What might happen if a wet spring follows the Modesto watershed’s plentiful winter?
Stanislaus County had its wettest year on record in 1983, thanks to spring storms that kept piling onto the impressive winter total.
Forty years later, another winter has brought plenty of rain to the county, and snow to its Sierra Nevada watershed.
Could history repeat itself this spring? Maybe.
California tends to have most of its storms from November through March. But they can happen in April and May, too, as 1983 showed. The extra runoff that year bolstered reservoirs and aquifers but did not cause widespread flooding.
As of Friday, the central Sierra snowpack stood at 196% of the historical average for the date, the California Department of Water Resources reported. It was at virtually the same point on March 3, 1983, and would peak at 230% in May.
The snowpack is the main source for cities and irrigation districts in the Northern San Joaquin Valley, and for parts of the Bay Area.
The National Weather Service can make a very rough guess at conditions three months out, at its Climate Prediction Center. It sees equal chances of wet or dry days through May 2023, based on calculations involving distant ocean temperatures and other variables. The models show that March by itself has above-average odds for rain and snow.
The forecast a few days ahead is more reliable. The weather service predicted up to 0.5 inches of rain in Modesto over the March 4-5 weekend, with up to 30 inches of snow in the watershed. Sunday’s seven-day forecast showed at least a slight chance of rain every day through Saturday.
The 2023 snowpack will be memorable even if it falls short of the record.
“It’s very possible we’ll end up vying for one of the top two snow years on record in parts of the state,” said Daniel Swain, a climate scientist at the UCLA Institute of the Environment and Sustainability. He provided a Feb. 20 update on YouTube.
Break from storms eased flooding
The January storms caused major flooding on some California waterways and backed up street gutters in Modesto and elsewhere. A mostly dry February eased the pressure. Much of the runoff went into reservoirs and aquifers strained by three years of drought.
Swain noted two ways the flood threat could return in spring. One is sunny weather melting the snow faster than what the dams and levees can handle. The other is warm storms, where rain can loosen the snowpack. The latter helped cause Modesto’s last major flood in 1997, although it was in January rather than spring.
Swain said he does not see either threat in the forecast through mid-March. And he noted that the 2023 snowpack is relatively cold, so raindrops would not dislodge much of it.
A wet spring could delay the need to irrigate farms and gardens, which helps with the recovery from drought. But the storms also could raise the risk of hail or disease damage to crops.
Hoping for ‘a long, slow melt’
The last very wet year was 2017, best known for a dam spillway failure on the Feather River that forced a mass evacuation in Butte County. The Tuolumne River ran high for several months past Modesto due to Don Pedro Reservoir releases to prepare for the snowmelt. The Stanislaus River was lower because New Melones Reservoir had more space.
On Friday, the state DWR held its monthly media event at a Highway 50 snow survey site. It found 177% of average conditions at this spot in the American River watershed.
Even more storms are welcome, if they are of the right sort, said Sean de Guzman, manager of snow surveys and water supply forecasting, in a news release.
“We are hopeful that we will see more cold storms to add to our snowpack for the next month and help set up a long, slow melt period into spring,” he said.
