FXUS66 KMTR 281658
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
958 AM PDT Fri Aug 28 2026
...New UPDATE, MARINE...
.KEY MESSAGES...
Updated at 1041 PM PDT Thu Aug 27 2026
- Slight chance of rain showers and thunderstorms through Friday
afternoon focused on the Central Coast.
- Cooling trend begins Friday
&&
.UPDATE...
Issued at 957 AM PDT Fri Aug 28 2026
Have increased rain shower chances to include the San Francisco
Bay Area and the North Bay through this afternoon as precipitation
(albeit light) is reaching the surface. The greatest rainfall
over the past 6 hours has been across the Central Coast with up to
0.05" at Chews Ridge (just above 5000 feet in elevation). There
still is a chance for thunderstorms across the Central Coast as
there is some instability aloft, however the 12Z KOAK sounding
reported only 3 J/kg of MUCAPE with the KVBG (Vandenberg AFB)
reported 459 J/kg MUCAPE. The threat diminishes this afternoon and
moreso by late evening as the remnant moisture from Tropical
Storm Iselle shift to the east. Thus, the Red Flag Warning remains
in effect through 3 PM this afternoon for much of the Central
Coast and the Santa Cruz Mountains. For more, please see the Short
Term discussion below.
RGass
&&
.SHORT TERM...
Issued at 1041 PM PDT Thu Aug 27 2026
(Tonight through Saturday)
As the remnant moisture from Tropical Storm Iselle moves over the
region we must analyze the risk for high-based thunderstorms.
This type of thunderstorm forms high in the atmosphere above a
dry layer of air. Most of the rain evaporates in the dry layer,
but lightning has no trouble getting to the ground. This dry
lightning is a well-known wildfire ignition source, so let's take
a close look. The 00Z sounding measured a PWAT of 1.24", with 307
J/kg of MUCAPE. The best lapse rate was 850-500 mb at 7.6 C/km,
with the moisture all above 700 mb. As this moisture gradually
moves lower in the atmosphere, the instability will increase.
That, combined with subtle lift from the trough and terrain could
trigger some thunderstorms today.
Models are not doing a good job analyzing the (307 J/kg of
MUCAPE) instability measured on the 00Z weather balloon sounding.
For example the 00Z ECMWF model initialization at OAK has 0
MUCAPE. The lapse rates and precipitable water are in the
ballpark, but not lined up as well as reality. It's the same story
with the HRRR and NAM. Specifically the models aren't resolving
the moisture at the base of the unstable layer. As such, we've
increased the probability of thunderstorms to 15% across the
Santa Cruz Mountains and most of Monterey County from 5AM to 2PM.
The two big questions are will this instability be realized, and
will the rain reach the ground. The instability piece looks
better as the night goes on. Now that the balloon data is getting
into the models, they are gradually getting more interesting. The
06Z HRRR is just starting to come in and the lightning flash
density has trended up. It's now showing a potential peak of 1
flash every 5 mins across parts of Monterey County, with more
scattered activity across the rest of the Bay Area. The 00Z ECMWF
has a similar trend across the Central Coast, but still doesn't
show much in the Bay Area.
One factor working against convection is the timing. While the
upper level support peaks in the mid to late morning, peak surface
heating will lag behind by several hours. If these lined up,
there would be some more help from surface based instability. On
the other hand, if the storms preform well in the morning, the
increasing surface temperatures may continue to support convection
that keeps some activity going longer into the afternoon than
currently expected.
As the 500 mb trough axis approaches Saturday it will usher in a
much drier air mass. This will end the threat of thunderstorms,
while ushering in some gusty onshore winds and cooler
temperatures this weekend.
&&
.LONG TERM...
Issued at 1041 PM PDT Thu Aug 27 2026
(Saturday night through next Thursday)
The marine layer will gradually reform and deepen through the
weekend. An active pattern will bring several short waves into the
Pacific Northwest this week, keeping the Bay Area on the cool and
breezy side. There is even a chance for some light rain midweek.
&&
.AVIATION...
(12Z TAFS)
Issued at 432 AM PDT Fri Aug 28 2026
VFR will prevail through the day and evening hours. Stratus is
expected to return late tonight into early Saturday morning. There
is a slight chance of isolated thunderstorms (primarily for the
South Bay and Monterey Bay terminals) with shower activity later
this morning through the afternoon. Low confidence in how well the
marine layer organizes in the wake of todays shower activity.
Vicinity of SFO...VFR. There's a slight chance of showers with
isolated thunderstorms later this morning into the afternoon.
Expect breezy onshore flow later today behind any shower activity.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR expected to prevail through the day
and evening hours. There's a slight chance of showers with
isolated thunderstorms this morning and afternoon. Moderate
confidence that MVFR/IFR ceilings will return around midnight and
persist through the remainder of the forecast period.
&&
.MARINE...
(Today through Wednesday)
Issued at 957 AM PDT Fri Aug 28 2026
Fresh to strong northwest breezes persist through the weekend and
diminish to moderate by the beginning of the week. A long period
southerly swell peaks on Saturday around 17 seconds and gradually
subsides by the end of the weekend. Moderate wind driven seas
persist into next week.
&&
.FIRE WEATHER...
Issued at 1041 PM PDT Thu Aug 27 2026
A Red Flag Warning is in effect from 5 AM through 3 PM for dry
lightning potential across the Santa Cruz Mountains and Central
Coast. Refer to the Short Term section for more details. By
Saturday a cooling trend will kick off as the marine layer
gradually deepens.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning until 3 PM PDT this afternoon for CAZ512-516-
517-528>530.
PZ...Small Craft Advisory from 9 AM Saturday to 3 AM PDT Sunday for
Mry Bay.
Small Craft Advisory until 9 AM PDT Sunday for Pigeon Pt to Pt
Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt
Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory until 3 AM PDT Sunday for Pigeon Pt to Pt
Pinos 0-10 nm.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...JM
MARINE...Navarrete
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Prepared by Boulder-Creek.com Weather at: Fri Aug 28 10:30:03 PDT 2026
From the National Weather Service
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