Southern California heat wave spikes humidity, heat advisories issued across Los Angeles basin

National Weather Service issues heat advisories for LA County beaches, valleys and mountains as temperatures near 100°F and humidity climbs, with an extreme heat warning extending through Thursday

A potent combination of soaring temperatures and unusually high humidity is set to blanket the Los Angeles area through Thursday, prompting the National Weather Service to keep heat advisories in place for the LA Basin and to expand them into Ventura, Santa Barbara and San Luis Obispo counties. The forecast calls for daytime highs climbing to the upper 90s and even reaching 100°F in Pasadena, while heat index values are expected to exceed 110°F, creating hazardous conditions for outdoor workers and vulnerable residents.

The heat surge follows a weekend of lingering moisture from the remnants of former hurricane Marie, which has lifted precipitable water values well above normal and is sustaining a 10-20 percent chance of scattered showers across the four-county region. Even as the rain threat wanes, the moisture layer is amplifying humidity, a key factor that the NWS says will drive the heat risk from advisory to extreme warning levels by Wednesday afternoon.

The latest Area Forecast Discussion from the Los Angeles/Oxnard office notes that weak offshore flow this morning is limiting the usual sea-breeze cooling effect, allowing temperatures to climb rapidly. The discussion highlights that advisory-level heat concerns are likely across the LA Basin today and that the heat will intensify on Wednesday and Thursday as an upper ridge settles over the region. The forecast calls for highs of 98°F in downtown Los Angeles this afternoon, climbing to near 100°F in Pasadena, with heat index values projected as high as 109°F in the city center and up to 114°F in the Pasadena valley, according to National Weather Service.

The NWS also issued an Extreme Heat Warning for a swath of the county that includes Calabasas, Agoura Hills, the eastern San Fernando Valley, the eastern Santa Monica Mountains, the Los Angeles County beaches, the inland coast, the San Gabriel Valley, Malibu and the Palos Verdes Hills. The warning runs from 8 AM Wednesday to 8 PM Thursday, covering temperatures between 92°F and 108°F and emphasizing that humid conditions will add to heat discomfort. The warning details match the advisory expansion described in the GovOneStop report.

In addition to the heat, the forecast indicates a 30-percent chance of brief, moderate-to-heavy showers this afternoon across the coastal waters and a 10-15 percent chance of isolated thunderstorms over the northern VTA county mountains on Wednesday. However, the primary story remains the heat, with PWAT values near 1.5 inches, about 0.86 inches above normal, fueling the humidity that is expected to keep overnight lows in the 70s and even the low 80s in foothill locations.

Health officials warn that the combination of high temperature and humidity dramatically raises the risk of heat-related illnesses, especially for the elderly, children and those without reliable air-conditioning. The extreme heat warning advises residents to drink plenty of fluids, stay in air-conditioned spaces, avoid outdoor activity during peak heat hours, and check on neighbors who may be vulnerable. The advisory also cautions that car interiors can reach lethal temperatures within minutes, underscoring the need for vigilance when transporting pets or children.

Pasadena’s local forecast mirrors the regional outlook, projecting a high near 100°F on Wednesday with heat index values as high as 114°F. The city’s heat advisory, issued by the National Weather Service, covers coastal valleys and beaches through Thursday, warning of dangerous conditions driven by the combination of heat and humidity. Residents are urged to limit outdoor exertion, seek shade, and use fans or cool showers to lower body temperature, as detailed by Pasadena Now.

The weather.gov portal confirms that the heat advisory began at 12:10 PM on September 8 and will remain in effect until 8 AM on September 9, with the extreme heat warning taking over from that point through Thursday evening. The site also notes that heat index values could reach 108°F in the broader Los Angeles area, reinforcing the urgency of the public health messaging. Local shelters and cooling centers are being readied, and the Los Angeles County Department of Public Health has activated its Heat Relief Hotline to field calls from residents experiencing heat stress.

The heat wave underscores how quickly a lingering tropical moisture plume can turn a typical September into a life-threatening event for Angelenos. With outdoor dining, street festivals and construction crews still active, the advisory forces businesses and city services to adapt plans, protect workers and keep cooling resources available. The overlap of heat and humidity also raises fire-risk concerns as any rain that does fall will be light, leaving dry fuels primed for ignition later in the week.