Technical NOAA Weather Discussion

FXUS66 KMTR 100952
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
152 AM PST Wed Dec 10 2025

...New SHORT TERM, LONG TERM, BEACHES...

.KEY MESSAGES...
Updated at 1224 AM PST Wed Dec 10 2025

 - Hazardous beach conditions through Friday.

 - Tule fog and stratus continue for the North and East Bay.

 - Light rain remains possible next week.

&&

.SHORT TERM...
Issued at 1224 AM PST Wed Dec 10 2025
(Today and tonight)

Tule fog and stratus continue with very similar coverage to 24 
hours ago. In addition to the entire Central Valley, most of the 
North Bay and the interior East Bay are impacted by this 
persistent radiation fog. This is caused by unmoving high pressure
bringing clear, dry, and calm conditions to the troposphere. 
Combined with long December nights, nocturnal radiative cooling 
from both the Earth's surface and cloud tops are enhanced. Since 
the cooling is coming from the bottom up, it's very stable (cold 
air sinks). And since the cooling at night is enough to reach 
saturation and form or bolster the existing clouds, a positive 
feedback loop sets up where the clouds then block incoming 
sunlight during the day. All this works to keep the day-time 
temperatures cool under the clouds, and the models have been 
struggling to capture just how cool. Kentfield stayed between 43 
and 46 degrees all day Tuesday. The forecast is challenging 
because there is very warm air just above the clouds. The 850 
temperature has blown past the 90th percentile is now approaching 
the daily max on record. Boulder Creek stayed clear and reached 
78 degrees yesterday. This big discrepancy between the thin cold 
layer and everything else is really tough to nail down. When the 
advanced weather models let us down, we often fall back on the 
most rudimentary form of forecasting in existence: persistence. In
other words, what happened yesterday will happen today. 
Specifically Tuesday's high temperatures are making up 66% of the 
forecast for Wednesday. With an unchanging pattern, that's not a 
bad guess. If the clouds clear early though, watch out for 
temperatures to spike. Areas outside of the clouds (Central Coast,
SF Peninsula) will have cooler mornings and warmer afternoons. 
Maybe just a couple degrees warmer than yesterday.

&&

.LONG TERM...
Issued at 1224 AM PST Wed Dec 10 2025
(Thursday through Tuesday)

Temperatures should climb a degree or two each day through Friday
before the ridge axis finally moves through on Saturday. That will
mark the end of this prolonged boring pattern as a more zonal jet
stream moves in. We should finally see some high clouds by Sunday
with a chance for rain starting Monday or Tuesday. While we are 
finally losing the ridge, we're not getting a substantial trough, 
so any rain bearing systems should be relatively light. On the
other hand the active zonal flow could bring these weaker systems
in quick succession. CW3E shows both the ECCC and NCEP AR 
activity anomaly peaking around +2 AR days per week from 
12/15-12/21 before falling back to near the climatological norm 
for the following 2 weeks. In other words if we don't get any 
substantial rain next week, there is no strong signal for it to 
come during the rest of the month. Perhaps the best chance is 
around the 20th, but there's a lot of uncertainty. The 00Z ECMWF 
ensemble members range from 0.0" to 8.4" of total rain at SFO 
through the 21st. So the total dispersion is massive, but the 
more reasonable interquartile range is 0.4" - 2.0". The GEFS is 
drier with a medial closer to 0.5". Temperatures will cool this 
weekend, but should remain warmer than normal through next week. 
The CPC temperature outlooks are about as confident as we ever 
see that it will remain warmer than normal for the next couple 
weeks, but that doesn't necessarily take our famous microclimates
into account.

&&

.AVIATION...
(06Z TAFS)
Issued at 944 PM PST Tue Dec 9 2025

Additional warming in the lower levels of the atmosphere per 
recent Oakland upper air sounding data and Bodega Bay profiler 
data, strengthened the lower level temperature inversion today. 
However, the difference between ground level and the base of the 
temperature inversion so far has not changed much since early
evening, at least not yet. The SAC-SFO pressure gradient is 1.5 
mb and the WMC-SFO 1.6 mb. The WMC-SFO gradient is forecast to 
strengthen tonight and Wednesday, both gradients support ongoing 
offshore winds tonight and Wednesday. 

