FXUS66 KMTR 210008
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
508 PM PDT Mon Apr 20 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 1210 AM PDT Mon Apr 20 2026

 - Cool temperatures and beneficial rainfall today through
   Wednesday

 - Isolated to scattered thunderstorms on Tuesday

 - Seasonal temperatures and dry conditions for the later part of
   the week

&&

.SHORT TERM...
Issued at 200 PM PDT Mon Apr 20 2026 
(This evening through Tuesday)

The much advertised wet weather has finally arrive - began with a
few showers last night through early this morning and more
widespread precip this afternoon. A cold front knocking on the Bay
Area's doorstep has led to an increase in precip over the North
Bay stretching south to SF Bay and finally Santa Cruz the last 
3-6 hrs. Monterey Bay and points E and SE remain dry at this time.
Decent precip totals over the last 12 hrs with 0.25-0.75 inches 
North Bay Coastal Mts and a few hundredths to 0.25" elsewhere. In
addition to the precip, the frontal structure and associated 
pressure gradient led to gusty southerly winds. Gusts since 
midnight range from 20-40 mph most areas with gust 40-50 mph along
the coast and and higher terrain. 

Rest of today through tonight: The front off the coast will
finally makes its way inland with a nudge from an upstream sfc low
and upper level jet. As such, the more stratiform precip will
move through the Bay Area and Central Coast with impacts to the 
evening commute. Wet roads and windy conditions. By late this 
evening the steadier rainfall will transition to more convective
nature/shower behind the front. 

Tuesday: Post frontal regime on Tuesday will be mainly be driven
by the trailing upper level trough moving overhead, cold pool 
associated with trough, a phasing jet aloft around 200-250mb, and
a jet max near 850-700mb. Lastly, some increased insolation 
during the afternoon due to less clouds. The combo of colder air 
aloft and daytime heating will increase sfc and ML CAPE. Factor in
some of the wind fields and there's an increase for late 
morning/early evening thunderstorms. While there is some large 
0-6km shear values it appears to be more confined to the 3-6km 
layer. Would like to see high shear below 3km for 
waterspout/tornado concerns. That being said, we'll be watching 
closely. Regardless, stronger storms will be capable of producing
gusty winds, heavy rain, and small hail given the temp profile. 
SPC is not highlight tor/hail/wind like they were earlier in the 
month. They do have a general mention of thunder though. 

&&

.LONG TERM...
Issued at 219 PM PDT Mon Apr 20 2026 
(Tuesday night through next Sunday)

Once the sun sets Tuesday evening the threat for thunderstorms
quickly fades, but lingering showers will persist through early
Wednesday. We will have to wait for the upper trough and
embedded/trailing weak shortwave to exit before shower activity
ends Wednesday.

Thursday into next weekend. Warm and drier conditions will be 
trend. However, that trend will be tempered by persistent 
trough/broad upper low settling over the EPAC and CA. Conf with 
the forecast decreases for next weekend. Current forecast has a 
passing shower late Saturday due to another trough swinging 
through CA. Ens members are scattered on amounts, but most members
at least show something. This will be fine tuned over the next 
few days, but for now cannot rule out some precip.

&&

.AVIATION...
(00Z TAFS)
Issued at 451 PM PDT Mon Apr 20 2026

The cold front is now right along the coast, with prefrontal rain
ongoing across the majority of the terminals. This light to
moderate steady rain will continue through the night before a
break in the early morning hours. This won't last long, however,
as a band of stronger showers and possible thunderstorms is
expected to roll through late morning through early afternoon.
After this finale, showers will become much more isolated as the 
sun comes back out.

Vicinity of SFO...Prefrontal rain will continue through the night 
with moderate southerly winds. There will likely be a break in the
rain from around 12-17Z as the southerly winds start to increase.
The main band of showers and possible thunderstorms is expected 
to reach the terminal vicinity around 17Z and continue for 2-4 
hours. In addition to the heavy rain, lower visibility, and 
lightning potential, winds will be gusty and erratic during this 
window. Conditions will improve in the early afternoon, but there 
is a chance for the atmosphere to recharge for another round of 
showers in the late afternoon or early evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...The impacts are more or less the same
across the southern terminals, although the timing is slightly
later with the onset of rain this evening, the break mid Tuesday 
morning, and the arrival of the strong band of showers late 
Tuesday morning. 

&&

.MARINE...
(Tonight through next Saturday)
Issued at 451 PM PDT Mon Apr 20 2026

Expect breezy to gusty southerly winds to continue as a cold front
and low pressure move through the area. Light to moderate rains
will move along the front with spotty showers expected in its
wake. Winds begin to reduce from the north to south this
afternoon and into the night a moderate southwesterly breeze by
Tuesday. Shower chances increase again into Tuesday with some
chances for thunderstorms lasting into that night. As the weather
breaks, winds will gradually shift back to the northwest
Wednesday before increasing to a strong northwest breeze by
Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt 
     to Pt Pinos 10-60 NM-Pt Reyes to Pigeon Pt 0-10 nm-SF Bay N 
     of Bay Bridge.

     Small Craft Advisory until 3 AM PDT Tuesday for Mry Bay-Pigeon 
     Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 
     nm.

     Small Craft Advisory from 9 AM to 9 PM PDT Tuesday for Mry Bay-
     Pigeon Pt to Pt Pinos 0-10 nm.

     Small Craft Advisory from 9 AM to 3 PM PDT Tuesday for Pt Pinos 
     to Pt Piedras Blancas 0-10 nm.

&&

$$

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

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Prepared by Boulder-Creek.com Weather at: Mon Apr 20 20:30:02 PDT 2026