Issued at 434 PM PDT Wed Apr 8 2026
High based showers have developed across the higher terrain this afternoon. Although most of the resulting precipitation has not reached the ground, and no lightning has been reported with these cells, a couple of the strongest showers could still result in precipitation, with the possibility for brief periods of rain and gusty winds to 30 mph through sunset. Otherwise, no changes to the forecast.
DialH
, 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.
(00z TAFS) Issued at 434 PM PDT Wed Apr 8 2026
Generally VFR conditions through the region with some convective clouds and virga developing over the higher elevations. Moderate onshore flow continues into the evening hours, diminishing to become light overnight. MVFR-IFR stratus will return to much of the inland regions through the evening and overnight hours, with LIFR and patchy fog at the coastal terminals. Stratus will lift and retreat to the immediate coast through Thursday morning, with mid- to high level clouds across the region as a system approaches, causing southwest winds and isolated showers through Thursday afternoon and evening.
Vicinity of SFO, VFR with some high clouds and moderate to fresh west-northwest winds through the evening hours. IFR stratus returns to the terminal late this evening as winds become light overnight. Ceilings will lift late Thursday morning into the early afternoon, with moderate winds resuming from a southwesterly direction in advance of a system that will bring MVFR-IFR ceilings and some isolated showers to the region on Thursday evening, but with most of the rain expected after the end of the TAF period.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, VFR with some high clouds through the evening as moderate northwest winds continue. IFR-LIFR stratus will develop across the region this evening through the overnight period, with the stratus gradually lifting through Thursday morning and afternoon. Mid-to high level clouds will then persist through the end of the TAF period. Winds will become light overnight and Thursday morning before an onshore breeze resumes in the afternoon.
(tonight through next Monday) Issued at 434 PM 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 Thursday 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.
