textproduct: San Joaquin Valley

KEY MESSAGES

1. Changing weather patterns bringing cooler weather and the next regional chances for rain.

2. Potential for thunderstorms over the Sierra Nevada crest continues for this afternoon and evening, with 20-30% probability today and additional chances Wednesday and Thursday.

3. Potential for wildfires with cured fuels still remains a threat this week.

DISCUSSION

An intriguing synoptic pattern is establishing over California: combined troughing digging down the West Coast and high pressure riding over the Pacific Ocean and Rocky Mountains. This blocking pattern as it were is set to push moist air from the south (and the Gulf of California to a lesser extent) right into California for the next several days. As of the time of writing, this has already began to manifest via growing status and cumulus cloud cover traveling up the San Joaquin Valley and a few popup rain showers likewise traveling north from the Ventura mountains. For the region, this also means the end of triple digit heat for much of the region by Thursday.

Further into the afternoon, more widespread thunderstorms (approaching 25-30% probability of lightning along the Sierra Crest) are set to begin after 11AM local time, with slighter chances (10-15%) across the Mojave Desert and mountains of southern Kern County. These probabilities and areal coverage are currently showing even higher for Wednesday. As the ridge across the Rocky Mountains pushes further east on Thursday, probabilities drop to below 5% for the Kern County desert and mountains, with Wednesday's elevated probabilities still holding for the Sierra Nevadas.

As for the rainfall outlook, the main complications are two- fold: mediocre low-level moisture transport, and marginal convective potential outside of topographic uplift. Looking at the latest probabilistic National Blend of Models, much of the forecast area is unlikely to receive even a trace of precipitation. The San Joaquin Valley is currently placed under 10-25% probability for a trace or greater precipitation between Wednesday evening and late Thursday afternoon, with probabilities approaching 30-40% in the foothills north of Tulare County and downwind of the San Emigdio Mountains.

Probability of a wetting rainfall (tenth of an inch or greater), meanwhile, will stay more likely (>50%) above 6,000 in the Sierra Nevada, approaching 70% along the Crest. The best timing also appears to be Thursday. Meanwhile, wetting rainfall along and downwind the San Emigdios is most likely between Tuesday afternoon and Wednesday afternoon, with probabilities approaching 70% along the Kern/Ventura County line near Pine Mountain Club. But as the larger pattern shifts by Thursday, wetting rainfall probability here decreases to around 40%. Though given the wildcard of our unique topography and just the right convective initiation, the 5-10% probability of a wetting rainfall in the San Joaquin Valley on Thursday appears valid, though highly unlikely.

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 25-30% percent chance from 21Z Tuesday until 03Z Wednesday across the Sierra Nevada Crest.

FIRE WEATHER

While the upcoming pattern change is set to bring cooler and rather pleasant temperatures and a nice bump in relative humidity, there is concern for lightning-initiated fires given how cured the region's fuels are this late into the summer. Upon regional collaboration this morning, our neck of the woods are such that conditions for Red Flag or Fire Weather products are not sufficient. But given how dry fuels are, we are starting to communicate the uniquely elevated fire risk starting Wednesday from more widespread probability of lightning. Updates on the forecast will [of course] be provided in the Forecast and Fire Weather discussions later today.

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

None.