FXUS66 KMTR 122331
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
431 PM PDT Sun Jul 12 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 130 PM PDT Sun Jul 12 2026
- Showers continue across the region into Tuesday
- Warming trend through midweek
- Large tidal swings to bring localized coastal flooding at high
&&
.SHORT TERM...
Issued at 130 PM PDT Sun Jul 12 2026
(This evening through Monday night)
As you read this remember the word nuance.
The atmosphere is many moving parts in the x, y, and z axis, as well
as time. Mid to upper level monsoonal moisture has been working its
way around the western periphery of an area of high pressure
dominating the center portion of the US. As it does so this moisture
is encountering an upper trough over the East Pacific. The path
between the ridge and the trough are providing a road for the
moisture to travel, while the trough and embedded shortwaves in the
ridge are allowing for some vertical ascent. Models typically
struggle with this type of pattern and this scenario is no
exception. That said, it is all we have to go on as we look out
beyond about 24 hours. In the short term forecast we can look at
satellite and upstream soundings and radar. Today's satellite is
showing a steady stream of moisture from the south cutting across
Monterey County and extending up to Eastern Santa Clara County
before exiting out to the NE. The remainder of our service area
generally has thin clouds or is clear, which is not always good in
an environment like this, because surface heating provides lift.
That said, remember this moisture is in the mid to upper levels of
troposphere. Even so, surface based heating can have an influence on
the mid levels. A shortwave has shown up nicely in satellite data
this afternoon over the San Diego and Orange County areas, tracking
northward while following the coast. This could prove interesting as
this shortwave is taking on a meso-low look, which simply means that
it could add a little vertical kick to an already receptive
environment. As this shortwave embeds within the larger moisture
flow, we could see a a traveling pulse of increased mid to upper
level instability this evening and overnight.
This evening and tonight expect bubbly cumulus clouds in the mid
layers, meaning the base of the clouds starting at about 10000-15000
feet. This will likely produce virga (rain out of the bottom of the
cloud that never reaches the ground). The question is, will it cause
lightning? Models (again, which don't handle these patterns well)
have been showing a vertical temperature profile that does not
support a robust hail growth zone. The -10C range doesn't even start
until around 19000 feet in the Vandenberg sounding from this
morning. Models do not show a descending hail growth zone during our
times of peak instability. So at this point the probability of
squeezing out a lightning strike is probably less than 10%. In all
likelihood we will probably simply experience virga and an occasional
drop of water on the ground. All that said, the probability is not
zero. While odds are low, don't be surprised if you see a flash from
a cloud or a rumble of thunder.
Monday is looking a little more robust for convective development.
With warming temperatures and higher humidity levels, we'll start
the day a little warmer than Sunday, meaning we'll have a little
jump start on breaking a convective cap. When models do show
convective parameters in these high based situations, it is
important to take notice. By Monday afternoon models are all
showing plenty of moisture across our entire service area with PWAT
values in the 1-1.5" range. MU CAPE pops in some select area to a
couple hundred J/kg, Total Totals jump into the mid 40s, and K index
pulls into the 30s. All the ingredients are there for a convective
afternoon on Monday, but again, the depth of convection will be the
limiting factor as to any lightning development. Expect more virga,
more chances for a few drops to hit the ground, but continued less
than 10% chance of lightning. If surface heating and/or an embedded
shortwave gives an afternoon kick to break the mid level cap, we
could see some solid deep convection. If that occurs these will be
dry thunderstorms and outflow boundaries could approach 30-40 mph.
In other words, if thunderstorms do develop, expect lightning and
locally gusty and erratic winds.
Other things through Monday. Coastal flood advisory remains in place
due to high astronomical tides with a little anomaly on top. Warming
trend is beginning and will run through mid week. Cloud cover could
mute the warming signal, but outside the clouds the warming will be
strong. Tuesday and Wednesday will bring inland areas into the 90s
and surpass 100 in a few spots.
&&
.LONG TERM...
Issued at 130 PM PDT Sun Jul 12 2026
(Tuesday through next Saturday)
Convective threat will continue Monday night into Tuesday with the
overall pattern shifting East through the day Tuesday. This shift
will bring an end to the threat of high based thunderstorms for the
Bay Area and Central Coast. The first part of Tuesday will be
interesting as the warm minimum temperatures will give us a jump
start on upward buoyancy in the low levels, potentially impacting the
upper levels. The question will be, is there enough moisture left
around as some convective parameters spike again on Tuesday? As it
stands now, there could be a small window Tuesday afternoon where we
can see another threat of dry thunderstorms, but those odds are in
the 5% or less range.
Overall, don't count anything out. With poor model guidance for high
based convection and the ever present threat of nocturnal
thunderstorms, the atmosphere can and does surprise us. All it takes
is one of the parameters breaking from expectation and we could be
off to the races.
After the warm up into midweek, the pattern takes on a much more
normal look for the second half of the week.
Early next week (yup, talking about that far out), we could see more
monsoonal/sub-tropical moisture work its way into the area. Stay
tuned.
&&
.AVIATION...
(00Z TAFS)
Issued at 431 PM PDT Sun Jul 12 2026
Radar and satellite show a band of high based showers over the
southern part of the forecast area; showers are moving northward.
A few patches of coastal stratus and fog may develop tonight and
Monday morning. Clouds will slow radiative cooling and redevelopment
of coastal stratus and fog tonight and result in a hazy, partly
to mostly cloudy sky Monday. Overall conditions favor VFR with the
highest probability of LIFR-IFR along the immediate coastline.
Vicinity of SFO...VFR. Northwest to west wind 5 to 15 knots.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR except patchy stratus and fog /LIFR-
IFR/ nearby late tonight and Monday morning. Winds mainly varying
from northwest to southwest 5 to 15 knots.
&&
.MARINE...
(Tonight through next Friday)
Issued at 425 PM PDT Sun Jul 12 2026
Moderate to fresh northerly winds are outer waters leading to
hazardous conditions. Otherwise, expect light to moderate north-
northwest winds with moderate northwest swell. Mainly northerly
winds become fresh to strong mid to late week, which will have
near gale force gusts. Seas will build in response, leading to
overall hazardous conditions.
&&
.FIRE WEATHER...
Issued at 130 PM PDT Sun Jul 12 2026
Warming temperatures this week will cause fuel moistures to
plummet. At the same time we are tracking monsoonal moisture
moving across the area and any threat of dry thunderstorms. While
dry thunderstorms have a 10% or less of development, any strike
can start a fire. Not to mention, in and around active
thunderstorms winds can become gusty and erratic. For more
information on the thunderstorm threat, see discussion above.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 4 AM PDT Thursday for CAZ006-506-
508.
Coastal Flood Advisory until 4 AM PDT Wednesday for CAZ505-509-
529-530.
PZ...Small Craft Advisory until 3 PM PDT Wednesday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...BFG
LONG TERM....BFG
AVIATION...Canepa
MARINE...KR
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Prepared by Boulder-Creek.com Weather at: Sun Jul 12 18:30:02 PDT 2026
From the National Weather Service
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