High pressure remains locked in place in the near term as hot days and near-record warm nights continue through Friday night. Slight chance of showers and thunderstorms for the mountains this afternoon and Friday afternoon. High pressure weakens and slides east this weekend with temperatures cooling to near average by Sunday and below average by early next week. For Tuesday and Wednesday of next week, increased high cloud cover is likely along with a 20 to 30 percent chance of precipitation from a tropical system that has yet to develop.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
The strong upper ridge centered over Arizona will remain in place through Friday with 500 mb heights hovering around 594 dm over southern California. High temperatures generally overperformed yesterday with daily record high maximums set in a few inland valley and desert locations. This overperforming trend will have to compete with increased cloud cover today, courtesy of the remnants of Tropical Storm Iselle, so forecast highs are few degrees lower than yesterday's temps. Regardless of exact temperatures, hot days in combination with near-record warm nights will produce major to locally extreme HeatRisk through Friday for inland valleys and lower deserts. Moderate to locally major HeatRisk is expected for the mountains, High Desert, and areas near the coast.
There is a slight chance (10-20%) for thunderstorms over the mountains this afternoon and Friday afternoon. There is ample moisture present (PWATs around 1.4-1.5"), but the 12z KNKX sounding indicates the presence of substantial mid-level capping, which will keep convection quite limited.
By Saturday, the ridge begins to weaken and slide eastward as a robust upper trough pushes into the PNW. This will usher in a welcome cooling trend across the region. Temperatures are expected to return to near average by Sunday with highs further cooling to around 3 to 7 degrees below average early next week. This pattern change will also produce breezy westerly winds for the mountains and deserts and deepen the marine layer for Sunday and Monday.
From next Tuesday onward, the forecast becomes more uncertain. The synoptic set up will remain generally unchanged on the synoptic scale with troughing over the PNW and ridging over the south-central CONUS resulting in southwesterly flow the region. Simultaneously, global models indicate that a tropical system will pass well to the southwest early next week. The exact evolution of this large scale set up and the track of the tropical system will determine how much moisture will be drawn into this region for the middle of next week. Most ensembles indicate a notable increase in PWATs over southern California next week, but only about 20-30% have any precipitation at this time.
271800z, Coast, Low clouds this morning have largely cleared out, but remain right along the immediate coast of extreme southwestern San Diego County. These clouds are generally based around 500ft MSL and within south-southwesterly flow, will linger in the vicinity of KSAN into the afternoon. Guidance suggests marine layer clouds will be even patchier tonight than this morning, with only stray patches off the northern SD/Orange County coasts. Best chances for CIGs overnight remain across the southern San Diego County coasts, with KSAN having a ~70% chance for CIGs between 11-15z Friday, with a ~40% chance at KCRQ. Bases for any clouds would be around 500ft with VIS restrictions to 3-6 SM in BR. Any clouds that do push onshore will clear back out offshore by 16z.
Mountains/Deserts, Cumulus based around 9000 feet MSL over the mountains this afternoon with isolated CB cloud tops rising to 40000 feet MSL. There is a 15-20% chance of TSRA over the mountains and a 5-15% chance in the deserts until 01z. Any TS that develops could produce gusty, erratic winds.
Regionwide, High clouds AOA 15000 feet MSL will continue today and tonight, decreasing early Friday.
No hazardous marine conditions are expected through Monday.
A 4-foot southerly swell (from 170 degrees) at 12-13 seconds will continue to produce elevated surf through tonight, particularly for south-facing beaches of Orange County and northern San Diego County. Surf of 3 to 5 feet, with sets up to 7 feet is expected. See the Coastal Hazard Message for details on beach hazards.
Ca, Beach Hazards Statement until 11 PM PDT this evening for Orange County Coastal Areas-San Diego County Coastal Areas.
Heat Advisory until 8 PM PDT Friday for Orange County Coastal Areas-Riverside County Mountains-San Bernardino County Mountains-San Diego County Coastal Areas.
Extreme Heat Warning until 8 PM PDT Friday for Apple and Lucerne Valleys-Coachella Valley-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire- San Diego County Deserts-San Diego County Mountains-San Diego County Valleys-San Gorgonio Pass near Banning-Santa Ana Mountains and Foothills.
PZ, None.
