, Issued at 1118 AM PDT Wed Apr 8 2026 (This afternoon through Thursday)
We didn't expect any rain today, but were surprised by a few small showers in the North Bay this morning. These are associated with a remote outer rain band from the cut-off low pressure system over the Eastern Pacific. These early showers demonstrate how moist the atmosphere is. The 12Z sounding found a PW of 0.92", well above the 90th percentile for this time of year. This moisture is being fed by large scale southerly winds pumping a band of tropical moisture to all areas east of the cut-off low. While the widespread clouds and rain are still well off-shore, the column is already gaining moisture effectively. When the weather does roll in to this moist environment, rain showers and thunderstorms will likely overperform earlier forecasts.
I'm getting ahead of myself, focusing back on the short term forecast, the marine layer is starting to mix out with cooler temperatures aloft decreasing the stability. As a result temperatures today will remain above normal, and broadly similar to yesterday. We'll trade the stratus for passing mid level clouds associated with the outer rain bands of the cut-off low.
As the low drifts closer to the coast on Thursday, the troposphere will start to become saturated and a blanket of high clouds will move overhead. Despite the continued southerly winds, the increased cloud cover will cause max temperature to drop around 5 degrees. Light prefrontal rain is possible Thursday, particularly in the North Bay. We could even see a couple thunderstorms up there, but the main event begins Friday.
..issued at 1118 AM PDT Wed Apr 8 2026 (Thursday night through next Tuesday)
Bottom line, the forecast has taken a turn towards the wetter and more active direction for Fri-Sat. The Friday rain is caused by the cut-off low finally approaching the coast. This will bring stronger southerly winds, enhancing the advection of tropical moisture in the warm sector. The exact speed and eventual location of this low is hard to pin down due to a lack of steering flow. The greatest impacts will be near the low pressure center, where the lift and vorticity is strongest. Rain will likely start well before the Sun comes up Friday morning and continue, off-and- on, through the day. The best chance for stronger showers and thunderstorms is during the afternoon hours.
The Saturday rain is more dynamic. It's caused by a robust, reinforcing short wave trough quickly moving from the Gulf of Alaska directly towards the Bay Area. While this feature is bringing a drier air mass, the lingering moisture from the stalled low will remain in place as the two features combine. We're basically adding the jet stream back into this cut off low. That means colder air aloft and much stronger winds. The shear between the surface southerlies and upper level westerlies creates favorable conditions for longer lived thunderstorms that can separate their up-drafts and down-drafts. The low-level shear also looks favorable (20-25 kt 0-1 km) and we can't rule out rotating storms capable of producing tornadoes. Again the most active time looks to be in the afternoon during peak day-time heating. Meanwhile this system will also bring a colder air mass. The 850 mb temperature should get close to freezing and surface temperatures will struggle to get out of the upper 50s or low 60s.
By Sunday the showers should become more scattered or isolated as the drier air takes over. The upper levels will continue to cool, and we could even get a couple novelty snow flakes across the highest mountain peaks Sunday morning. There is still a chance for thunderstorms due to steep lapse rates, but the decreasing moisture will be a major barrier to convection. The Sun will likely make a return in the afternoon or early evening. All told we are expected to pick up around 0.5" to 1.0" across the valleys and 1.0" to 2.0" in the coastal mountains. Overall that's a beneficial amount of rain, but some localized flooding is possible due to the showery nature of this rain. This impact is most likely in urban areas with poor drainage.
The pattern stabilizes Monday-Tuesday as the trough moves inland and ridging starts to take its place. That will kick off a warming trend that lasts at least through the middle of the week before the uncertainty grows late week. There's roughly an equal chance of the ridge hanging on or a new trough moving in late next week.
(18z TAFS) Issued at 1037 AM PDT Wed Apr 8 2026
VFR conditions look to persist through much of the day with onshore flow increasing. LIFR/IFR/MVFR ceilings return late this evening near the coast and more so into Thursday morning across the remainder of the region. Fairly high confidence for MVFR/VFR ceilings return by mid morning on Thursday.
Vicinity of SFO, VFR. Onshore winds increase this afternoon out of the west-northwest to between 10-15kt before easing a bit late in the night. Moderate to high confidence for sub-MVFR conditions to late tonight and potentially lowering to LIFR at times (lower confidence for this to occur). Fairly high confidence for MVFR ceilings to return by 18Z Thursday and VFR by 21Z.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, VFR. Onshore winds increase this afternoon out of the northwest before easing this evening. High confidence for IFR ceilings to return late this evening. There is moderate to high confidence for LIFR conditions overnight gradually improving to MVFR by mid morning.
(today through Monday) Issued at 1037 AM PDT Wed Apr 8 2026
Gentle to moderate NW winds across the coastal waters will gradually shift to southwest over the next 24-48 hours as a low pressure system approaches the coast. This system will result in showers and a chance of thunderstorms Friday through Sunday, with SW winds increasing to a strong breeze on Saturday. As high pressure builds behind the system, fresh northwest winds will develop Sunday-Monday.
Ca, None. PZ, None.
