FXUS66 KMTR 211823
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1123 AM PDT Tue Apr 21 2026

...New AVIATION, MARINE...

.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...
(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.   

&&

.MARINE...
(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.

&&

.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.

     Small Craft Advisory from 3 AM to 3 PM PDT Thursday for Pt Arena 
     to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...MM
MARINE...RGass

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Prepared by Boulder-Creek.com Weather at: Tue Apr 21 14:30:02 PDT 2026