Area Forecast Discussion

KEY MESSAGES

1. Another round of monsoonal moisture will bring chances for thunderstorms across the Sierra Nevada crest through the middle of the week.

2. Extreme Heat Watch issued for the Mojave Desert and Indian Wells Valley Thursday through Monday. Major HeatRisk expected with afternoon highs 108 to 112 each day.

3. High pressure over the region will maintain temperatures near season averages for the remainder of Central California for most of the week.

DISCUSSION

High pressure remains the driving force behind the weather pattern affecting central California for the foreseeable future. Today, the high is located over the Great Basin/Four Corners region, leading to southerly flow across our area as winds rotate anticyclonically around the system. This flow aloft is driving moisture into California from the warmer tropical regions, particularly from the Gulf of California. As the moisture encounters the local terrain in the Sierra Nevada, it will condense as it rises, resulting in increased cloud cover over the mountains. Warm air from the San Joaquin Valley will be upsloping into the Sierra during the afternoon, pushing the moisture higher into the atmosphere, and creating showers and thunderstorms across the higher elevations. For today, there is a 30 to 40 percent probability for thunderstorms to occur across the Sierra Nevada, with these being the greatest probabilities of the week. Gusty and erratic winds, locally intense rainfall rates, small hail, and cloud-to-ground lightning are all threats with convective development. To the second point listed, these storms will be on the wetter side, with ensemble guidance from the National Blend of Models expressing an 80 to 90 percent probability for rainfall to exceed 0.10" through 5 AM Wed. While the pattern will remain mostly unchanged for tomorrow, thunderstorm probabilities fall to around 20 to 30 percent.

Later this week, the ridge will begin to slide southward, centering itself over western Texas and eastern New Mexico, strengthening over central California and turning the flow out of the southwest. This will cause warmer temperatures to develop across the area, particularly over the Mojave Desert. Beginning Thursday, high temperatures in this area will be approaching 110 degrees each afternoon through at least next Monday, with minimum temepratures providing little relief from the heat at 75 to 80 degrees each morning. Because of this, an Extreme Heat Watch has been issued for a portion of the Mojave Desert and Indian Wells Valley.

In the San Joaquin Valley, the high pressure will not have as large an impact, with afternoon highs remaining near season averages for this time of year. Each afternoon through Monday, there is a 50 to 60 percent probability for temperatures to exceed 100 degrees in the valley areas, albeit on Thursday when there is a 20 to 30 percent probability.

AVIATION

25 to 50 percent chance of thunderstorms over the Sierra Nevada beginning around 17Z and will be a problem through at least 6Z Wednesday. Otherwise, VFR conditions will prevail 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

Extreme Heat Watch from Thursday morning through Monday evening for CAZ337-339.