textproduct: San Joaquin Valley

KEY MESSAGES

1. Heat Advisory in effect for the San Joaquin Valley, Sierra Foothills and West Side Hills along the Coastal Range.

2. High probability (70 to 90 percent) for triple digit highs in the San Joaquin Valley today and Thursday.

3. Medium chance (60 to 70 percent) for a wetting rainfall in the Sierra Nevada near Mount Whitney both today and Thursday.

4. Stronger winds expected to develop in the Mojave Desert Slopes later this week.

DISCUSSION

Clear conditions abound across central California this morning as high pressure begins to build in from the Desert Southwest. This increased pressure will result in a warming trend across the local area, with afternoon highs rising around seven to eight degrees above season averages today, then around ten degrees above average on Thursday. Ensemble guidance from the National Blend of Models gives a 70 to 80 percent probability for maximum temperatures to exceed 100 degrees this afternoon in the San Joaquin Valley, which rise to 80 to 90 percent for Thursday. Valley lows are expected in the upper 60s while the larger urban areas and the surrounding foothills rise into the mid 70s. The combination of above-average highs and warmer overnight lows will result in widespread Moderate Heat Risk with some areas of Major Heat Risk, particularly in the West Side Hills along the Coastal Range. This level of heat brings a Major Risk of heat-related illnesses for much of the population, especially those without effective cooling or hydration and there will be little to no relief at night: air conditioning is necessary, fans and open windows at night will not be enough.

While high pressure is building, moisture in the low levels from the previous few days will linger into the afternoon. The warming temperatures will allow the saturated air to rise into the Sierra Nevada, particularly in the area around Mount Whitney. This afternoon, there is a 5 to 15 percent chance for thunderstorms to develop in this area. Through 5 AM Thursday morning, there is a 60 to 70 percent chance for more than one- tenth of an inch of rainfall to occur. Any storms that develop may contain cloud-to-ground lightning, small hail, gusty and erratic winds, and locally intense rainfall rates. Warm temperatures continuing on Thursday will bring a similar chance for thunderstorms in the same location tomorrow afternoon.

In the latter part of the week, the aforementioned high pressure ridge will degrade as a low pressure trough begins to move into the Pacific Northwest, returning the synoptic pattern to zonal flow over central California. This pattern will result in a cooling trend across the region, likely returning to seasonable temperatures by Saturday in the low 90s. However, the incoming trough will cause gustier winds to develop in the wind-prone areas of the Mojave Desert Slopes; 60 to 70 percent probability for maximum gusts to exceed 45 mph in this area both Friday and Saturday. As the trough moves closer to the local area Saturday, stronger wind gusts are also poised to enter the valley areas, with a 50 to 60 percent probability for maximum gusts to exceed 25 mph. Cooler temperatures will extend into next week in the wake of the trough progressing eastward.

AVIATION

VFR conditions expected for the next 24 hours. A 5 to 15 percent chance of thunderstorm across the Sierra Nevada Crest around Mount Whitney from 19Z Today through 03Z Thursday.

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

Heat Advisory until 10 PM PDT Thursday for CAZ300>317-319-321.