, Issued at 1114 AM PDT Sun Sep 20 2026 (This afternoon through Monday)
A cut-off low at 500 mb has allowed the marine layer to deepen (currently 2,000 ft) and ushered in a blanket of high clouds above the eroding stratus. The radar is actually showing pretty decent elevated returns over the ocean, and I just switched the scanning strategy into "precip mode". While there is some precip in the upper atmosphere, the air between these clouds and the marine layer is very dry. Most, if not all, of these rain drops will evaporate before reaching the surface. The combination of these cloud decks and a cooler upper atmosphere is keeping max temperatures notably cooler than yesterday. The coast will be stuck in the 60s all day, with inland ares broadly in the 70s this afternoon. The low will continue to approach from the west, moving overhead by Monday morning. This will allow the marine layer to deepen further and brings a good chance for drizzle Monday morning. Temperatures will be very similar Monday before a warming trend starts Tuesday. Before we get to that,
Could we get surprised by high based thunderstorms today? Probably not, but it's worth keeping an eye on. As the cut-off low approaches from the west, the associated clouds moving in from the SW look better than expected. Cloud tops are around 30,000 feet, and there are even some 20-30 dBz radar returns around 15,000'. Those numbers don't scream thunderstorms at all, but there is an outside chance that this system could become more energetic as it approaches the Central Coast over the next few hours. The 18Z HRRR shows a distinct blob of 500 mb vorticity moving into Monterey County around 1 PM this afternoon before pushing north. This HRRR also shows a signal across Monterey and San Benito County in the lightning flash rate product.
On the other hand, there is virtually no LightningCast activity, and the reflectivity at -10C and -20C is far too weak to support the amount of ice particles needed for thunderstorms to develop at the moment. With ample shear associated with the cut-off low (45 kts 0-6 km), CAPE is the limiting factor. There is a conditionally unstable layer with lapse rates around 7 C/km, but most of the moisture is above that. As the low approaches, the moisture will lower and that may open the door. Again there is a very small chance of lightning, but if it does happen, it's unlikely to be accompanied by any rain. This poses an obvious fire weather concern as dry lightning can cause new ignitions.
..issued at 1114 AM PDT Sun Sep 20 2026 (Monday night through next Saturday)
The low will continue to slide east, and we should be firmly on the back side by Tuesday morning. This will allow a low amplitude ridge to build over the next few days. This pattern change will result in a warming trend while compressing the marine layer. 850 mb temperatures are expected to climb above 20 C Wednesday - Thursday, but the trend has been a little cooler since yesterday's forecast. The widespread moderate HeatRisk is now mostly confined to Thursday. It used to look like Wednesday and Thursday. While it will certainly be warm through the middle of the week, if the recent trend continues we may not need a Heat Advisory.
Heat Advisories are somewhat common in late September, but they typically come with offshore winds and a longer duration event. This is a quick warm-up with continued onshore winds and some, albeit shallow, marine layer surviving along the coast.
The pattern changes again by Friday as another trough approaches the Pacific Northwest. This will reverse our warming trend, and more seasonal temperatures are expected through next weekend.
(18z TAFS) Issued at 1100 AM PDT Sun Sep 20 2026
High clouds are slowing the burn off of low cigs so far this morning but clearing of low clouds still forecast at most terminals away from the coast by 19z. As low clouds burn back it will be replaced by extensive high clouds passing over the region in association with upper low offshore. This low will keep the marine layer deep with low cigs spreading inland early this evening and lasting at least through 18-19z Monday.
Vicinity of SFO, Low cigs forecast to mix out around 19z with extensive high clouds and west winds around 15 kt this afternoon/evening. Low cigs spread back inland around 04z lasting overnight through 19z Monday.
SFO Bridge Approach, Lingering haze and cigs at least through 19-20z this morning before improving conditions later this afternoon and early evening.
Monterey Bay Terminals, Low cigs may not clear for KMRY today or only intermittently from 20-23z this afternoon. KSNS to clear by 19z but mid and high clouds linger through the day. Low cigs to spread back inland early this evening and linger through 18-19z Monday.
(today through Friday) Issued at 416 AM PDT Sun Sep 20 2026
Fresh to strong northerly breezes will create hazardous conditions for small craft across most of the coastal waters through Monday morning. Moderate to fresh northerly breezes will prevail Tuesday through Thursday with locally strong breezes across the northern outer waters. Slight to moderate seas will prevail through Monday, building to become moderate to rough Tuesday through Thursday.
Ca, None. PZ, Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this evening for Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory until 3 AM PDT Monday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory until 9 AM PDT Monday for Pt Arena to Pt Reyes 10-60 NM.
