California forecast models show unsettled weather pattern could persist through mid-May

San Francisco Chronicle​

01-05-2025

A lingering upper-level trough will keep California locked in a springtime pattern of cooler than average temperatures, scattered showers and occasional high-elevation snow through at least mid-May.
The setup, a continuation of the same broad-scale pattern that made April cooler and drier than normal, remains unusually far south for this time of year, said Scott Handel, a forecaster at the Climate Prediction Center.
'The flow is cut off from the main storm track, which is why we're seeing this kind of unsettled weather reach deeper into California than usual for May,' Handel said.
That pattern won't ease quickly. Forecast models suggest a ridge will build across the central and eastern U.S., reinforcing a trough along the West Coast and keeping much of California in a cooler, more unstable regime through the first half of the month.
Across California, temperatures will stay below seasonal averages for much of the state during the first part of May, especially in the northern part of the state. Inland highs in the Sacramento and San Joaquin valleys will see days with temperatures in the 70s to low 80s, while coastal areas will often stay stuck in the 50s and 60s. Southern California will be milder overall, especially away from the coast, with interior highs reaching well into the 80s at times, but nothing especially hot by May standards.
'It's going to be a cooler and more unsettled pattern for much of the West, and definitely cooler than normal for this time of year across California,' Handel said.
The pattern for the first half of the month will also allow for periodic precipitation. Most of it will be light, with occasional showers or isolated thunderstorms across the northern half of the state and the Sierra. Snow levels may dip below 6,500 feet at times, with light accumulations over higher passes. With cold air aloft and weak disturbances rotating around the trough, day-to-day instability could linger through the second week of the month. Still, May systems tend to be moisture-starved as the jet stream begins its seasonal retreat north.
Cloud cover will remain a persistent feature near the coast, with a deeper marine layer settling in at times. In Southern California, ' May Gray ' conditions could be particularly stubborn, with low clouds pushing inland overnight and slow to clear, if at all, during the day.
By mid-month, the larger-scale pattern should begin to shift, but not cleanly. High pressure is expected to build across the desert Southwest, bringing gradual inland warming to parts of California. But some long-range ensemble guidance suggests a weaker trough could linger offshore, limiting inland heat and reinforcing coastal marine clouds.
'That could still feed some unsettled weather along the West Coast. But given the time of year, that's more likely the farther north you go. It could be a transition period from this unseasonably cool, unsettled pattern to a more normal scenario," Handel said.
By late May, long range models point to a gradual drying and warming trend, but not the sharp seasonal handoff that California sometimes sees. Even as inland temperatures rise, that warming will likely reinforce coastal cloudiness by increasing the contrast between land and ocean temperatures.
However, confidence remains low for the final stretch of the month. Model guidance diverges on whether California might get one last brush of unsettled weather or simply slide into a familiar late-spring rhythm. A sustained heat wave doesn't look likely, but brief warm spells are possible, especially if high pressure strengthens inland and drives temperatures into the 90s for a few days.
'We're seeing a lot of model spread as we get toward the end of the month,' Handel said. 'There's no strong signal right now for a major warmup or a return to storminess.
That means California's transition into summer may not come with a clear break, just a gradual shift, with inland heat building and coastal clouds hanging on a little longer each day.