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December 15, 2019

More rain on the way

‘Atmospheric river’ dumped five-plus inches on South County

South County weather conditions went from fire hazard to soaking wet as November closed out, and more rain is expected this weekend and throughout the month of December.

A series of storms that started Tuesday, Nov. 26 have dumped more than four inches of rain throughout Morgan Hill, San Martin and Gilroy, according to reports from the Santa Clara Valley Water District. Some areas up in the hills received even more rain during the six-day period from Nov. 26 through Dec. 2; Uvas Canyon County Park saw a whopping 7.2 inches fall during that time.

Rain measurements from surrounding areas suggest that South County took the brunt of the latest storm Nov. 30-Dec. 2. San Benito County has seen much lower rainfall totals over the last week, with about 1.5 inches falling from recent storms, according to the San Benito County Water District. 

In west Morgan Hill, where local weather observer Chris Henry takes measurements, more than five inches of rain fell from Nov. 26 through Dec. 2, Henry said. That brings the season’s precipitation total to about 5.6 inches for the year—notably more than the approximately 3.82 inches that had fallen on Morgan Hill the same date last year.

Henry has been tracking the latest storms on his Facebook page “Morgan Hill Rainfall.” He said one unusual feature of the latest of the two recent storms was the “atmospheric river” that lingered from Saturday morning through Monday evening.

Henry observed that this storm largely stayed in the Central California region from south of San Jose to King City, and northeast through the Sierra foothills up to Lake Tahoe. On Monday morning, he posted a comparison of rain totals for the previous 48 hours in Morgan Hill (3.10 inches) with those of nearby cities; San Jose only received .21 inches, and San Francisco only .32 inches during the same two-day period.

“This has been one of the most remarkable and bizarre winter storms in memory,” Henry posted Dec. 2. “Rainfall totals are not spectacular. But what is noteworthy is the duration of the front stalling over our area since early Saturday morning!”

He also posted National Weather Service radar images from throughout the day Sunday to demonstrate the size and shape of the atmospheric river storm did not change as the rain continued to steadily fall in the same Central California region.

The slow and steady nature of the rainfall likely prevented flooding or heavy accumulations of standing water. Spokespeople for the Santa Clara County and San Benito County sheriff’s offices reported no rain-related incidents as of Monday morning.

Valley Water’s reservoir gauges show that the recent storms did not cause South County’s Chesbro and Uvas reservoirs to rise significantly. Both reservoirs were at about 39 percent of their capacity as of Dec. 3.

Wet December expected

Another cold, wet weather system is on its way to Santa Clara County, according to National Weather Service Meteorologist Ryan Walbrun. And long-term forecasts show the rest of December could remain wet, except for a few dry days coming up.

By Friday night, a “true cold front” is expected to arrive locally, bringing steady rain showers throughout the evening and into Saturday, Walbrun said. By Saturday afternoon, the rain is expected to taper off with up to a 30 percent chance of precipitation lingering Sunday morning.

Walbrun described the coming storm as “a fairly cold system” with high temperatures in the low to high 50s Dec. 6-8.

“This one is kind of a different setup” than last weekend’s atmospheric river, Walbrun explained. “It’s coming in from the Gulf of Alaska, so it has colder origins.”

The more typical cold front will likely result in more equal rainfall distribution throughout the valley, rather than starkly varying volumes of rain from the hillsides to the valley floor, Walbrun said. The latter phenomenon is known as “rain shadowing,” which often occurs during an atmospheric river with strong south winds.

Before the next cold front arrives, Walbrun said the NWS was expecting moderate rainfall in South County on Wednesday. Thursday, Dec. 5, “looks dry,” he added.

After Dec. 8 will be a few dry days, at least through Dec. 11, Walbrun added. But the rain is expected to return shortly after that.

“Longer range models suggest the wet pattern will stay active, with the potential for more storms through the month,” Walbrun said.

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