FXUS66 KMTR 130616
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1116 PM PDT Sun Jul 12 2026
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 1104 PM PDT Sun Jul 12 2026
- Monsoon moisture brings a slight chance for thunderstorms
through Tuesday
- Minor coastal flooding continues through Thursday morning due
to large tidal swings
- Warmer temperatures through Wednesday
&&
.SHORT TERM...
Issued at 1104 PM PDT Sun Jul 12 2026
(Tonight through Tuesday)
You may have noticed some colorful sunrises and sunsets over the
last two days. Those are being caused by monsoon moisture pushing
up from the south bringing us mid level clouds. Even if it's
overcast, there is enough room underneath the clouds (about 10,000
feet) for the setting sun to illuminate the bottom of these
altostratus and altocumulus clouds and create the beautiful
sunsets. It also helps that the low-level marine layer clouds,
which typically only sit about 1,000 feet above the surface and
famously ruin sunsets, were not nearly as widespread as we saw
last week. So that explains the sunset, but what about the
monsoon?
While the word invokes many different feelings, "monsoon" actually
describes a seasonal wind direction reversal. The North American
Monsoon primarily impacts NW Mexico, Arizona, and New Mexico as
the high summer temperatures in the desert induce thermal low
pressure that brings in moist air primarily from the Gulf of
California. This moisture, combined with the instability driven by
high surface temperatures, often generates thunderstorms over the
desert states in the Summer. In fact, Tuscon gets 52% of its rain
from July-September, while we are lucky to get anything. Every
once in a while the weather pattern aligns in such a way to bring
some of this monsoon moisture to the Bay Area. In this case it's
driven by a very strong high pressure system over the Rockies.
This system is so strong that Salt Lake City just set their all
time record high temperature of 109F. Since winds flow clock-wise
around a high, this system is pushing the monsoon moisture from
Arizona in a circular pattern towards Southern California and then
northward to the Bay Area. This final push of southerly winds is
enhanced by a low pressure trough over the Eastern Pacific. As the
moisture moves along this conveyer belt it also moves higher in
the atmosphere (humid air is less dense than dry air). This
explains where the uncommon mid level clouds are coming from.
While this moisture was mostly harmless today, there is a slight
chance for thunderstorms through Monday. If that happens it could
bring significant impacts because the high cloud base and much drier
lower atmosphere means that most of the rain would evaporate before
it hits the ground. When lightning strikes without much rain,
wildfires can start. Thunderstorms need 3 ingredients: instability,
moisture and a lifting mechanism. We have moisture, check that off
the list. The instability looks pretty meager. The 00Z weather
balloon from Vandenberg found the steepest lapse rates to be -7.3
C/km between 850 and 500 mb. That's not terrible, but most of the
moisture is above 600mb, where it's more like -6.6 C/km, which is
pretty bad. To add to the issues, there's not really an obvious
trigger either. Surface heating is the typical trigger for monsoon
thunderstorms in the desert, but these would be high based
thunderstorms that wouldn't really feel that impact. More likely
is nocturnal cloud top radiational cooling, combined with cloud
base heating from surface radiation. While surface cooling makes
things more stable, cloud top cooling does the opposite. If the
top of the cloud gets colder and the bottom gets warmer, the
instability increases due to the steeper lapse rate. The HRRR
soundings also suggest that in the morning the the moisture plume
will move further down towards the -10 to -20 C sweet spot where
the combination of supercooled water and ice maximize particle
collisions and cloud charging potential. So the best chance is
probably early Monday morning. Even then it's a long shot. We're
talking like somewhere between 5 and 10%. Don't be surprised if
you see another good sunrise and even a few drops of rain,
however. Otherwise Monday will feel pretty much like Sunday. Maybe
a couple degrees warmer.
&&
.LONG TERM...
Issued at 1104 PM PDT Sun Jul 12 2026
(Tuesday night through next Sunday)
The slight high based thunderstorm threat continues through at least
Tuesday morning. There is some evidence that a short wave will move
through that could make things more interesting, but the moisture
also starts to clear by then, so we're still talking about single
digit percentages. The bigger story is the heat. As the moisture
clears out, so do the clouds. The full sunshine will bring
temperatures into the mid 90s for several of our hotter cities, with
mid 70s in the more coastal locations. That's about 5-10 degrees
above normal. Strong onshore winds will arrive in the afternoon and
help cool the coast quickly. Wednesday will be very similar to
Tuesday, just without the thunderstorm threat. By Thursday the
Eastern Pacific trough that helped pump the moisture earlier in
the week will arrive and bring a weak cold front, kicking off a
late week cooling trend.
A quick note on the tropics. As the El Nino Advisory continues, the
National Hurricane Center is currently tracking 5 disturbances in
the Central and Eastern Pacific. While these systems rarely
impact California directly, the residual moisture can bring
impacts similar to the monsoon with high based thunderstorms.
Refer to Tropical Storm Fausto, the 2020 lightning outbreak and
subsequent wildfires for a good example of this. Additionally, the
ocean swell generated from these storms can bring hazardous beach
conditions. While there is no immediate threat, there is a high
chance for an active hurricane season in the Pacific and we'll be
watching closely for these impacts.
&&
.AVIATION...
(06Z TAFS)
Issued at 903 PM PDT Sun Jul 12 2026
Radar and satellite show isolated 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 Saturday)
Issued at 901 PM PDT Sun Jul 12 2026
Moderate to fresh northerly winds continue in the 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...Flynn
LONG TERM....Flynn
AVIATION...Canepa
MARINE...Canepa
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Prepared by Boulder-Creek.com Weather at: Mon Jul 13 04:30:04 PDT 2026
From the National Weather Service
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