FXUS66 KMTR 131153
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
453 AM PDT Mon Jul 13 2026

...New AVIATION, MARINE...

.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...
(12Z TAFS)
Issued at 448 AM PDT Mon Jul 13 2026

VFR is expected to prevail through the forecast period with the
exception of KHAF that may toggle between VFR and IFR/LIFR through
mid-morning. Slight chance of low end MVFR cigs early Wednesday
morning for terminals on the immediate coast.

Vicinity of SFO...VFR, with breezy onshore flow this afternoon and
slight chance of low end MVFR cigs early Wednesday morning.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR is expected to prevail through the
TAF period with a slight chance of low end MVFR ceilings early
Wednesday morning.

&&

.MARINE...
(Today through Saturday)
Issued at 448 AM PDT Mon Jul 13 2026

Moderate to fresh north-northwest winds across the northern outer
waters continue with hazardous conditions, along with moderate
northwest swell. Northerly winds over the coastal and outer waters
become fresh to strong mid to late week, with near gale force
gusts. Expect building seas in response to the increasing winds
and long period southerly swell late week into next weekend.

&&

.FIRE WEATHER...
Issued at 448 AM PDT Mon Jul 13 2026

Warming temperatures this will cause fuel moistures to plummet for
the first half of the week. At the same time we are tracking 
monsoonal moisture moving across the area and any threat of dry 
thunderstorms today and Tuesday. While dry thunderstorms have a 
10% or less chance 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 from 3 PM Tuesday to 3 PM PDT Wednesday for 
     Pt Arena to Pt Reyes 0-10 nm.

     Small Craft Advisory until 3 PM PDT Wednesday for Pt Arena to Pt 
     Reyes 10-60 NM.

&&

$$

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

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Prepared by Boulder-Creek.com Weather at: Mon Jul 13 06:30:03 PDT 2026