Another hot day Sunday was forecast for inland San Diego County, and increasing monsoon moisture is expected to create more humid conditions and a chance for showers and thunderstorms each day through Wednesday, the National Weather Service said.
A heat advisory was issued until 9 p.m. Sunday for San Diego County mountains and valleys and an excessive heat warning was issued until 9 p.m. Sunday for county deserts.
A flash flood warning was in effect until 4:30 p.m. for Hwy 79 Between Warner Springs and Oak Grove, Hwy 79 Between Oak Grove and Aguanga, S7 – East Grade Rd, Palomar Mountain, Lake Henshaw, Warner Springs, Hwy 79 Between Santa Ysabel and Warner Springs, Aguanga, Oak Grove, La Jolla Indian Reservation and Los Coyotes Indian Reservation.
A strong thunderstorm is moving across SD & Riverside counties. FLASH FLOOD WARNING in red until 4:30pm. FLOOD ADVISORY in green / SPECIAL WEATHER STATEMENT possible sever weather in beige. EXCESSIVE HEAT WARNING pink, HEAT ADVISORY in orange until this evening. @nbcsandiego pic.twitter.com/RvLFX2bEHN
— Dagmar Midcap (@DagmarNBCSD) August 29, 2021
High heat and low relative humidity were expected to create elevated fire weather conditions through Sunday, especially north of San Diego County, forecasters said. The main threat is plume-dominated fires. Gusty winds along the sea breeze front and near-desert passes will also be a concern Sunday afternoon, the NWS said.
High temperatures along the coast Sunday were predicted to be 75-80 degrees and inland 82-87 with overnight lows of 63-68. Highs in the western valleys will be 86-91 and 95-100 near the foothills with lows of 64-71. Mountain highs were expected to be 92-98 with overnight lows of 66-74. Highs in the deserts will be 107-112 with overnight lows of 84-89.
Storms over the lower deserts and mountains were expected Sunday afternoon and there could even be a few stray showers tumbling into the foothills west of the peninsular ranges of San Diego County, forecasters said.
First showers forming in the lower San Diego mountains – activity will move east to west and increase over the mountains in the afternoon with thunderstorms forming #monsoon #CAwx note: Poway is ground clutter pic.twitter.com/tni11b6HM3
— NWS San Diego (@NWSSanDiego) August 29, 2021
Minimum relative humidity of near 20% will trend upward into Monday due to the monsoon flow.
Cooler weather was forecast to spread farther inland after Sunday, as the marine layer rebuilds with periods of night and morning low clouds and fog.
For Tuesday, the flow turns more southerly, and instability aloft was expected to increase over the entire area as another surge of subtropical moisture pushes northward.
Southwest flow should dry San Diego skies later this week, and rebuilding high pressure will bring warming into the Labor Day Weekend, the NWS said.