FXUS66 KMTR 281824
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1124 AM PDT Fri Aug 28 2026

...New AVIATION...

.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...
(18Z TAFS)
Issued at 1123 AM PDT Fri Aug 28 2026

VFR across all terminals, except for HAF where it will remain IFR 
until the early afternoon. Gentle to moderate onshore winds increase 
through the day and ease overnight. High level clouds associated 
with a moisture plume persist and gradually decrease this 
evening. Drizzle/light rain is being measured across the region 
which is expected to continue into the early afternoon as well.

As the moisture remnants continue on their eastward track out of 
our region, the marine layer will begin to build back to its usual 
pattern. There is currently low confidence on how widespread and 
organized it will be tonight as the stratus feed looks patchy on 
satellite to our northwest. Rather than locking in IFR to MVFR 
ceilings, the TAF hints at the possibility with SCT decks. High 
confidence for coastal sites to develop IFR ceilings later this TAF 
period, though the exact timing is uncertain.

Vicinity of SFO...Moderate northwest winds persist through the 
afternoon with some light rain nearby the terminals. Onshore winds 
ease through the evening with low confidence for MVFR ceilings to 
develop around midnight.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with gentle northwest breezes through 
the day. The potential for light rain near the airports persists 
this afternoon and gradually diminishes by sunset. Overnight, winds 
become more west to southwest, but remain light. Medium confidence 
for IFR ceilings to develop with an arrival around midnight. 

&&

.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...Navarrete
MARINE...Navarrete

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Prepared by Boulder-Creek.com Weather at: Fri Aug 28 12:30:02 PDT 2026