Area Forecast Discussion
Issued by NWS San Joaquin Valley, CA
Issued by NWS San Joaquin Valley, CA
329 FXUS66 KHNX 212303 AFDHNX Area Forecast Discussion National Weather Service Hanford CA 403 PM PDT Thu Aug 21 2025 .UPDATE... Updated Aviation Section. && .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 beginning tomorrow morning. 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 Saturday 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 tomorrow due to an influx of monsoonal moisture across the area, with the greatest chances along the Sierra Crest. Dry thunderstorms are possible tomorrow afternoon. && .DISCUSSION... An area of high pressure building over central California, centered over the Four Corners region, will continue a warming trend across our area into the weekend. Maximum temperatures tomorrow afternoon have a 40 to 80 percent chance to exceed 105 degrees in the San Joaquin Valley, with the greatest chances in the southern valley, and over a 90 percent chance for the Kern Desert; tomorrow`s highs are forecast to be around ten degrees above season averages for the date. A slight cooldown to low triple digits is expected on Saturday, however minimum temperatures across the lower elevations will remain much above normal in the low to mid 70s. Foothill areas along the thermal belt, urban areas of the San Joaquin Valley, and the Mojave Desert are projected to be in the upper 70s to low 80s through Sunday morning. Due to the Major Heat Risk these temperatures are causing, an Extreme Heat Warning has been issued for the Kern County Desert (now in effect until Sunday morning), in addition to an Extreme Heat Warning for the foothill and urban areas of the San Joaquin Valley and a Heat Advisory for the remaining portions of the lower elevations, which both go into effect Friday morning through Sunday morning. Southwesterly flow moving anticyclonically around the aforementioned high will bring chances for isolated to scattered thunderstorms to the Sierra Nevada each afternoon through the weekend and into early next week. Drier conditions ahead of this monsoonal moisture will result in a risk for dry lightning to occur tomorrow afternoon before wetter conditions arrive for the weekend. Each subsequent afternoon starting Saturday has a Marginal Risk for excessive rainfall along the Sierra Crest, with the area expanding into the foothills of Mariposa, Madera, and Fresno counties on Monday. && .AVIATION... 00Z Update: 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 CAZ337>339. Extreme Heat Warning from 11 AM Friday to 8 AM PDT Sunday for CAZ300-304-307-308-313-314-316-317-319-321. Heat Advisory from 11 AM Friday to 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/EW idss/pio.............EW weather.gov/hanford