Heavy rain and thunderstorms likely to drench parts of San Diego County will bring increased humidity and heighten the risk for dangerous flash flooding in some areas. A National Weather Service flash flood watch for the valleys, mountains and deserts is scheduled to last until the afternoon of Sept. 22. Although the coast was not included under the watch, forecasters said showers, thunderstorms and humidity are likely today, Sept. 21, as temperatures push into the low 80s. Forecasters predicted rainfall in excess of an inch in the deserts and the inland valleys and more than two inches in the mountains. The heaviest rain is likely through the morning of Sept. 22 as low pressure combines with an approaching plume of deep moisture, increasing the possibility of flash flooding. “Areas most affected will be the mountains, low deserts, the San Diego County valleys and the Inland Empire,” according to the weather service. “However, heavy rain is possible for all other areas as well. The risk of flash flooding will diminish as the upper low moves off to the northeast Tuesday evening into Wednesday morning.” Locales in and below recently burned areas will be particularly susceptible to heavy runoff and debris flows should strong thunderstorms develop nearby. Forecasters advised residents with outdoors plans or who live near a flood-prone area to “be alert to rapidly changing weather conditions.”