The ongoing heatwave in San Diego County will spread to include all of Southern California from now through Thursday. A combination of above average temperatures and high humidity will create major to extreme heatRisk through the coastal basin. Moderate to locally major HeatRisk expected elsewhere. A cooling trend starting Friday will bring temperatures back to near or slightly below normal from Sunday onward. Scattered thunderstorms expected in the mountains this afternoon. Only a slight chance for showers or storms Wednesday and Thursday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
A number of factors came together to create a scorching afternoon in San Diego County yesterday. First, the tropical moisture that has been funneling into the region for a few days effectively wiped out any cooling marine layer influence. Also, subsidence was likely sourced from multiple locations yesterday; the building high pressure to the east and the compensating sinking air associated with the shower activity over Orange County and areas further north.
As high pressure continues to build into Southern California, tropical clouds and showers will be pushed northward and anomalously hot conditions will expand to include the entire region from today through Thursday. Additionally, southeasterly flow today and Wednesday will bring exceptionally high humidity to the region, which will not only exacerbate the heat during the day, but will also limit the amount of cooling expected overnight. High and low temperatures running around 5 to 15 (locally 20) degrees above normal along with high humidity will create major to extreme HeatRisk throughout the coastal basin today and Wednesday. Similarly anomalous temperatures will create moderate HeatRisk in the mountains and deserts with areas of major HeatRisk in the low deserts. Note that this set up will be particularly anomalous and uncomfortable for coastal areas where the typical marine layer influence will be weak or absent. It is advised to seek shelter in air-conditioned buildings and stay hydrated during this heatwave.
As flow turns more southerly today, there will be sufficient moisture for isolated to scattered monsoon showers and thunderstorms along the mountain ridges from San Diego County northward up through the Big Bear Lake area. The morning sounding at KNKX recorded PWATs around 1.8 inches, but modest CAPE values and capping will limit how much of that moisture can be realized as precipitation. Still, thunderstorms may locally produce rain rates over 0.5" per hour along with gusty winds, mainly in San Diego County. As high pressure moves overhead on Wednesday, mountain thunderstorm chances will become quite limited (10-20%). Further capping on Thursday may inhibit any shower or storm development entirely.
The high pressure slowly begins to weaken from Thursday onward. Moderate to major HeatRisk is expected to linger through Thursday across the region and into Friday for the deserts. Thereafter, temperatures continue to cool over the weekend, returning to near or slightly below normal from Sunday onward. Additionally, marine layer clouds will finally return beginning on Friday or Saturday. Increasing onshore flow will allow the marine layer to deepen into early next week.
081800z, Mostly VFR conditions expected across the region through the TAF period. SCT to locally BKN clouds between 10-20 kft. CBs based at 9 kft MSL leading to isolated SHRA/TSRA over mountains with tops to 35 kft 19-01Z. Localized gusty winds in and near any TSRA.
No hazardous marine conditions are expected through Saturday.
A large south swell from Tropical Storm Marie will continue to produce large surf of 3 to 6 feet, with sets up to 8 feet today, highest on south and southwest facing beaches. This will result in strong rip and longshore currents, creating hazardous swimming conditions. Additionally, high tide this evening will combine with the high surf to produce minor tidal overflow, coastal flooding, and localized beach erosion. Swell and surf will gradually diminish overnight tonight. Check the Coastal Hazard Message for details.
Ca, Coastal Flood Advisory until 11 PM PDT this evening for Orange County Coastal Areas-San Diego County Coastal Areas.
High Surf Advisory until 11 PM PDT this evening for Orange County Coastal Areas-San Diego County Coastal Areas.
Extreme Heat Warning until 8 PM PDT Wednesday 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 from 8 PM Wednesday to 8 PM PDT Thursday 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 Thursday for Riverside County Mountains-San Diego County Mountains.
Extreme Heat Warning from 10 AM Wednesday to 8 PM PDT Friday for Coachella Valley-San Diego County Deserts-San Gorgonio Pass near Banning.
PZ, None.
