Temperatures in the 90s are nothing new to the northern San Joaquin Valley. But having them in March, as expected this week, is unusual and potentially dangerous.
An exceptionally strong ridge of high pressure will bring record-breaking heat to the entire region, the National Weather Service said. Temperatures in the San Joaquin Valley are likely to reach 90 degrees by Tuesday and 95 by Friday. That's 20 to 25 degrees above normal.
UC Merced experts explained why such heat now, when it's still technically winter, is more hazardous than the same temperatures later in the year.
"If the forecasts come to fruition, we will be in for a March heat wave with temperatures 20-30 degrees above normal - an event without precedent in the modern era," said climatologist Professor John Abatzoglou. "This comes on the heels of the warmest or second-warmest winter since at least 1895 for a huge portion of the western U.S."
While these temperatures are not near the triple digits the area routinely sees in July and August, early-season heat waves can have outsize impacts.
"First, people are not acclimated to the heat yet, as they would be in mid-summer, which increases the risk of heat impacts on human health," Abatzoglou said. People who are sensitive to heat are advised to avoid being outside during the warmest hours of the day. Everyone should use sunscreen and stay hydrated.
Unusually hot temperatures may also impact the agricultural sector, affecting both outdoor workers and crops that are flowering or fruiting, Abatzoglou said.
Lauren Parker, managing director of Secure Water Future at UC Merced, has extensively studied how heat can affect agriculture.
"In orchard crops that might still be blooming, heat exposure can reduce fruit set as a function of effects on pollen quality and pollinator activity," she said. "Fruit set" is the process in which a flower is successfully pollenated before becoming a fruit. "Crops also need more water when it is hot. Water stress during early or young fruit stages can cause trees to reallocate resources away from young fruit, aborting some fruit development and ultimately lowering yields."
Growers could use more water to irrigate early in the season, leaving the sources depleted in the summer months.
"That has real dollar implications for their energy and water consumption - especially if this is a harbinger of heat waves to come," Parker said.
Early heat waves like this one could exacerbate and lengthen wildfire seasons, which already are overlapping globally, new research co-authored by Abatzoglou and postdoctoral researcher Cong Yin shows.
Early heat will deplete the snowpack much of California relies on for its water. According to the state Department of Water Resources, the statewide snowpack has been melting an average of 1 % a day since early March. Generally, the snowpack lasts well into the spring.
"While fuels are not yet widely available to carry fire, this heat wave will accelerate the drying of vegetation and the melt-off of the remaining Sierra snowpack, pushing us closer to fire season," Abatzoglou said.
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