textproduct: San Joaquin Valley

KEY MESSAGES

1. The Heat Wave will continue to be the major weather concern as the Extreme Heat Warnings will be in effect until this evening.

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

3. Potential for thunderstorms over the Sierra Nevada crest continues for this afternoon and evening, with chances 10-20% today and chances spreading to the Kern County mountains on Tuesday and Wednesday. Chances also spread to the lower Sierra, and adjacent foothills on Wednesday.

DISCUSSION

Today will be another very hot day across the lower elevations with low daytime humidity (10-20 percent), including in the San Joaquin Valley, Mojave Desert and higher terrain adjacent to the Valley. While not as hot, Tuesday will be another day of triple digit heat across the Valley and Desert. A lowering trend in temperatures occurs Tuesday through the weekend, based on the latest probabilities of highs below 100 degrees (increases from 15 to 20 percent on Tuesday to 60 to 70 percent on Wednesday in the Valley). Further cooling continues with below average temperatures by Thursday. Highs lower to the 90's across much of the San Joaquin Valley by Wednesday, but will remain near seasonal averages. On Thursday, highs reach mainly the lower to mid 90s across the Valley and even into portions of the Mojave Desert, which would be considered at least a few degrees below seasonal averages. Similar temperatures take place into the weekend and early next week as an upper-level trough pattern persists along the West Coast. Humidity will gradually improve as we progress through this week, although additional monsoonal moisture arrives Tuesday and Wednesday.

Thunderstorm chances (10-20 percent) will be limited to the Sierra Nevada crest, mainly towards Mt. Whitney today. However, thunderstorm chances spread to much of the Sierra Nevada crest, as well as the Frazier Park area on Tuesday. More coverage occurs on Wednesday, including into the Sierra Nevada foothills. There will even be a chance of light rain or sprinkles in the San Joaquin Valley on Wednesday, but confidence remains low. Otherwise, thunderstorm chances retreat to the higher elevations of the Sierra Nevada for Thursday and Friday with a lowering trend in coverage and chances.

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 10-20 percent chance from 21Z Monday until 03Z Tuesday across the Sierra Nevada Crest.

FIRE WEATHER

Hot temperatures with the current heat wave will be observed again 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. Mixing heights will rise above 9000 feet again today, causing concern for plume-dominated fires. Thunderstorms will be limited to mainly the Sierra Nevada crest today, but chances spread to the Kern County mountains on Tuesday and Wednesday with chances into the lower Sierra foothills by Wednesday.

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 this evening for CAZ300>319-321-332-334>339.