textproduct: San Diego

SYNOPSIS

Minor to major HeatRisk continues for much of the region through Sunday before cooler weather returns Tuesday and beyond. Thunderstorm chances each day in the mountains Sunday through Wednesday, with best chances and highest rain rates Monday and Tuesday. Patchy coastal fog through Sunday morning with a deeper marine layer after Monday.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

Mid level water vapor imagery shows high pressure firmly in place over the Four Corners, with ongoing heat impacts for Southern California. Highs both today and tomorrow will be around 3-7 degrees above normal with warm overnight lows 4-10 degrees above normal, resulting in minor to moderate HeatRisk for inland and valley areas and moderate to major HeatRisk for the deserts. A Heat Advisory and Extreme Heat Warning remain in effect through Sunday evening.

Outside of the heat, attention returns to monsoonal moisture as the upper level ridge will elongate and stretch this coming week, allowing for multiple weak waves to move across. Forcing this afternoon looks limited with lackluster low-level moisture for areas other than the southern mountains. If any storms manage to develop, they will likely be confined to the higher terrain this afternoon with limited impacts. The ridge will become a bit more compressed tonight and, as seen Friday morning, another very weak easterly wave may be able to move across Baja late tonight. Localized light rainfall will possible in San Diego County Sunday morning, although the best chances for any early morning precipitation will be in the San Diego County Mountains and foothills, or eastern I-8 corridor. This window will also allow for low and mid level moisture to begin to surge further northward, reaching Riverside and San Bernardino Counties by early Sunday afternoon as PW values return above 1.15. While not sensational, this, along with the slight breakdown in the ridge, may allow for a few decent thunderstorms to develop over the mountains, especially in Riverside and San Diego counties, Sunday afternoon (30-40% chance). Brief, heavy rainfall with rain rates up to 0.50-0.75"/hr and lightning will be the main threats. There will be around a 10-20% chance for a storm to fire off of the San Bernardino Mountains around this time too, but rain rates that far north will likely be 0.40"/hr or less. If you are spending time in the mountains this weekend, ensure you are prepared to take shelter in a sturdy, enclosed structure should you find yourself caught in a thunderstorm. Trees are not a safe place to shelter from lightning!

The pattern shifts cooler beginning Monday as high pressure slides east, with a deepening marine layer into the end of the week. The position of the high centered over the South Plains and a separate high off the coast will establish upper level troughing for much of CA as we cool back down to near normal for mid August by Tuesday. Monday and Tuesday will once again feature afternoon monsoonal convection in the mountains as moisture is pulled northward and PWs reach ~1.5. Both of these days could see more widespread storms and higher rain rates, with a slightly higher potential for storms to move off the mountains and into the deserts compared to Sunday. Afternoon storms over the mountains will be possible each day through the end of next week, but chances decrease after Tuesday as a more robust trough takes effect.

You've been patient, and the cooler weather is finally in view!

AVIATION... 081730Z....Coast

Patchy low clouds with bases 800- 1200 feet MSL will develop gradually but intermittently overnight, with cigs most likely between 11-15Z Sunday. Any cigs should scatter out Sunday by 15-16Z.

Valleys/Mountains/Deserts...Mostly clear across the region through tonight, except for SCT cumulus at 10000 feet MSL over mountains 20- 01Z.

Regionwide...Increasing clouds AOA 10000 feet Sunday morning.

MARINE

No hazardous marine conditions are expected through Wednesday.

SGX WATCHES/WARNINGS/ADVISORIES

CA...Heat Advisory until 8 PM PDT Sunday for Orange County Inland Areas-Riverside County Mountains-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Mountains-San Diego County Valleys-Santa Ana Mountains and Foothills.

Extreme Heat Warning until 8 PM PDT Sunday for Apple and Lucerne Valleys-Coachella Valley-San Diego County Deserts-San Gorgonio Pass near Banning.

PZ...None.