textproduct: San Joaquin Valley

KEY MESSAGES

1. Seasonally average conditions Monday

2. High confidence in a pattern change starting Tuesday.

3. Temperatures well below average next week with moderate confidence of rain and high Sierra snow early Wednesday morning through Thursday.

DISCUSSION

Sunday, October 18 is still on track to be the last day of 90 degree temperature in 2026. In anticipation of a major pattern change later this week, high temperatures Monday are expected in the low-to-mid 80s across the San Joaquin Valley and Mojave Desert. Minimum relative humidity will likewise improve, with all areas outside the desert expected above 20 percent. Hopefully, this indicates the rest of 2026 is also free of large fire concerns.

Now for the exciting part of the discussion: rain and snow. Ensemble models are in good agreement a closed low approaches from the Pacific Northwest on Tuesday, then slowly traverses California west to east on Wednesday. As this low will be approaching from continental areas rather than the Pacific Oceans, temperatures in much of the forecast area will struggle to reach 75 degrees and higher, with a less than 10 percent chance by Thursday across the Mojave Desert and San Joaquin Valley. Precipitation chances first enter the forecast along the Sierra Nevada on Tuesday morning, before spreading region-wide and peaking Wednesday morning. The entire area has a 70-90% chance to see at least a trace of rain on Wednesday, with chances for a tenth of an inch or main ranging from 40-50% in the Mojave Desert, 50-70% in the San Joaquin Valley, and 80-90% in the Sierra Nevada. These chances take a 5-10% hit on Thursday, before significantly decreasing by Friday.

As this upcoming closed-low is on track to be the first meaningful event of Water Year 2027 (and closed lows bring forecaster woes), a few sore-spots are possible. Firstly, some ensemble model members are signaling elevated wind gusts along Tejon Pass, where the National Blend of Models (NBM) indicates a 30-40% chance for maximum wind gusts above 40 MPH. If this comes to fruition, slick roads and gusty winds will make travel along the Grapevine difficult. Secondly, the Weather Prediction Center indicates a marginal (5 percent chance) for excessive rainfall Wednesday and Thursday, as a few ensemble members place as much as an inch of rain across the entire forecast area on Wednesday and Thursday. Because of this, we have some concern for flash flooding in poorly-draining hydrophobic areas, including the Mojave Desert and Tehachapi pass.

The last sore-spot? Snow! Model runs this weekend have steadily increased the snow level closer to 8,000 feet. Semantics aside, there is high confidence (greater than 80 percent) for at least an inch of snow to accumulate on Wednesday into Thursday. Scenario-wise, a worst-case, 10% chance of occurrence scenario places 6-10 inches of snow above 8,000 feet. Whichever ends up the winner, travel will also be difficult across Sierra Nevada passes in reduced visibility and slick roads as well.

AVIATION

VFR conditions and light winds will prevail for the next 24 hours.

AIR QUALITY ISSUES

None

CERTAINTY

The level of certainty for days 1 and 2 is high. The level of certainty for days 3 through 7 is medium.

Certainty levels include low...medium...and high. Please visit www.weather.gov/hnx/certainty.html for additional information and/or to provide feedback.

HNX WATCHES/WARNINGS/ADVISORIES

None.