FXUS66 KMTR 280724
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1224 AM PDT Fri Aug 28 2026
...New SHORT TERM, LONG TERM, FIRE WEATHER...
.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
&&
.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...
(06Z TAFS)
Issued at 925 PM PDT Thu Aug 27 2026
It's VFR with widespread mid and high level clouds over much of
the forecast area. High based showers have decreased in coverage.
A few light showers/sprinkles reached the ground over the north
Central Coast based on earlier webcam and spotter reports. Increased
cloud cover i.e. water vapor will interfere with and decrease
radiative cooling; coastal stratus and fog will be limited tonight
and Friday.
Vicinity of SFO...VFR. A few showers are possible tonight and/or
Friday. Currently, the 06z TAF advertises vicinity showers (VCSH)
11z-20z Friday. West to northwest wind 8 to 14 knots.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR. Vicinity showers (VCSH) tonight, with
tempo showers 10z-15z Friday morning and low confidence if showers
will continue during late Friday morning. Winds will be a mix of west
to northwest 5 to 15 knots and light and variable.
&&
.MARINE...
(Tonight through next Wednesday)
Issued at 918 PM PDT Thu Aug 27 2026
Northwest breezes gradually diminish to moderate by Monday.
Moderate wind driven seas prevail into next week. A long period
southerly swell impacts our coastal waters by Friday and persists
into the weekend.
&&
.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.
&&
.FIRE WEATHER...
Issued at 125 PM PDT Thu Aug 27 2026
One more day of near-critical fire weather conditions above 1000
feet, especially across the Central Coast, as warm and dry
conditions continue above the marine layer inversion and overnight
humidity recoveries remain poor into Friday morning. There still is
a non-zero chance for thunderstorms across the Central Coast today
through tonight. By tomorrow, with the increasing potential for
lightning, we have issued a Red Flag Warning for much of the Central
Coast including the Santa Cruz Mountains from 5 AM to 3 PM Friday
given the low confidence for wetting rains to develop. By Friday
night and into the weekend, the cooling trend will continue and
bring cooler temperatures and likely a deeper marine layer, bringing
progressively more relief to some of the higher elevations.
RGass
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 5 AM to 3 PM PDT Friday for CAZ512-516-517-
528>530.
PZ...Small Craft Advisory until 9 AM PDT Saturday for Pigeon Pt to Pt
Pinos 0-10 nm-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.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Canepa
MARINE...Canepa
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Prepared by Boulder-Creek.com Weather at: Fri Aug 28 00:30:03 PDT 2026
From the National Weather Service
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