Issued at 1200 AM PDT Wed Aug 13 2025 (Today and tonight)
Let`s talk about moisture first. Tropical Storm Ivo dissipated early Monday in the tropical Eastern Pacific and most of the moisture is being reabsorbed by the monsoon trough. Despite this, a long loop of water vapor imagery shows that a small band of this moisture split off and pushed north towards the Bay Area. This moisture is now manifesting as upper level clouds currently streaming across the Bay Area (well above the marine layer stratus). These clouds are somewhere between 20,000 and 25,000 feet based on a comparison between IR imagery cloud top temperatures (-11C) and the latest vertical sounding (temperature vs. height). Outside of these high clouds, this moisture is too high and thin to have any other effect on the weather at the ground. The vertically integrated precipitable water is only 0.61". That puts us in the 25th percentile for this time of year. So the overall moisture is still well below normal. The reason for this is very dry air between the marine layer and these high clouds. The PGE 2km WRF cross section across the Bay Area is highlighting RH around 10% above 3,000 feet. This dry air is being dragged to lower elevations east of the Diablo Range due to mountain wave activity. As a result, a few inland stations are reporting red flag conditions with RH still in the teens at midnight and moderate westerly winds around 15 mph.