The synoptic pattern remains favorable for plentiful tule fog and
low stratus to continue over the Central Valley, corresponding to
ongoing chilly temperatures and higher surface pressures in the 
valley. There'll be very little if any interference in outgoing 
radiative cooling tonight; it's dry above the lower level temperature
inversion and cirrus clouds will be at a minimum tonight. Expect 
redevelopment of fog and stratus /LIFR-IFR/ in our area including
the potential for dense tule fog /VLIFR/ to be transported into the
Bay Area on light offshore winds tonight and Wednesday. Key to mix
out times, if they are earlier tomorrow than recently observed,
will be the base of the lower level temperature inversion, if it
does not move downward much then it's a persistence forecast with
late day break out times if at all. In areas where VFR has prevailed
recently the probability of VFR or mixing out from late night and
morning fog and stratus /VFLIFR-IFR/ is greater for Wednesday.
This is a very challenging forecast, forecast confidence regarding 
mix out times Wednesday is low. 

Vicinity of SFO...VFR initially forecast tonight, then low to
moderate confidence IFR tempo stratus ceiling develops 08z-12z 
then IFR prevailing to 18z Wednesday. The combination of radiative
cooling and light offshore surface winds increases the probability
of stratus /IFR/. Will need to closely monitor satellite and surface
observations for potential fog development as well. Offshore winds
are forecast to prevail during the 06z TAF.

SFO Bridge Approach...Slant range visibility moderate to poor at 
sunrise and sunset due to trapped hazy conditions. Light offshore 
winds may end up bringing low stratus and fog to the SFO approach 
later tonight and Wednesday morning.

Monterey Bay Terminals...The highest probability of VFR is here,
though radiative cooling may result in patchy fog to patchy dense
fog /LIFR-VLIFR/ in the Salinas Valley tonight and Wednesday
morning. Cold air drainage winds will result in southeasterly 
winds tonight and Wednesday, possibly shifting to light and 
variable temporarily Wednesday afternoon. Cold air drainage winds
redevelop Wednesday night and Thursday morning. 

&&

.MARINE...
(Tonight through next Monday)
Issued at 929 PM PST Tue Dec 9 2025

High pressure will result in northerly winds over the coastal
waters. High pressure will also build over the Great Basin tonight
and Wednesday resulting in additional offshore winds across the
Delta to the San Francisco Bay. Winds will strengthen across all
outer waters by midweek. A new, long period northwesterly swell is
expected Wednesday, lasting into the early weekend.

&&

.BEACHES...
Issued at 1224 AM PST Wed Dec 10 2025

Long period swell will reach the coast Wednesday - Friday. This 
energy brings strong rip currents, an increased risk for sneaker 
waves, and large breaking waves. The Bodega Bay Buoy is reporting 
a WNW swell of 9 feet with a 13 second period. That translates to 
breaking waves of 12-16 feet along west facing beaches. This 
coincides with nice warm weather, which will likely lure more 
people to the beach under a false sense of security. A Beach 
Hazard Statement is in effect through Friday evening. Respect the
power of the ocean and never turn your back on it.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement from 7 AM PST this morning through 
     Friday evening for CAZ006-505-509-529-530.

PZ...Small Craft Advisory until 9 AM PST this morning for Pt Pinos to 
     Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory until 3 AM PST Thursday for Pt Arena to Pt 
     Reyes 10-60 NM.

     Small Craft Advisory until 3 PM PST this afternoon for Pigeon Pt 
     to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Canepa
MARINE...Canepa

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Prepared by Boulder-Creek.com Weather at: Wed Dec 10 02:30:02 PST 2025
From the National Weather Service