FXUS66 KMTR 211139
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
439 AM PDT Wed Oct 21 2026
...New AVIATION, MARINE...
.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...
(12Z TAFS)
Issued at 418 AM PDT Wed Oct 21 2026
Shower activity is picking up across the marine environment with
increasing confidence in thunderstorm development along the
Central Coast. Thunderstorm potential increases for MRY and SNS
by late morning as the bulk of the convective activity moves
onshore. Highest confidence in thunderstorms impacting MRY/SNS
but if the HRRR is correct thunderstorm activity may move into the
Bay Area by early this afternoon as the main line moves through.
Moderate to at times heavy rain is likely with the first round of
showers across the Central Coast starting around 17Z. It will
spread slightly slower into the Bay Area (reaching it around
~20-03Z) with recent guidance slowing down the arrival of rain
even further. Confidence remains low in the overall wind forecast
given the close proximity of the low offshore. Winds will likely
stay variable for much of today but we may see periods of
stronger, erratic winds particularly if a thunderstorm is able to
develop. Expect temporary reductions in visibility and ceiling
heights as stronger showers move through. Showers may become more
scattered overnight Wednesday into Thursday but the prevailing
trend will be for showers to continue through the end of the TAF
period.
Vicinity of SFO...Showers will reach SFO by late morning/early
afternoon with temporary reductions in visibility and ceiling
heights likely as stronger showers impact the airport. A low
potential for thunderstorms continues through the day with
confidence too low to include TS in TAF as of this moment. This
may change as convective activity to our south gets within range
of KMUX and we get a better idea of how robust thunderstorm
activity will be today. If thunderstorms do reach SFO it would
likely be between 18 to 00Z as the main convective region moves
through. Confidence in wind directions remains low due to the
close proximity of the low.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Increasing confidence in thunderstorms
impacting MRY and SNS this morning/afternoon as showers move in.
Thunderstorms will result in erratic winds and reductions in
visibility/ceiling heights below what is currently in the TAF.
The timing of rain arrival to MRY/SNS remains on track with
guidance suggesting the arrival of rain/thunderstorms may be
speeding up. The bulk of the convection is currently to the south
of Point Piedras Blancas but will reach the Monterey Bay region
within the next few hours. The heaviest showers are likely this
morning but lighter, at times more scattered, showers will prevail
through the end of the TAF period.
&&
.MARINE...
(Today through Monday)
Issued at 418 AM PDT Wed Oct 21 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.
Thunderstorms are possible across the coastal waters now through
Thursday with gusty, erratic winds, lightning, and steep seas
possible should a thunderstorm develop. Waterspouts are possible
with the highest potential across the southern coastal waters.
&&
.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...Kennedy
MARINE...Kennedy
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 08:30:02 PDT 2026
From the National Weather Service
|