east toward the Central Coast. Normally, winter storms from the Pacific tend to follow predictable paths, dictated by the position and strength of the upper-level winds.

Without the jet stream’s guidance, it becomes difficult to almost impossible to predict the speed and direction of these rebellious weather systems. I’ve seen these lows remain nearly stationary for days, or even move westward back out to sea. The different numerical weather prediction models that meteorologists consult for guidance can diverge wildly, much like the cutoff lows themselves. 

Last Wednesday and Thursday, a large and slow-moving cutoff low in the upper levels of the atmosphere moved south along the California coast and progressed into Southern California. This high-level system brought cold air aloft and significantly decreased atmospheric stability. 

My PG&E colleague Scott Strenfel, who taught meteorology at San Jose State University, told me all the ingredients came (phased) together at the same time to produce the thunderstorms and abundant lightning strikes that occurred on Thursday: cold air aloft, cyclonic flow, abundant moisture and an ignition method.

“An ignition method can be orographic forcing, daytime (differential) heating, an outflow boundary from another thunderstorm or a front. This lighting outbreak, however, was a result of the synoptic or dynamic forcing coupled with large-scale instability associated with the upper-level low-pressure system.”  Strenfel said.

On Thursday morning, the air rose, condensed and formed cumulus clouds that towered over the Earth’s surface. This condensation process released latent heat and, like turning on the burner in a hot-air balloon, it warmed the surrounding air, which rose at surprisingly high speeds.

A band of potent cumulonimbus clouds with the slate gray bases and tall billowing towers moved toward the Cuesta Grade from the east. The air turned electric and fashioned an intense display of lightning. Flashes of light were seen in the eastern sky and the distant rumble of thunder was followed by hail and rain showers. This may have been your wake-up call Thursday morning.

As the day progressed, a 10-to-30-mile-wide band of moderate rain stretched from Creston southwestward over San Luis Obispo toward Los Osos and out over the Pacific Ocean.

Historically, storms coming from the northeasterly direction drop most of their moisture as they pass over the Santa Lucia Mountains with little rain in the coastal areas. However, this system was much higher in altitude and was only slightly affected by the mountain topography. 

The main band of moisture was centered at about 25,000 feet with the cloud ceiling at about 10,000 feet. From the Irish Hills, I could see sunshine to the south toward Muscle Point and to the north toward San Simeon while the rain came down on top of me. At the surface, we only had gentle northwesterly winds, unlike the gusty southerly surface winds that normally accompany storms that produce this magnitude of rainfall. 

This high-elevation band of moisture dropped a surprising amount of rain over a narrow area as it stalled. Cal Poly, home for climatology for San Luis Obispo, recorded 1.05 inches of rain, ending a long dry spell. Southeast of Creston on Highway 58, a rain gauge at Jennifer Best’s home reported 1.5 inches.

Areas north and south of this relatively narrow band of rain recorded only a few hundredths of an inch. For example, the Paso Robles Municipal Airport reported 0.03 inches of rain, while the Santa Maria Airport only reported trace amounts.

Indeed, the contrast was even sharper between Atascadero and Templeton, just six miles apart. Mickey’s rain gauge in south Atascadero recorded 1.12 inches of rain, while the Templeton rain gauge at the Sheriff’s Office station near the intersection of Highway 101 and Main Street only recorded 0.04 inches of rain.

Today’s forecast

High pressure in the upper levels of the atmosphere will build over the Eastern Pacific and dominate the weather for this week. This condition will produce gusty northeasterly (offshore) winds along the coastal sections of San Luis Obispo County during the night and morning hours. These offshore winds will produced spectacular Indian Summer weather across the Central Coast with warm temperatures and clear skies.

Today’s high temperatures will be in the 80s in the North County and Coastal Valleys. Maximum temperatures along the shoreline will be in the low 80s along the southwesterly facing beaches (Cayucos, Avila Beach and Shell Beach) and the high 70s along the westerly facing beaches (Pismo Beach, Grover Beach and Cambria). The northwesterly facing areas (Morro Bay, Los Osos, Montaña De Oro and the Nipomo Mesa) will range from the low to mid-70s.

The ridge of high pressure is expected to break down by Friday and will bring relief from heat. The marine layer will return along the coast Friday into next Sunday with night and morning low clouds and fog and temperatures returning to normal.

Today’s surf report

Strong to gale-force (25- to 38-mph) afternoon northwesterly winds will generate 3- to 5-foot northwesterly (320-degree deep-water) sea and swell (with a 4- to 12-second period) today. 
A 974-millibar Gulf of Alaska storm has transferred a lot of energy to the ocean in the form of seas on Friday.

A northwesterly (300-degree deep-water) swell from this storm will arrive along the coast Monday and combined with locally generated northwesterly seas will produce a combined 4- to 6-foot northwesterly sea and swell (with a 4- to 17-second period) Monday, increasing to 6 to 8 feet (with a 5- to 14-second period) Tuesday. This northwesterly (320-degree deep-water) sea and swell will decrease to 5 to 7 feet (with a 5- to 12-second period) Wednesday.

A 3- to 5-foot (310-degree deep-water) sea and swell (with a 7- to 11-second period) is forecast along the coast Thursday, decreasing to 2 to 4 feet (with an 8- to 10-second period) Friday. 

    
Seawater temperatures

Seawater temperatures will range between 56 and 58 degrees through Wednesday.

•  •  •

PG&E delivers some of the nation’s cleanest electric power. More than half of the electricity we provide to our customers comes from sources that are renewable and/or emit no greenhouse gases. In fact, PG&E’s electricity creates only one-third as many greenhouse gas emissions per kilowatt-hour compared to the industry average.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question, send him an email at pge
weather@pge.com.

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