textproduct: San Joaquin Valley

KEY MESSAGES

1. Triple digit heat continues for the San Joaquin Valley this week. Moderate to Major Heat Risk expected with a Heat Advisory in effect through Wednesday evening, mainly Fresno County southward.

2. Extreme Heat Warning in effect for the Mojave Desert through Wednesday evening with Major Heat Risk expected. An 70 to 80 percent probability of temperatures of 105 degrees or hotter with little overnight relief.

3. Moisture tracking over the region in the late week period will bring a 10 to 15 percent chance for thunderstorms to the Sierra Nevada crest.

4. Moisture will also bring a 20 to 30 percent probability for a hundredth of an inch of rainfall to the northern San Joaquin Valley on Friday.

DISCUSSION

High pressure remains firmly in place over the Four Corners region of the Desert Southwest this evening and will persist through much of the upcoming work week. Meanwhile, a low pressure trough is located off the coast of the Pacific Northwest which will gradually move onshore through the latter portions of the week. With the local area caught between these two air mass systems, prevailing flow is from the southwest, similar to this past weekend. The result is warmer temperatures with sporadic high- level cloud cover tracking over the region. For Tuesday and Wednesday, there is an 80 to 90 percent probability for maximum temperatures to exceed 100 degrees in the San Joaquin Valley south of Fresno and, similarly, there is a 70 to 80 percent probability for highs to exceed 105 degrees in the Mojave Desert.

In the mid-to-late week period, there will be a slight pattern change as the aforementioned trough tracks into Washington and British Columbia. The influence will reach southwards along the Pacific Coast, helping to shear portions of moisture from current Tropical Storm Iselle and push it over the north and central Sierra Nevada. Through 5 AM Saturday, there is a 60 to 70 percent probability for at least a hundredth of an inch over 24 hours in the central Sierra, including Yosemite National Park. Some probabilities also extend into the northern San Joaquin Valley, which have a 20 to 30 percent probability for the same amounts over the same time frame. The warmer valley temperatures interacting with the flow of moisture aloft will also result in a 10 to 15 percent chance for thunderstorms across the Sierra Crest Friday afternoon.

As the weekend approaches, a second wave of influence from the aforementioned trough will pass through California, eroding the high pressure ridge and starting a cooling trend that will continue into next week. Temperatures will have less than a 20 percent probability to exceed 100 degrees on Sunday.

However, before reaching the weekend, warm and dry conditions will persist. Minimum relative humidities across the valley areas are expected between 15 and 25 percent through most of the week, with desert RHs flirting with single digits once again. RHs in the Sierra Nevada will be slightly better than in previous heat events due to the flow of moisture being shallow enough to reach the peaks of the range. MinRH values are expected between 25 and 35 percent in the higher elevations and will improve in the latter portions of the week near 40 percent.

AVIATION

VFR conditions prevailing 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 11 PM PDT Wednesday for CAZ337>339. Heat Advisory until 11 PM PDT Wednesday for CAZ304>316.