FXUS66 KMTR 211124
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
424 AM PDT Tue Jul 21 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 154 PM PDT Sun Jul 19 2026
- High based showers through Tuesday.
- Breezy onshore winds return for the second half of the week.
- Temperatures will hover around normal through the week.
&&
.SHORT TERM...
Issued at 105 AM PDT Tue Jul 21 2026
(Today and tonight)
Isolated to widely scattered high based showers mainly continue
over the coastal waters and the Bay Area. A few reports via social
media and webcams Monday showed patchy light sprinkles resulting
in a mix of trace reports and a couple isolated 0.01". Almost all
of the forecast area has remained dry. The precipitable water (pw)
on late Monday afternoon's Oakland upper air sounding was 1.42",
the max moving average for July 20. Earlier over the weekend
Saturday evening's pw was 0.50", 10th percentile. Deep southerly
winds currently transporting remnant tropical moisture into the
forecast area is largely distributing the moisture between 15,000
feet and 26,000 feet. Between 15,000 feet and the ground, the layer
is relatively drier, likely resulting in some evaporation of rain
drops before reaching the ground.
No lightning has been observed so far over our forecast area,
including no lightning activity since Sunday evening over the
Pacific all the way out across the remnants of Elida. An ASCAT
wind pass at 0511z (10:11 pm PDT Monday) showed the remnant
surface low pressure circulation ~ 500 miles southwest of Monterey
with winds 15 to 30 knots with a few higher gusts to 35 knots.
The surface low center will slowly fill and weaken while moving
northward. The area out ahead of the low has pw upwards of 1.50"
to 1.60" which is not far from the July daily maximum pw (1.74")
on the period record since 1948. This pw arrives today and
forecast soundings show this will continue to be largely
distributed in the middle to upper troposphere. The highest mid to
upper level pw will be in our area when there is weak positive
vorticity at and above the 500 mb level this afternoon/evening
otherwise negative vorticity will be present in the 700-500 mb
layer. Given model trends, a lack of lightning seen so far,
temporary weak positive vorticity at and above 500 mb and weak
convergence at 250 mb later today, it's a persistence forecast
today. The atmosphere is very complex though, could it still
convect and produce an isolated lightning strike? Yes. Stay tuned
to the latest updates.
00z GFS and ECMWF forecast the 700-500 mb layer drying out
tonight. However, elevated pw will remain from the surface to
lower levels.
The recent increase in pw (water vapor) will largely slow radiational
cooling through morning, possibly slowing how warm it gets today too
as well, though lower level temperatures around the 925 mb level are
forecast to increase a few degrees Celsius today. Daytime highs may
get above normal today, varying from the 70s/80 coastside to the 80s
bayside and 90s/100 far inland.
&&
.LONG TERM...
Issued at 105 AM PDT Tue Jul 21 2026
(Wednesday through Monday)
As mentioned in the short term discussion, elevated pw will remain
from the surface to the lower levels tonight. This will carry over
through Wednesday. This is tropical moisture that'll keep surface
dewpoint temperatures in the upper 50s/60s Wednesday. Dewpoints in
this range will make it feel muggy Wednesday while the mid to upper
levels will be drier. If low clouds return by early Wednesday, patchy
light measurable drizzle may develop in the tropical air mass. Weak
cool air advection, onshore winds and gentle upsloping may help with
the development of patchy drizzle.
Daytime onshore breezes return mid to late week. It'll otherwise
be dry, warm to hot inland and coolest at the coast and bays later
in the week. The high pressure system over the interior west will
gradually shift southward then slowly move to the Four Corners by
the weekend.
&&
.AVIATION...
(12Z TAFS)
Issued at 420 AM PDT Tue Jul 21 2026
Currently VFR with mid-level clouds and elevated rain showers
streaming in from the south. Both of these features are contributing
to VFR by mixing out the marine layer and limiting radiational
cooling. As such, high confidence in VFR through the afternoon. The
marine layer is expected to reform tonight around 1,000 feet which
would bring ceilings on the cusp of IFR/MVFR to bayshore and coastal
terminals with lower confidence farther inland. This may be generous
given the over performance of guidance and models currently, it'll
all depend on if mid-level clouds can clear overnight. Elevated rain
showers moving in from the south will bring virga to the region.
Diurnal winds will prevail.
Vicinity of SFO...Currently VFR with westerly flow. Moderate to high
confidence on VFR prevailing through the TAF period. If a ceiling
were to develop tomorrow night, it'll likely be MVFR.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently VFR and calm at MRY and VFR with
drainage flow at SNS. High confidence in VFR through the morning
with low to moderate confidence on ceilings on the cusp of IFR/MVFR
returning tonight.
&&
.MARINE...
(Today through Sunday)
Issued at 420 AM PDT Tue Jul 21 2026
Isolated elevated rain showers may result in erratic and gusty
winds capable of generating steep seas today. Light northwesterly
breezes are expected today, backing to become southerly tomorrow.
Hazardous conditions for small craft may return as early as
Thursday as northwesterly breezes return and increase to become
moderate to fresh through Saturday. Moderate southwesterly swell
from tropical cyclone activity in the Eastern Pacific will bring
mixed seas and potentially hazardous conditions through the
weekend.
&&
.FIRE WEATHER...
Issued at 154 PM PDT Mon Jul 19 2026
No significant impacts expected as sub-tropical moisture moves
over the Bay Area and Central Coast. There has been some
discussion of dry thunderstorms, but as of this afternoon models
are not indicating an environment that would support their
development. At this point I would give dry thunderstorms a
probability of less than 1% this week. What should be expected is
an isolated shower here or there with little to no accumulation.
The marine layer will remain fairly well mixed out through Tuesday
afternoon, but should start to build back in late Tuesday and
Tuesday night. Expect it to reach around 1000 feet by midweek.
Strong onshore winds will develop each afternoon for the second
half of the week. They typical places will gusts like they do
every summer afternoon.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...Sarment
MARINE...Sarment
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Prepared by Boulder-Creek.com Weather at: Tue Jul 21 04:30:03 PDT 2026
From the National Weather Service
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