Seasonal Lag Explains Why West Coast Cities See Hot Weather in September

While the mercury has finally starting to drop across the West, residents in the region were met with unseasonably warm weather to welcome the month of September. However, forecasters have been quick to remind people that this type of heat is not particularly uncommon this late in the season in California. Here is what you need to know about what weather experts refer to as a "seasonal lag."

September Heat Wave Out West Not an Abnormality

It is not unusual for many coastal West Coast cities to not experience the hottest weather of the summer until September. That is exactly what happened last weekend when places such as downtown Los Angeles saw the temperatures soar into the triple digits. You have to go back to the summer of 2020 to find temperatures this hot throughout the Los Angeles basin.

The heat was the result of a zone of high pressure that was set up over California for several days. The middle of this high was anchored over the southern part of the Golden State, translating to the scorching weather for cities that typically enjoy moderate temperatures.

However, Southern California typically does cool down slower than locations farther to the north. This is because the cooler waters over the Pacific Ocean need the entirety of the summer to heat up. The water ocean water temperatures also work to mitigate the cooling marine layer that is often present during the beginning and middle of the summer season.

For example, coastal communities such as Santa Monica typically record the warmest three months of the year from August through October. San Diego also sees its hottest temperatures of the year during these three months with readings coming in warmer on average than June and July.

Although the downtown area of Los Angeles generally sees it warmest weather in August, the warmest average September day as measured by the mean maximum comes in on top for the year. You will find a similar situation to the north in San Francisco with this metro area also recording its warmest month in September.