, Issued at 725 AM PDT Tue Apr 21 2026 (This afternoon through Wednesday)
We have pretty good thunderstorms chances today. The surface cold front has passed through the cwa, ending the continuous rain we had last night. Our attention now shifts to the post-frontal environment. Scattered showers are expected through the day with a a 30-40% chance of thunderstorms around midday. The 12Z sounding featured high shear, but low instability. Specifically the 0-6 km shear was measured at 82 kts, but the surface based CAPE was only 12 J/kg. While that wouldn't support anything other than gusty winds, the best environment is not here yet. Cold fronts tend to stack back (westward) with height, meaning while the surface front has moved through, cold air aloft is still filtering in. The 500 mb temperature was measured at -21C on the 12Z sounding, but is expected to drop to -26C by 18Z. That rapid cooling of the upper levels changes the instability profile drastically. The latest HRRR model sounding for Watsonville brings the CAPE up to 713 J/kg at 17Z (10 AM). This instability is followed by a post-frontal trough that will serve as a trigger to release the potential energy as upward vertical motion. We are already starting to see this main band of thunderstorms develop roughly 130 miles SW of Big Sur, moving NE at around 20-25 mph.
While the instability is rapidly increasing, the shear is decreasing. The same model sounding for Watsonville only has 8 kts of 0-1 km shear, and the 0-6 km shear drops from 78 kts at 12Z to 43 kts by 17Z. So this morning will be a transition from low CAPE, high shear, to high CAPE, low shear. While there may be a sweet spot in the interim, by late morning it looks like any thunderstorms will be the garden variety. We are not expecting any severe criteria including tornadoes, damaging wind, or large hail. The biggest hazards will be lightning strikes and heavy downpours. These storms will also bring small hail and gusty, erratic winds.
As far as timing goes, after scattered showers this morning, high resolution models are highlighting the 11AM - 3PM window for the main band of showers and thunderstorms to pass through, with more isolated coverage through the rest of the day. Coverage looks pretty widespread across the cwa, with the best chances anywhere south of the Golden Gate.
..issued at 725 AM PDT Tue Apr 21 2026 (Wednesday night through next Monday)
The drying trend will continue into Wednesday, although there could still be a lingering shower or two in the morning. By the afternoon it will be a nice, cool, sunny day with highs in the mid 60s. A very weak ridge will build on Thursday and Friday increasing the temperature a bit, but this will quickly be overtaken by another trough moving in from the Eastern Pacific. This next feature brings a slight chance of light rain through the weekend, but it won't be anything like what we're going through yesterday and today. The disturbed weather and slight rain chances will continue through at least the first half of next week.
(18z TAFS) Issued at 1100 AM PDT Tue Apr 21 2026
VFR this afternoon with occasional MVFR/IFR due to passing showers/lower cigs. The convective threat is looking less for the heart of the Bay Area and more for SNS/MRY. Expect a brief break late this afternoon and early tonight. Will need to wait for another upstream circulation to bring another round of -shra and MVFR cigs for a few hours early Wednesday. VFR returns by Wednesday afternoon.
Vicinity of SFO, VFR with pockets of blue skies through the afternoon. That being said, cannot rule out a passing shower dropping CAT to MVFR/IFR briefly. Another round of precip and lower CIGS will be possible during the AM rush for Wednesday.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, VFR generally with MVFR/IFR due to passing showers and low chc (20%) for thunderstorms this afternoon.
(today through Sunday) Issued at 1100 AM PDT Tue Apr 21 2026
A moderate breeze will gradually shift from SW to westerly today. Showers and thunderstorms will bring locally gusty and erratic winds to the coastal waters through the day. As the weather breaks on Wednesday, winds will shift to a moderate NW breeze, before increasing to a fresh breeze on Thursday.
Ca, None. PZ, Small Craft Advisory until 9 PM PDT this evening for Mry Bay- Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory until 3 PM PDT this afternoon for Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory from 3 AM to 3 PM PDT Thursday for Pt Arena to Pt Reyes 10-60 NM.
