textproduct: San Joaquin Valley

KEY MESSAGES

1. High confidence of a line of heavy rainfall, flooding, and embedded strong thunderstorms arriving from 4am through 8am Wednesday, moving from southwest to northeast, starting in Kern County. Burn scars are at risk.

2. High Wind Warning and Wind Advisory areas impacted by downed trees or power outages, moderate confidence. Travel will be difficult, especially for high profile vehicles.

3. 40% Probability of winds gusting to 30 mph from Merced to Hanford, will also topple trees and move loose objects.

4. High confidence of heavy snow above 7000 feet, with snow load impacts on roofs in the high Sierra. Travel will be impossible with 60 mph winds and whiteout conditions.

5. Wednesday afternoon will feature thunderstorms, with the highest probability of severe weather from Fresno county and points north.

DISCUSSION

A potent atmospheric river has arrived along the southern California coast, bringing with it significant and impactful precipitation as well as strong winds. Expanding upon this threat, the Weather Prediction Center's Excessive Rainfall Outlook highlights the Kern County Mountains as the primary threat area for heavy precipitation with a Slight risk (15-40% chance). The risk increases to Moderate (40-70%) along the far southern edge of the forecast area including I-5 through Grapevine. Accumulated rainfall amounts continue to track steady from the previous forecast cycle, with a reasonable high end scenario for the high terrain nudging upward slightly. The northern Sierra Nevada are poised to see at least (90% chance) 3 inches of precipitation with as much as 8.5 inches (10% chance) at the higher end. The upper end potential tapers to 5 inches (10% chance) at the southern end near Alta Sierra and Lake Isabella. Significant spread remains at the far southern edge of the forecast area with a reasonable best/worst case scenario along I-5 being 1.5 to 6 inches of liquid precipitation.

Given this continued level of confidence, Flood Watches remain in effect as we head into Wednesday morning. Expect rises along waterways and minor flooding to occur due to this precipitation. Recent burn scars, especially those due to the Basin, Garnet, French, and Borel Fires, will be particularly vulnerable to excessive runoff and debris flows. Also worth noting that the Storm Prediction Center has the entire forecast area under a General Thunder outlook with a low probability of thunderstorm development in the forecast, and a marginal risk for severe storms limited to the west of the forecast area at this time.

Snow levels continue to run fairly high given the warm airmass accompanying this atmospheric river, with high confidence that the snow level will sit at around 6800-7000 feet. This is driving a high confidence forecast for multiple feet (3+) of snow across much of the high Sierra, with confidence greatest for the highest peaks. It is worth noting that there is a 25% chance that the snow level could push as high as 8600 feet during the day on Wednesday, especially further south along the range.

Wednesday night into Thursday, the likelihood of valley precipitation decreases greatly as we transition into more of a terrain-forced mode. This will shift focus for continued precipitation towards the terrain on Thursday, with early indications of the storm wrapping up as early as Friday morning (10% chance), but almost certainly over after midnight Saturday into Sunday morning (90% chance). In the wake of this system, ensemble forecasts are in good agreement with the deep upper level trough closing off and sticking around into early next week before shifting out of the area. This will lead to a period of cooler temps and unsettled conditions into mid next week.

AVIATION

VFR/MVFR conditions beginning to deteriorate in some areas, with the potential for IFR conditions to develop early Wednesday, especially between 10- 19Z driven by the period of heaviest precipitation and low ceilings. Gusty southerly winds continue through the period across the San Joaquin Valley. Upper end wind gusts along the leading edge of this system are forecast to crest as high as 40-50 kts, but quickly reduce to 20-25 kts across the valley after 14-15Z Wednesday.

AIR QUALITY ISSUES

On Wednesday December 24 2025, Fireplace/Wood Stove Burning Status is: No Burning Unless Registered in Fresno and Kern Counties. Burning Discouraged in Kings, Madera, Merced, and Tulare Counties, and Kern (Greater Frazier Park Area) and Sequoia National Park and Forest.

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

Flood Watch through Friday afternoon for CAZ300>339.

High Wind Warning until 4 PM PST Thursday for CAZ333>339.

Wind Advisory until 4 PM PST Thursday for CAZ300>304-308-309- 313-314-316>318-324-332.

Winter Storm Warning until 10 PM PST Friday for CAZ323-326>331.