SHORT TERM... Issued at 132 AM PDT Tue Oct 21 2025 (Today and tonight)
Current satellite shows southerly surge of moisture and associated stratus pushing into the Monterey Bay region and up the coast of Santa Cruz and San Mateo counties. These locations can expect areas of dense fog this morning. The marine layer is still very compressed along the coast and well mixed inland thanks to weak offshore flow, so expect another day of mostly clear skies and dry conditions. The offshore pressure gradient begins to reverse ahead of a pattern change slated to take place starting this evening. Deep moisture from an EPac upper low begins advecting into the region this evening, first made apparent by increased mid-to-upper level cloud cover. Good surface moisture spreads much further inland tonight with RH 70-100% region-wide and PWAT approaching 1" by Wednesday morning (approx 160% of normal for this time of year). Deep moisture and marginal lift associated with this upper low will potentially lead to a few isolated showers and thunderstorms overnight tonight into Wednesday morning. Weak flow aloft doesn`t promote much in the way of large-scale forcing, and there is still a hint of a dry layer between the moisture aloft and surface moisture. While these aren`t terribly noteworthy alone, we are keeping the mention of dry lightning simply because any rain that does fall will likely be below the wetting rain definition of 0.1". Highest likelihood for these impacts to occur looks to be interior Monterey and San Benito counties, coastal waters off the Central Coast and Santa Clara, San Mateo, and Santa Cruz counties. Any shower and thunderstorm activity that does occur should be winding down by just after sunrise Wednesday. Despite a slight risk for dry lightning, overall fire risk remains on the lower end given the moist fuels and recent wetting rains last week.
