Strengthening high pressure aloft will bring a warming trend with a long-duration heat wave likely for this weekend through much of next week into the following weekend. The coastal basin will be particularly hot through this weekend as offshore flow reinforces hot and dry conditions. This will also produce elevated fire weather conditions for inland valleys and mountains through this weekend. Major HeatRisk is expected for most of the coastal basin from Friday through next week with areas of extreme HeatRisk for coastal zones and western valleys. Moderate HeatRisk through next week in the low deserts with minor HeatRisk elsewhere.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
A building strong high pressure today will center itself over southern NV and northern AZ this weekend. Easterly flow aloft will allow for periods of offshore winds through the weekend, especially in San Diego County. This will amplify already hot and dry conditions in the coastal basin. Model guidance suggests 500 mb heights will peak around 590 dm and 850 mb temps will reach 24-25 Celsius over the weekend. With offshore flow in place, this translates to surface temperatures well over 100 degrees through inland areas of the coastal basin Friday through Sunday. For western valleys, highs will locally reach 105 or higher, especially in San Diego County on Saturday and Sunday. This will combine with lows in the upper 60s to low 70s to create major HeatRisk throughout the coastal basin from Friday onward. Areas of extreme HeatRisk expected for coastal areas and western valleys.
Easterly flow of 10-20 kts at 700 mb through the weekend will support weak Santa Ana winds gusting 25-45 mph (locally higher) along west-facing slopes, adjacent foothills and locally into the inland valleys. The orientation of winds will favor San Diego County and downwind of the Banning Pass. Offshore winds in combination with heat and afternoon RH values of 15-25% will create elevated fire weather conditions for inland valleys and mountains through the weekend.
Outside of the coastal basin, heat will be less anomalous. High temperatures will range from the 70s in the mountains to near 100 in the low deserts, which is only about 5-10 degrees above normal. Thus, HeatRisk is minor outside of moderate risk in the low deserts.
Heading into early next week, the mean of model guidance backs off somewhat with the heat in the coastal basin as the high pressure weakens a touch and slides east. This will likely reduce or remove offshore wind potential for Monday and Tuesday. However, high temperatures in the 90s to near 100 inland from the coastline will keep major HeatRisk in place for most of the coastal basin. By mid to late next week, the general consensus is that upper ridging will strengthen slightly again, keeping this heatwave firmly in place.
011845z, Coast/western valleys: Very patchy low clouds based 700- 1000 ft MSL with local vis reductions (1-5SM) inland edge of the low clouds. Clouds will be far patchier and intermittent when they reform late Thursday evening, impacting coastal sites late, if they do at all(30-50% chance for cigs, highest probability vcnty KSAN).
Otherwise, clear skies today and tonight.
No hazardous marine conditions are expected through Monday.
Ca, Extreme Heat Warning from 10 AM Friday 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.
PZ, None.
