FXUS66 KMTR 210733
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1233 AM PDT Wed Oct 21 2026
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 801 PM PDT Tue Oct 20 2026
- Cooling trend continues through the end of the week
- Beneficial light rain showers expected Wednesday through Thursday
- Thunderstorms possible Wednesday afternoon into Thursday morning
across Central Coast
- Warm and dry next weekend
&&
.SHORT TERM...
Issued at 1230 AM PDT Wed Oct 21 2026
(Today and tonight)
The center of the 500 mb cut-low is inching closer to the coast as
the sub-tropical jet stream supports the left portion of the system,
bringing enhanced momentum and moisture flux. This is where the most
instability and lift can be found. As of 11:30 PM PDT, KMUX radar is
picking up on a few cells offshore exhibiting a ~10-25% chance of
lightning with a few lightning strikes observed along the occluded
front (~250-300 miles SW from Monterey). The current offshore
activity is foreshadowing what is to come early Wednesday morning.
The first few rain bands are expected to make their way over the
Central Coast just before sunrise. Going into the afternoon,
precipitation gets wrapped around the system's center and eventually
spreads through the rest of the Bay Area. Precipitable water is
expected to rise above the 90th percentile for this time of year,
which prompted WPC to include the southern Central Coast in a
marginal risk of excessive rainfall for Wednesday.
To further assess the potential for severe thunderstorms, three high-
resolution model soundings were analyzed: the 3km NAM, 10km RDPS,
and the RRFS. Keep in mind that the NAM tends to be the most
pessimistic model out of the three in terms of moisture. Upon
evaluation of the near-storm environment for each, there are a few
inconsistencies across the models in terms of the severe
thunderstorm threat and whether the environment is conducive to
tornado spin-ups.
Before getting into what *is* consistent across the models, let's
run through a quick overview of a couple notable stability indices:
the Total-Totals and K index. The Total-Totals index measures
atmospheric instability by taking the thermal difference between 850
to 500 mb to identify the potential for severe thunderstorms or
tornadoes. The K index is used to assess atmospheric buoyancy by
measuring the low to mid-level moisture as well as the 850-500 mb
thermal difference. The greater the K index value, the greater the
potential is for widespread thunderstorms (K > 20 = scattered
thunderstorms, K > 35 = numerous thunderstorms). Similarly, higher
Total-Totals (TT) index values indicate an enhanced potential for
severe thunderstorms and/or tornadoes (TT > 48 = severe
thunderstorms, TT > 50 = tornadoes).
Why am I talking about stability indices? Well, according to these
indices and other environmental parameters, the three high-res model
soundings that were analyzed all indicate a heightened risk for at
least scattered, severe thunderstorms along the Big Sur coastline.
The NAM estimates TT ~ 56 and a K index ~ 36, RDPS estimates TT ~ 54
and a K index ~ 30, and RRFS estimates TT ~ 50 and K index ~ 31.
Another consistency found across the model soundings is the amount
of low-level wind shear. For our area, a small amount of shear goes
a long way and can support low-topped supercells and/or brief spin-
ups of waterspouts/tornadoes. From Wednesday afternoon through early
Thursday morning, the models agree on low-level 0-1 km shear in
excess of 10-15 kt (12-17 mph). This time frame coincides with when
the greatest severe weather threat may occur. The NAM wants to fully
lock in the tornado threat, though the RDPS and RRFS are split
50/50. The SPC issued a 2-4% tornado probability for the Big Sur
coastline, thereby indicating increased potential for some low-
topped supercells and near-coastal, potentially landfalling,
waterspouts.
If severe weather does develop, here are some safety reminders:
* Have multiple ways to receive emergency alerts (i.e. NOAA weather
radio, follow NWS Bay Area on social media, and/or ensuring your
phone's emergency alert system is on)
* Move to an interior room away from windows, if a waterspout makes
landfall
* Have an emergency action plan ready if you live downslope of
wildfire burn scars
* Avoid flooded roads. Turn around, don't drown!
By late Thursday morning, some lingering showers persist through the
region with the potential for general thunderstorms and no
waterspout/tornado threat across the Central Coast. Max temperatures
remain in the mid 50s to low 70s as onshore winds gradually increase
through the evening.
&&
.LONG TERM...
Issued at 1230 AM PDT Wed Oct 21 2026
(Thursday through Tuesday)
The rain and thunderstorm threat are gone by Friday. With the trough
to our east, building high pressure brings warm and dry conditions
into early next week. Euro and GFS ensemble members suggest warming
to near seasonal averages meanwhile precipitable water values are
forecast to dip below the 25th percentile. Monday onward, there is a
great deal of uncertainty on when the weak ridge breaks down. An
energetic Central Pacific plus the most recent 6-10 day Climate
Prediction Center precipitation outlook suggests another trough
coming our way with the potential for more rain.
&&
.AVIATION...
(06Z TAFS)
Issued at 955 PM PDT Tue Oct 20 2026
VFR with increasing high clouds and light winds through the next few
hours. A low pressure system offshore will bring showers to the area
on Wednesday with a slightly delayed onset time compared to the
previous forecast. As before expecting a main rain band to come up
from the Central Coast through Wednesday with convective activity
behind the band. Low confidence (20-30% probability) for
thunderstorm activity along the coast south of MRY. Confidence in
the wind speeds beyond the next few hours remains low, as hinted at
by the large differences in the wind field output from subsequent
runs of the high resolution models. With the low pressure system so
close to shore, the wind pattern may very well depend on how the low
travels as it meanders off the coast and how it interacts with local
terrain. Any thunderstorm activity on Wednesday is also liable to
bring localized strong gusts and visibility reductions to the
immediate area.
Vicinity of SFO... VFR with high clouds and light winds continues
into the overnight hours. A band of showers arrives Wednesday
morning and afternoon followed by a period of more scattered
showers. . Low confidence in the winds on Wednesday as high
resolution models are diverging on the direction of the flow, which
could depend critically on the evolution of the low pressure system.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... VFR with high clouds and light drainage
winds through the overnight hours. Isolated showers will start to
build early Wednesday morning before an organized band arrives
later that morning, bringing the chance for MVFR ceilings along with
it. Low confidence in the winds throughout Wednesday, and for a
isolated thunderstorm (10-20% probability) to impact the terminals
Wednesday.
&&
.MARINE...
(Tonight through next Monday)
Issued at 955 PM PDT Tue Oct 20 2026
A storm system off the California coast will maintain light to
gentle breezes through early Thursday with southeast winds
gradually backing to a more northerly flow Wednesday into
Thursday. Fresh to strong north winds return Thursday and
continue into the weekend with gale force gusts possible across
the northern outer waters. Moderate seas prevail with localized
rough seas across the outer waters Friday into the weekend. Rain
showers with a chance of thunderstorms are expected through
Thursday, especially south of Pigeon Point.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM Thursday to 9 AM PDT Friday for
Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 9 PM Thursday to 9 AM PDT Friday for
Pigeon Pt to Pt Pinos 10-60 NM.
&&
$$
SHORT TERM...Navarrete
LONG TERM....Navarrete
AVIATION...DialH
MARINE...DialH
Visit us at www.weather.gov/sanfrancisco
Follow us on Facebook, X, and YouTube at:
www.facebook.com/nwsbayarea
x.com/nwsbayarea
www.youtube.com/nwsbayarea
Prepared by Boulder-Creek.com Weather at: Wed Oct 21 04:30:03 PDT 2026
From the National Weather Service
|