textproduct: San Joaquin Valley

KEY MESSAGES

1. Extreme Heat will continue to be the major weather concern until Monday. Extreme Heat Warnings continue until Monday evening.

2. Persistent hot and dry conditions will also continue to promote the potential for wildfires with cured fuels.

3. Potential for thunderstorms over the Sierra Nevada crest continues for this afternoon and evening, with chances of 20 to 30% today and additional chances Tuesday through next weekend.

DISCUSSION

Temperatures were very hot across much of the forecast area yesterday, and another day of extreme heat is in store across the lower elevations today. Highs in the San Joaquin Valley yesterday were 102 to 108 degrees; today's highs will rise by another couple of degrees with the hottest temperatures near 110 degrees, such as over portions of Kern County in both the valley and desert. The upper-level ridge of high pressure remains in control over the Southwest today into Monday, but a downward trend in temperatures begins Monday. However, Monday's temperatures won't be much lower than today's. A more noticeable lowering of temperatures begins Tuesday, with cooler temperatures by later in the week. In addition, a threat of high Sierra thunderstorms (20-30 percent chance with isolated dry lightning) continues today, with a brief break on Monday. In addition to the heat, we continue to see an elevated fire weather risk in our lower elevations due to low minimum humidity (10-20 percent) until Monday. There will also be increased winds with gusts 20-30 mph near the passes along the hills and mountains adjacent to the west side of the San Joaquin Valley, as well as over ridge tops and along the Mojave Desert slopes.

Beyond Monday, similar chances to today's forecast of thunderstorms return Tuesday and Wednesday, including along the Sierra Nevada crest. We will see additional moisture later this week, or beginning Thursday, so coverage of thunderstorms expands to the Kern County mountains and lower Sierra Nevada. Some sprinkles or light showers are also possible later this week or next weekend in the Central Valley, but confidence is low regarding this scenario. At least we will be much cooler by late next week, with temperatures at least five degrees below average. We will have to monitor for increased winds in the typical prone areas along with this cooling. With this cooling, we will finally see some relief from the lower daytime humidity.

AVIATION

VFR conditions will prevail for the next 24 hours with light northwest winds during the day and variable winds overnight. Thunderstorms will have a 20-30 percent chance from 21Z Sunday until 03Z Monday across the Sierra Nevada Crest.

FIRE WEATHER

Hottest temperatures with the current heat wave will be observed this afternoon, resulting in lowered minimum relative humidity (minRH) values around 15 to 20 percent in the San Joaquin Valley, Sierra Foothills, and high Sierra Nevada where thunderstorms do not occur. Desert minRH values will be around 10 percent. The hot, dry conditions bring elevated fire concerns especially to those areas where the hottest temperatures occur, including the Mojave Desert, the lee of the Coastal Range, and the western San Joaquin Valley. Downsloping winds into this latter area due to onshore flow will also be a contributing factor. The hot conditions across central California will also raise mixing heights across the region; for the higher terrain and desert areas, heights will rise above 9000 feet, causing concern for plume-dominated fires. In addition, monsoon moisture tracking over the Sierra Nevada will bring a threat for isolated dry thunderstorms to occur this afternoon.

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 PM PDT Monday for CAZ300>317-319- 321-332-334>339.