FXUS66 KMTR 211436
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
736 AM PDT Tue Apr 21 2026

...New SHORT TERM, LONG TERM...

.KEY MESSAGES...
Updated at 725 AM PDT Tue Apr 21 2026

 - Scattered showers this morning with a good chance for
   thunderstorms midday

 - Lightning strikes, heavy downpours, small hail, and gusty
   erratic winds are the primary hazards

 - Dry weather Wed afternoon through Friday before disturbed
   weather returns this weekend and into next week

&&

.SHORT TERM...
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.

&&

.LONG TERM...
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. 

&&

.AVIATION...
(12Z TAFS)
Issued at 421 AM PDT Tue Apr 21 2026

Multiple rounds of light to moderate showers will continue to move
across our area through the morning hours into the afternoon.
There will be breaks at individual terminals at times between
rounds. North Bay terminals should be the first to see widespread
showers come to an end, especially KSTS by mid-afternoon. KAPC 
may be far enough south to see shower activity extend into late
afternoon. There continues to be a 30-40% chance of thunderstorms
after 8AM this morning through the afternoon hours. Expect gusty 
erratic winds and moderate to heavy rain with the strongest 
showers/thunderstorms.

Vicinity of SFO...MVFR to VFR is expected to prevail through the
TAF period with multiple rounds of scattered showers continuing
through the morning and afternoon, with some breaks in shower
activity at times. There is a chance of isolated thunderstorms as
early as 8AM this morning through late morning into the afternoon.
Expect gusty erratic winds and moderate to heavy rain with the
strongest showers/thunderstorms.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR is expected to prevail with periods 
of MVFR as several rounds of showers move across the peninsula 
this morning and afternoon. There's a chance of isolated 
thunderstorms late this morning into the afternoon, expect gusty 
and erratic winds and moderate to heavy rainfall with the 
strongest showers/thunderstorms. MVFR ceilings are possible later
this evening into the overnight hours.

&&

.MARINE...
(Today through Sunday)
Issued at 421 AM PDT Tue Apr 21 2026

Moderate to fresh westerly breeze will develop today. Scattered
light showers will increase in coverage later this morning into
the afternoon. Some of these showers will be strong with a 20-30%
chance of thunderstorms. Once the weather breaks by late Wednesday
afternoon, winds will become moderate out of the northwest, then
increase to strong out of the northwest on Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
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.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...JM
MARINE...JM

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Prepared by Boulder-Creek.com Weather at: Tue Apr 21 08:30:03 PDT 2026