Area Forecast Discussion
Issued by NWS San Joaquin Valley, CA
Issued by NWS San Joaquin Valley, CA
266 FXUS66 KHNX 222341 AFDHNX Area Forecast Discussion National Weather Service Hanford CA 441 PM PDT Fri Aug 22 2025 && .KEY MESSAGES... 1. A significant warming trend returns for the remainder of the week with triple digit heat in the warmest locations. Widespread Major Heat Risk are expected Friday with localized areas of Extreme Heat Risk Saturday. 2. An Extreme Heat Warning has been issued for the Kern Desert valid through Sunday morning. An Extreme Heat Warning has been issued for the Sierra Foothills and the urban areas of the San Joaquin Valley. A Heat Advisory has also been issued for the remaining areas of the San Joaquin Valley and for the Lake Isabella area. 3. A Red Flag Warning has been issued for the Frazer Park, Grapevine, and Fort Tejon areas in southern Kern County valid through tomorrow evening due to low relative humidity and locally breezy winds. There is elevated fire risk for the Mojave Desert due to expected minimum relative humidity less than 10 percent and breezy winds in the afternoon to evenings. 4. There is a 10 to 30 percent chance for thunderstorms each afternoon starting today due to an influx of monsoonal moisture across the area, with the greatest chances along the Sierra Crest. Dry thunderstorms are possible this afternoon due to lower minimum relative humidity for this area. && .DISCUSSION... High pressure building over central California this weekend will lead to a continuation of a warming trend, resulting in widespread Major Heat Risk across the lower elevations of the region today, including the San Joaquin Valley, Kern Desert, and some areas of the Sierra Foothills. The triple-digit temperatures are expected tomorrow as well, with localized areas of Extreme Heat Risk primarily in the foothill and urban areas due to above average minimum temperatures in the upper 70s to near 80 degrees. Extreme Heat Warnings are in effect for the Kern Desert, Sierra Foothills and West Side Hills, and the urban areas of the San Joaquin Valley through Sunday morning; a Heat Advisory is in effect for the remaining areas of the San Joaquin Valley and for Lake Isabella. The high pressure will also bring a chance for thunderstorms each afternoon along the Sierra through early next week due to the influx of monsoon moisture across the region. However, as the moisture approaches the region this afternoon, relative humidity values particularly in the Sierra are still expected to be around 20 to 30 percent, which may cause precipitation to evaporate from developed storms and ultimately result in the potential for dry lightning to occur. The increasing moisture through the weekend will improve relative humidity across the mountain areas, which will lessen concerns of dry lightning, but replaced by the risk for excessive rainfall. The Weather Prediction Center has given the Sierra a Marginal Risk (5 to 15 percent) for excessive rainfall leading to flash flooding for each afternoon from Saturday through at least Tuesday. Probabilistic guidance from the NBM gives a 30 to 70 percent chance for greater than one tenth of an inch across 24 hours ending 11 PM Saturday for a large portion of the Sierra. Though, high-resolution output from the HREF gives up to a 90 percent chance along the crests. Localized rainfall totals may be greater in areas under developed storms. && .AVIATION... 00Z Update: 10 to 20 percent chance of thunderstorms over the Sierra Nevada and a 5 to 15 percent chance of thunderstorms over the Mojave Desert for the next 24 hours. Otherwise, VFR conditions are expected in the San Joaquin Valley for the next 24 hours. && .FIRE WEATHER... Warming temperatures across the region will cause minimum relative humidity to lower for much of the lower elevations of the area (the influx of moisture in the mid levels will cause RH to increase in the Sierra). Widespread minimums 15 to 20 percent are expected for the San Joaquin Valley and Kern County mountains and less than 15 percent in the Kern Desert. Lowering RHs paired with stronger southerly wind gusts approaching 25 mph and mixing heights above 10000 feet in Frazier Park and Fort Tejon has led to the issuance of a Red Flag Warning as these conditions can cause rapid, plume-dominated fire spread if development occurs. && .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... Extreme Heat Warning until 8 AM PDT Sunday for CAZ300-304-307- 308-313-314-316-317-319-321-337>339. Heat Advisory until 8 AM PDT Sunday for CAZ301>303-305-306- 309>312-315-332. Red Flag Warning until 9 PM PDT Saturday for CAZ597. && $$ public/aviation....McCoy/BSO idss/pio...........BSO weather.gov/hanford