Minor to moderate HeatRisk continues through Monday with cooler weather Tuesday and beyond. Thunderstorm chances each day in the mountains through Wednesday, with best chances and highest rain rates Monday and Tuesday. Patchy coastal fog Monday morning with a deepening marine layer through the end of the week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Satellite shows high clouds streaming into Riverside and San Bernardino counties from the southeast after a collapsed MCS this morning, although any precipitation died off before reaching our borders. Much of the high clouds should move out by the afternoon, and high pressure situated over the Four Corners will keep temperatures well above normal and allow for monsoonal thunderstorms to develop over the higher terrain this afternoon. Pw values above 1" indicate that brief, heavy rainfall will be possible with any storm, but realistic maximum rain rates are likely closer to 0.60"/hr or less for Riverside and San Diego mountains and 0.40"/hr or less for San Bernardino this afternoon.
Soundings are a bit more optimistic for storm potential on Monday/Tuesday as the high becomes more compressed by upper level troughing over the PacNW, allowing for a more unstable environment and monsoonal moisture to surge up into the mountains of SoCal. Temperatures will also begin to cool on Monday as we shed the last of our heat products. While HeatRisk will be lower on Monday, highs will still be above 100 in the High Deserts and above 105 in the low deserts, so continue best practices when spending time outdoors - frequent water/AC breaks and avoid strenuous activity during the hottest parts of the day. If attempting to escape to the mountains to avoid the heat, be prepared for more widespread thunderstorms Monday afternoon with heavy rain, frequent lightning, and localized flooding possible as instability peaks on this day. With easterly flow and ample mid and low level moisture, PWs will be increasing closer to 1.5-1.6, and rain rates may potentially exceed 1-1.25"/hr at times with any anchored storm. The majority of the storms are expected to remain in the mountains, but a few stronger cells may make it off and migrate into the deserts.
Temperatures will cool further Tuesday back down to near normal as heights continue to fall and the marine layer deepens into the end of the week. One final day of scattered monsoonal thunderstorms in the higher terrain as PWs remain at or above 1.5" and instability lingers across southeastern CA. After Tuesday, more robust upper level troughing and the high sliding further eastward will usher in slightly below normal temperatures into the end of the week, along with lower storm chances (10-15% chance for a storm or two off the higher terrain will exist each afternoon Wed-Fri). This cooler weather will be a welcome change after temperatures have remained at or above normal since early July.
091800z. Coast, Low clouds are continuing to clear along the coast with a few clouds lingering around 1,200-1,500ft MLS. Low clouds with bases 1,000-1,500ft MSL will start to redevelop after 03- 05Z Monday, increasing in coverage and moving further inland by 08- 10Z. Intermittent lower cigs 500-800ft MSL along coastal sites 11- 15Z. Low clouds scatter out 16-18Z.
Valleys/Mountains/Deserts, SCT-BKN cumulus still expected to develop over the mountains this afternoon with a 20-35% chance of TSRA in the mountains. Any storms that may develop could produce lightning, UDDFS and associated vis reductions. Mid/high clouds may drift into the valleys and deserts with bases AOA 10,000-15,000 ft MSL.
No hazardous marine conditions are expected through Thursday.
Astronomical high tides are expected to exceed 7 ft Monday through Wednesday between 8-9:30 PM PDT. Surf is not expected to play a large role in elevating the tides further, with a surf forecast of only 2-3 ft. Regardless, tides exceeding 7 ft can result in minor tidal overflow into low-lying areas along the beach, including beach parking lots.
Ca, Heat Advisory until 8 PM PDT this evening 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 this evening for Apple and Lucerne Valleys-Coachella Valley-San Diego County Deserts- San Gorgonio Pass near Banning.
PZ, None.
