textproduct: San Joaquin Valley

KEY MESSAGES

1. Temperatures below seasonal averages will continue into next week before a warming trend returns temepratures to averages midweek.

2. Precipitation chances increase into the weekend, with the best chances on Sunday. The greatest probabilities will also be in the desert and southern mountain areas.

DISCUSSION

Cooler temperatures across central California are a welcome sight after prolonged heat through the latter portion of the summer. The below average conditions are largely due to a cutoff low pressure trough that is currently located off the coast of the Pacific Northwest. The low is pulling cooler air from the north Pacific as it rotates cyclonically around the system and is depositing it over portions of California and into the Great Basin. Temperatures for this afternoon are expected to remain below normal by around 10 degrees across the local area, and valley highs have only a 20 to 30 percent probability to exceed 85 degrees. A more southerly influence will emerge from the system on Thursday, raising temperatures by a few degrees, but remaining below average. Perturbations from the system as it remains centrally located over the Oregon/Washington coastline will fluctuate temperatures into the weekend as the prevailing flow shifts between multiple directions. Friday's highs drop near Wednesday's highs as the influence becomes more westerly, warms on Saturday into the upper 80s when the flow shifts from the south, then cools again Sunday with westerlies taking hold again.

With south to west flow taking hold into the weekend, warmer tropical moisture will entrain into the region, raising relative humidities and bringing chances for precipitation to some portions of central California. The location and strength of the trough is currently expected to push much of the moisture through towards the end of the holiday weekend; the main focus will be on Southern California, but influence may reach into South-Central California. As it stands, NBM ensemble guidance gives a 50 to 60 percent probability for rainfall to exceed one- tenth of an inch for portions of the desert, including the likes of Boron, Mojave, California City, Edwards AFB, and Ridgecrest between 5 AM Sunday and 5 AM Monday. For the same time frame and amounts, there is a 40 to 50 percent probability for the valley and mountain areas of Kern County and Tulare County. The high- confidence scenario (9 in 10 chance) shows no precipitation across any of the local region; meanwhile the low-confidence scenario (1 in 10 chance) shows more than 1 inch for the deserts and southern mountains, and between 0.5-0.75" for the valley areas. However, these scenarios will be determined based on the exact positioning of the trough through early next week.

A warming trend with relatively dry conditions (30-40 percent minRHs) is expected to gradually take hold through the middle of next week. This is due to the aforementioned trough progressing eastward, causing dry air to move in on the backside and warmer temperatures as winds shift out of the south-southwest. Though, temperatures will remain at or below average for most of the area, and there is just a 20 to 30 percent probability for valley temperatures to exceed 95 degrees next Wednesday.

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

None.