textproduct: San Joaquin Valley

KEY MESSAGES

1. Widespread Moderate Heat Risk, including triple-digit heat, in the lower elevations through Wednesday. Some pockets of Major Heat Risk, particularly near urban areas.

2. Hot and dry conditions will create increased fire risk across the region this week.

3. 15-25% probability for thunderstorms in the Sierra Nevada the next few days, which may include dry lightning.

4. Practice safety around lakes, rivers, and streams, as water is still running cold.

DISCUSSION

Hot and dry conditions continue over central California today, with similar afternoon temperatures expected for Wednesday, albeit with cooling morning lows leading to lesser areas of Moderate HeatRisk. As we get into the mid- to late-week period, a weak low pressure trough will approach the region, leading to a downtrend in temepratures that will last into the weekend, at which point afternoon highs will near season averages. The cooler conditions are expected to raise minimum relative humidity values near 30 percent for the San Joaquin Valley, however these conditions, on Saturday are an outlier, with much of the remaining week being at or below 25 percent. These values also don't apply to the desert areas, where humidities are expected between 10 and 20 percent, and will fall further to 5 to 10 percent early next week. Some of these lower relative humidities will also creep into the higher elevations of eastern Kern and Tulare County, especially for the Lake Isabella area and the southern Sierra Nevada. This continues the risk for grass fire developments across the region, however the high pressure overhead will limit wind speeds, with locally diurnal and terrain processes being the dominant sources for surface winds.

Slight chance (10 to 15 percent) for isolated thunderstorm activity remains over the Sierra crests each afternoon due to the warmer temperatures from the valley and foothill areas rising above cooler conditions at the higher elevations. Any thunderstorm that develops can have cloud-to-ground lightning, intense rainfall rates, gusty and erratic winds, and small hail associated with them. The incoming trough will have an influx of low- to- mid level moisture that has a better opportunity for thunderstorms in the Sierra on Thursday, with current chances standing at 15 to 20 percent from Yosemite National Park to Sequoia National Park.

AVIATION

VFR conditions expected in the San Joaquin Valley 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

Heat Advisory until 11 PM PDT this evening for CAZ300>321-332.