Monsoon moisture will spread across the region Sunday and Monday, increasing the possibility of mountain and desert thunderstorms, according to the National Weather Service. The mid-level moisture will inhibit formation of the marine layer in some locations and the added humidity will make the above-normal temperatures more uncomfortable.
Upper-level high pressure over the Four Corners is expected to weaken and shift to the east, beginning Sunday night. As a result, the flow aloft will switch to the southeast, pulling up monsoonal moisture from Mexico.
There is a slight chance of thunderstorms in the eastern San Gabriels on Monday afternoon, and the air is likely to feel a bit more sticky during Labor Day picnics, cookouts and other outdoor holiday activities.