textproduct: San Joaquin Valley

KEY MESSAGES

1. Extreme Heat Warning in effect for the Mojave Desert and Indian Wells Valley through Monday. Major HeatRisk expected with afternoon highs 108 to 112 each day.

2. A small increase in monsoonal moisture will bring a slight chance for thunderstorms across the Sierra Nevada crest again this Friday, but the rest of the week into next will see chances less than 10%.

3. High pressure over the region will maintain temperatures near season averages for the remainder of Central California before hotter conditions appear late next week.

DISCUSSION

In recent days, an area of high pressure has stationed itself over the Great Basin.Four Corners region, leading to an influx of monsoon moisture over California as saturated air tracked northwards from more tropical areas. Over the last day, the high has slid southward and now resides over north-central Texas. Over the next 24 to 48 hours, the system is expected to build westward into the Desert Southwest, causing temperatures to rise across the region. This area will include portions of the Mojave Desert and Indian Wells Valley, which have a 60 to 70 percent probability to exceed 110 degrees each afternoon through Monday. Meanwhile, there is an 80 to 90 percent probability for morning lows to exceed 75 degrees, offering little relief from the heat at night. The combination of daily highs and lows in the desert brings a Major Risk for heat-related illnesses to much of the population and has led to an Extreme Heat Warning now in effect for the area through Monday. For the remaining areas of central California, near average temperatures are expected.

Lingering moisture from the previous week will have one last gasp on Friday to develop into showers with a few isolated thunderstorms across the Sierra Nevada. There is a 20 to 35 percent probability for thunderstorms along the crests in Tulare and southeastern Fresno County, with lower chances across the remainder of the Sierra. After Friday, the probabilities for convective development fall to 10 percent or less each day through the next week.

As the high strengthens over the region into next week, warmer temperatures and drier conditions are expected, especially towards the start of next month. Starting Monday, minRH values in the San Joaquin Valley and Sierra Nevada will fall to 15 to 20 percent, with values in the Mojave Desert around 10 percent at most. The aforementioned high building over the region in the late week brings a 50 to 60 percent probability for afternoon highs to exceed 105 degrees in the San Joaquin Valley beginning next Friday.

AVIATION

A 5-10% chance of afternoon thunderstorms across the Sierra Nevada Crest from 21Z Thu through 03Z Friday. Otherwise, VFR conditions will prevail in the San Joaquin Valley 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 Warning until 9 PM PDT Monday for CAZ337-339.