Issued at 939 AM PDT Wed Oct 21 2026
We are starting to see the some of the first rain bands moving into the north Central Coast this morning as the core of the cut-off upper level low approaches the region. There aren't currently any lighting strikes as of yet across the region this morning. However, we are expecting that to change as we head into late morning and moreso this afternoon and evening. This is as instability increases with up to 400-800 J/kg of MUCAPE (Most Unstable Convective Available Potential Energy). The greatest instability is forecast to remain over the north Central Coast and points southward. All eyes remain on the burn scars of the Timber and Plaskett fires given the threat for debris flows. Rain rates across these regions matter and the scattered nature of these showers and/or thunderstorms gives us less confidence of exactly how much rainfall will occur over any given one area. Please see the AFD for the Short Term for additional details.
RGass
, Issued at 1230 AM PDT Wed Oct 21 2026 (Today and tonight)
The center of the 500 mb cut-low is inching closer to the coast as the sub-tropical jet stream supports the left portion of the system, bringing enhanced momentum and moisture flux. This is where the most instability and lift can be found. As of 11:30 PM PDT, KMUX radar is picking up on a few cells offshore exhibiting a ~10-25% chance of lightning with a few lightning strikes observed along the occluded front (~250-300 miles SW from Monterey). The current offshore activity is foreshadowing what is to come early Wednesday morning.
The first few rain bands are expected to make their way over the Central Coast just before sunrise. Going into the afternoon, precipitation gets wrapped around the system's center and eventually spreads through the rest of the Bay Area. Precipitable water is expected to rise above the 90th percentile for this time of year, which prompted WPC to include the southern Central Coast in a marginal risk of excessive rainfall for Wednesday.
To further assess the potential for severe thunderstorms, three high- resolution model soundings were analyzed: the 3km NAM, 10km RDPS, and the RRFS. Keep in mind that the NAM tends to be the most pessimistic model out of the three in terms of moisture. Upon evaluation of the near-storm environment for each, there are a few inconsistencies across the models in terms of the severe thunderstorm threat and whether the environment is conducive to tornado spin-ups.
Before getting into what *is* consistent across the models, let's run through a quick overview of a couple notable stability indices: the Total-Totals and K index. The Total-Totals index measures atmospheric instability by taking the thermal difference between 850 to 500 mb to identify the potential for severe thunderstorms or tornadoes. The K index is used to assess atmospheric buoyancy by measuring the low to mid-level moisture as well as the 850-500 mb thermal difference. The greater the K index value, the greater the potential is for widespread thunderstorms (K > 20 = scattered thunderstorms, K > 35 = numerous thunderstorms). Similarly, higher Total-Totals (TT) index values indicate an enhanced potential for severe thunderstorms and/or tornadoes (TT > 48 = severe thunderstorms, TT > 50 = tornadoes).
Why am I talking about stability indices? Well, according to these indices and other environmental parameters, the three high-res model soundings that were analyzed all indicate a heightened risk for at least scattered, severe thunderstorms along the Big Sur coastline. The NAM estimates TT ~ 56 and a K index ~ 36, RDPS estimates TT ~ 54 and a K index ~ 30, and RRFS estimates TT ~ 50 and K index ~ 31.
Another consistency found across the model soundings is the amount of low-level wind shear. For our area, a small amount of shear goes a long way and can support low-topped supercells and/or brief spin- ups of waterspouts/tornadoes. From Wednesday afternoon through early Thursday morning, the models agree on low-level 0-1 km shear in excess of 10-15 kt (12-17 mph). This time frame coincides with when the greatest severe weather threat may occur. The NAM wants to fully lock in the tornado threat, though the RDPS and RRFS are split 50/50. The SPC issued a 2-4% tornado probability for the Big Sur coastline, thereby indicating increased potential for some low- topped supercells and near-coastal, potentially landfalling, waterspouts.
If severe weather does develop, here are some safety reminders:
* Have multiple ways to receive emergency alerts (i.e. NOAA weather radio, follow NWS Bay Area on social media, and/or ensuring your phone's emergency alert system is on) * Move to an interior room away from windows, if a waterspout makes landfall * Have an emergency action plan ready if you live downslope of wildfire burn scars * Avoid flooded roads. Turn around, don't drown!
By late Thursday morning, some lingering showers persist through the region with the potential for general thunderstorms and no waterspout/tornado threat across the Central Coast. Max temperatures remain in the mid 50s to low 70s as onshore winds gradually increase through the evening.
..issued at 1230 AM PDT Wed Oct 21 2026 (Thursday through Tuesday)
The rain and thunderstorm threat are gone by Friday. With the trough to our east, building high pressure brings warm and dry conditions into early next week. Euro and GFS ensemble members suggest warming to near seasonal averages meanwhile precipitable water values are forecast to dip below the 25th percentile. Monday onward, there is a great deal of uncertainty on when the weak ridge breaks down. An energetic Central Pacific plus the most recent 6-10 day Climate Prediction Center precipitation outlook suggests another trough coming our way with the potential for more rain.
(18z TAFS) Issued at 1049 AM PDT Wed Oct 21 2026
Currently VFR with clouds and rain showers pushing in from the south. The best chances for thunderstorms are for the Monterey Bay terminals this afternoon and this evening, 22Z to 02Z. Elsewhere, rain showers are expected through the TAF period, spreading from south to north. The best chances for rain showers to occur at a terminal will be this afternoon and this evening as coverage and intensity increases; however, chances linger through the TAF period with wrap around rain showers expected overnight into tomorrow morning. Winds will be light and diurnal except when rain shower/thunderstorm driven. Low confidence on sub-VFR ceilings developing overnight; however, sub-VFR ceilings and visibilities can be expected briefly if a rain shower passes directly over a terminal.
Vicinity of SFO, Currently VFR with northwesterly flow. Clouds will continue to lower through the morning with rain showers expected by this afternoon. The best chances for rain showers directly at the terminal today are between 22Z and 04Z with wrap around rain showers continuing through the end of the TAF period. Conditions may improve towards the tail end of the TAF period at or after 20Z; however, another chance of rain showers is expected during this time as the upper-level low pulls east.
SFO Bridge Approach, Relatively higher chance for thunderstorms; however, the best chances remain south of SJC. Clouds will lower and rain showers will occur sooner than SFO. Clouds at or below FL070 are expected through the TAF period; however, conditions may briefly improve between rain showers.
Monterey Bay Terminals, Currently VFR with southeasterly flow at both terminals and rain beginning to be observed at MRY. There is a chance for thunderstorms today with the best chances being between 22Z and 02Z. Rain showers will increase in coverage and intensity this evening. Wrap around rain showers are expected overnight and into tomorrow morning, likely less in coverage and intensity. Winds will be light and diurnal except when rain shower/thunderstorm driven.
(today through Monday) Issued at 939 AM PDT Wed Oct 21 2026
There is a chance for thunderstorms today, especially south of Pigeon Point. Rain showers and thunderstorms will be capable of producing local gale force wind gusts, rough seas, heavy rainfall with reduced visibility, and waterspouts. Gentle variable breezes will prevail today. Moderate to fresh northerly breezes will prevail Thursday through Sunday with locally strong to near gale breezes across the outer waters Thursday through Saturday. Moderate seas build to become rough across the outer waters Friday.
Ca, None. PZ, Small Craft Advisory from 9 PM Thursday to 9 AM PDT Friday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory from 3 PM Thursday to 9 AM PDT Friday for Pt Arena to Pt Reyes 10-60 NM.
