FXUS66 KMTR 211639
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
939 AM PDT Wed Oct 21 2026

...New UPDATE, 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

&&

.UPDATE...
Issued at 939 AM PDT Wed Oct 21 2026

We are starting to see the some of the first rain bands moving into 
the north Central Coast this morning as the core of the cut-off 
upper level low approaches the region. There aren't currently any 
lighting strikes as of yet across the region this morning. However, 
we are expecting that to change as we head into late morning and 
moreso this afternoon and evening. This is as instability increases 
with up to 400-800 J/kg of MUCAPE (Most Unstable Convective 
Available Potential Energy). The greatest instability is forecast to 
remain over the north Central Coast and points southward. All eyes 
remain on the burn scars of the Timber and Plaskett fires given the 
threat for debris flows. Rain rates across these regions matter and 
the scattered nature of these showers and/or thunderstorms gives us 
less confidence of exactly how much rainfall will occur over any 
given one area. Please see the AFD for the Short Term for additional 
details. 

RGass

&&

.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 939 AM PDT Wed Oct 21 2026

There is a chance for thunderstorms today, especially south of 
Pigeon Point. Rain showers and thunderstorms will be capable of 
producing local gale force wind gusts, rough seas, heavy rainfall 
with reduced visibility, and waterspouts. Gentle variable breezes 
will prevail today. Moderate to fresh northerly breezes will 
prevail Thursday through Sunday with locally strong to near gale 
breezes across the outer waters Thursday through Saturday. 
Moderate seas build to become rough across the outer waters 
Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 PM Thursday to 9 AM PDT Friday for 
     Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm.

     Small Craft Advisory from 3 PM Thursday to 9 AM PDT Friday for 
     Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Navarrete
LONG TERM....Navarrete
AVIATION...Kennedy
MARINE...Sarment

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Prepared by Boulder-Creek.com Weather at: Wed Oct 21 10:30:02 PDT 2026