textproduct: San Joaquin Valley

UPDATE

Aviation section updated.

KEY MESSAGES

1. Widespread Moderate Heat Risk, including triple-digit heat in the lower elevations through Wednesday.

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.

5. Lake Wind Advisory in effect until 11PM for San Luis Reservoir

DISCUSSION

Hot,dry conditions continue over central California today. Afternoon temperatures hovering around 100 expected this afternoon, albeit with cooling morning lows leading to lesser areas of Moderate HeatRisk. As we get into the end of the work week, a weak low pressure trough will approach the region. This will decrease temperatures 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% for the San Joaquin Valley. However, these expected 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. The high pressure overhead will limit wind speeds however, with locally diurnal and terrain processes being the dominant sources for surface winds.

Slight chance (10-15%) for afternoon isolated thunderstorms remains over the Sierra Crest 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 the lasted model forecasts standing at 15-20% from Yosemite National Park to Sequoia National Park.

Morning wind observations at San Luis Reservoir have met the threshold for a Lake Wind Advisory, with gusts currently hovering around 25 mph. High-resolution probabilistic modeling (HREF, REFS) is highly confident (>70% probability) for gusts up to 35 mph later this afternoon across the area. As a result, a Lake Wind Advisory is in effect for San Luis Reservoir until 11PM tonight, and may be extended longer if later model runs increase confidence for high gusts continuing overnight.

AVIATION

VFR conditions expected across the area for the next 24 hours. 10-15% probability for thunderstorms across the Sierra Crest through local sunset.

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

Lake Wind Advisory until 11 PM PDT this evening for CAZ300.