DISCUSSION
FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

Thunderstorms ended in all areas by 7 PM with partly cloudy skies overnight. While high temperatures were mostly a bit cooler today versus Saturday, one place that was slightly warmer was San Diego Airport, which hit 90 today, the highest recorded temperature since 19 Oct 2022 when SAN hit 95. Several stations had new or tied record high temperatures for the date. At mid-evening, skies were generally partly cloudy from a combination of some lingering debris clouds from the thunderstorms, mainly in the deserts and areas of smoke both aloft and near the surface. Any marine layer fog will be minimal tonight and restricted to the immediate coast, and latest SAN AMDAR soundings show a surface-based inversion instead of the usual elevated marine layer inversion.

Model and ensemble guidance are very consistent with having thunderstorms form in the valleys, especially the Inland Empire where substantial rainfall could form near the Elsinore Convergence Zone, potentially over one-half inch. Moisture aloft will be peaking, along with modeled CAPE values over 1000 J/kg and strong convergence from the south winds through Temecula, west winds through Corona and east winds through Banning/San Gorgonio Pass. It will be another very hot day as well, with temperatures over 105 F in hottest parts of the Inland Empire. Strong winds will be a threat with these storms as they again will be high-based, with the LCL mostly at/above 10000 ft MSL. Rainfall rates between 0.40" and 0.75" per hour are possible in strongest storms. The timing, based on model guidance, will mostly be 11 AM to 7 PM, with little or no threat of surprise nocturnal convection. Looking ahead to Monday, as the upper high strengthens over the Pacific around 500 miles west of San Diego but weakens substantially over the 4-Corners area, the moisture flow will weaken over SoCal, though enough instability and moisture should be available for isolated thunderstorms, mainly along the mountain crests as the thermal troughing/convergence also shifts from the valleys back to the mountains.

The temperatures will be about as hot Sunday as today, and SAN could again reach 90 (ECMWF ensemble MOS actually has all solutions between 90 and 93, while NBM MOS has 93, though GFS MOS has only 88). Also of note is most of the San Diego County near-coastal buoys had water temperatures of 74-77 this afternoon, which will contribute further to the humid conditions near the coast as well as limiting somewhat the cooling effect of the sea breeze. Monday should be a few degrees cooler though still 10-15 deg F (locally more) above seasonal normals.

Confidence is even higher now of the Pacific NW trough moving into SoCal around Tue with temperatures falling rapidly in most areas to levels a little below normal by Wednesday, except the deserts might take until Thursday. There could be gusty west winds in some areas though starting Tuesday, especially the mountains and deserts, which could affect any wildfires that might still be out of control then.
The marine layer stratus should return to the coast. With the drier profile aloft, the thunderstorm threat should end by Tuesday.

AVIATION
080345Z
Coasts/Valleys
Marine layer looks to be absent from the coast overnight, although ongoing nearby fires may result in periods of hazy conditions at coastal and valley sites, especially KSBD. FEW Cu with bases around 15,000ft MSL prevail overnight and into Sunday morning, and very localized and brief dense fog possible at some beaches. Isolated TSRA possible Sunday afternoon and early evening primarily near the mountains, but a few storms possible in the valleys (20-30% chance) with gusty and erratic winds near any storm.
Storms should diminish by 02z Monday.

Mountains/Deserts...Ongoing smoke from nearby wildfires may create hazy conditions and some VIS reductions, especially near the San Bernardino Mountains where the Line Fire is burning. Ongoing SCT Cu around 12-000ft-15,000ft MSL near the mountains through Sunday, with localized lower bases near the fire. Around a 50% chance for TSRA Sunday afternoon and early evening near the mountains, with gusty and erratic winds and frequent lightning near any storm, with mod up/downdrafts possible. While confidence is low for impacts at any specific TAF sites, outflow from storms in the mountains may result in brief periods of strong winds and/or VIS reductions from smoke.
Any storms should diminish by 02z Monday.

MARINE

No hazardous marine weather conditions are expected through Thursday.

SKYWARN
Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions.



SGX WATCHES/WARNINGS/ADVISORIES
CA...Heat Advisory until 8 PM PDT Monday for Orange County Coastal Areas-San Diego County Coastal Areas.

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

PZ...None.