The hot and humid high pressure system over the desert southwest will continue to weaken into the weekend. Hot conditions this afternoon will bring a cooling trend with lower humidity over the next few days as an area of low pressure is expected to move through the West Coast by the weekend. Monsoonal moisture hangs on for one more day this afternoon with a slight chance of thunderstorms across the mountains and adjacent deserts. The weak troughing pattern will stick around into at leat the first half of next week, bringing a deeper marine layer and cooler weather.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Another hot day out there as high pressure continues to dominate the desert southwest. Weakening is seen in this system today though, as observations show better onshore flow closer to the coast. This is keeping temperatures a bit cooler than at this time yesterday with more places in the 80s instead of the 90s, so the high temperature forecast may be overdone in some places. One last chance for thunderstorms will occur later this afternoon across the mountain spine from the Mexican border to Big Bear, as well as some of the desert areas. Chances for precip range from 20-40 percent.
A troughing pattern begins to develop over the West Coast by Friday, which will help weaken the high pressure system further and bring in more cooling. High temperatures were raised slightly from the previous forecast, but still shows a steeper decline in temperatures mainly west of the mountains from 5-10 degrees cooler than today. A stronger low pressure system develops from this troughing pattern to the north, swinging through the Pacific Northwest this weekend. This will bring further cooling to our region with temperatures lowering closer to normal for the middle of September. Welcome highs in the 80s/90s for western valleys, most mountain areas, and locales in the high desert can be expected. Overnight temperatures and humid conditions will also decrease into the weekend as well as drier southwest flow moves into our area. Morning temperatures in the 70s will be replaced with 60s west of the mountains making it feel much more comfortable for us. The marine layer will also slowly return for coastal and western valley areas over the next few days as well.
The area of low pressure moves out of the Pacific Northwest by Monday, leaving behind a weak troughing pattern over the region for at least the first half of next week. This will bring similar temperatures each day with highs and lows near to slightly below average with dry weather.
101815z, VFR conditions expected across the region through this evening, with FEW clouds between 10-20 kft and SCT cumulus over mountains. ISO SHRA/TSRA over mountains and near desert slopes 19- 00Z, with CB clouds based 8-12 kft MSL and tops rising to 30 kft. Localized gusty winds and heavy rain in and near any TSRA. Areas of coastal low clouds based 600-1000 ft MSL and fog will gradually increase after 07Z Friday, with cigs most likely 11-16Z Friday in vcnty KSAN and patchy cigs 13-16z in vcnty KCRQ. Vis 1-5SM over higher coastal terrain. FG and vis 0-1SM locally in valleys east of I-15 (SD County) above 600 ft MSL. Scatter out 16-18z Fri.
No hazardous marine conditions are expected through Monday.
Ca, Extreme Heat Warning until 8 PM PDT this evening for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire- San Diego County Coastal Areas-San Diego County Valleys- Santa Ana Mountains and Foothills.
Heat Advisory until 8 PM PDT this evening for Riverside County Mountains-San Diego County Mountains.
Extreme Heat Warning until 8 PM PDT Friday for Coachella Valley- San Diego County Deserts-San Gorgonio Pass near Banning.
PZ, None.
