FXUS66 KMTR 090144
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
644 PM PDT Sat Aug 8 2026

...New UPDATE...

.KEY MESSAGES...
Updated at 104 PM PDT Sat Aug 8 2026

 - Warm to hot conditions will result in moderate to major 
   HeatRisk for the interior through Sunday

 - Near-critical fire weather conditions in the higher terrain 
   through Sunday

 - Minor coastal flooding for bayshore locations along the San 
   Francisco, San Pablo, and Monterey bays through Thursday 
   morning

&&

.UPDATE...
Issued at 633 PM PDT Sat Aug 8 2026

A deeper marine layer than the forecast combined with strong 
onshore flow contributed to cooler than expected temperatures 
across the lower elevations today. The focus shifts to tomorrow's 
high temperatures and heat potential. The current forecast has
significantly higher temperatures for tomorrow when compared to
today's observed highs, particularly in the interior regions 
where the Heat Advisory remains in effect. The 00Z (5 PM PDT) 
OAK sounding showed a marine layer inversion at around 250 to 500 
feet, which would limit the marine layer clouds to the immediate 
coast. It will be interesting to see how the marine layer evolves
overnight, particularly as a wedge of drier air clears much of the
stratus offshore the North Bay. Add to this the possibility for 
offshore flow down the northern Sacramento Valley and people 
across the interior might be in for some thermal whiplash 
tomorrow. The overnight shift will evaluate the forecast high 
temperatures and consider the need to issue a Heat Advisory for 
the North Bay interior valleys and the San Francisco Bayshore 
regions. 

DialH

&&

.SHORT TERM...
Issued at 104 PM PDT Sat Aug 8 2026
(This evening through Sunday night)

The deeper marine layer, around 1600 feet in depth per the OAK 
Upper Air Sounding, early this morning has resulted in slightly 
cooler temperatures across the lower elevations. As high pressure 
over the Four Corners region continues to build westward across 
the Bay Area and Central Coast, temperatures aloft remain very 
warm to hot! As such, the Heat Advisory remains in effect for much
of the interior through 10 PM PDT Sunday. This is as the marine 
layer is forecast to eventually compress to around 500 feet by 
Sunday morning under the building high pressure and 850mb 
temperatures are between 27-30C Sunday afternoon. Afternoon 
temperatures in the Heat Advisory are forecast to warm into the 
mid 90s and up to 105 deg F in the warmest interior locations 
where the most widespread moderate to isolated major HeatRisk are 
present. Overnight lows are also forecast to be warm and range 
from the mid 60s in the hills just above the marine influences to 
low 80s (especially in the higher elevations). Afternoon 
temperatures closer to the coast are forecast to be in the 60s to 
mid 70s largely due to the onshore winds and marine influences. 
Overnight lows near the coast are forecast to be in the 50s as 
stratus returns overnight. If the depth of the marine layer is 
higher than currently forecast, temperatures will be slower to 
warm Sunday afternoon.

Minor coastal flooding is also expected around the time of the 
highest high tide (evening/overnight) for bayshore locations along
the San Francisco, San Pablo, and Monterey bays through Thursday 
morning. The Pacific Coast may need to be added Sunday through 
Tuesday to the advisory. We are waiting to see how high the 
observed levels are tonight (Saturday) before making that 
decision. Please see the CFW product we have out for additional 
details. Also, for more on the fire weather concerns this weekend,
please see the Fire Weather Section of the AFD below. 

During this weekend's heat event, remember: 

* Stay hydrated and drink plenty of fluids.
* Wear lightweight and light-colored clothing.
* Reduce time spent outdoors or stay in the shade.
* Never leave people or pets unattended in vehicles.
* Use sunscreen if going to the coast or the pool.

&&

.LONG TERM...
Issued at 104 PM PDT Sat Aug 8 2026
(Monday through next Friday)

Ensemble clusters are in decent agreement that a mid/upper level 
shortwave trough will develop by Monday off of the West Coast. 
However, the high pressure aloft will persist over the Four 
Corners region and over the eastern Pacific. This will bring 
temperatures closer to near seasonal norms by Monday and through 
Wednesday, also lowering HeatRisk to the minor category across the
interior. 

By late in the week, temperatures cool to below normal as a 
mid/upper level through develops off of the Bay Area and Central 
Coast. As this low pressure approaches the coastline late in the 
week and monsoonal moisture advects northward around the high over
the Four Corners, there is the potential for high-based 
convection. The main question is if there will be enough 
instability. While this is still far out in time and low 
confidence, any dry lightning would be problematic from a fire 
weather standpoint. Be sure to check back for additional details 
as we move closer to midweek. 

&&

.AVIATION...
(00Z TAFS)
Issued at 408 PM PDT Sat Aug 8 2026

IFR-LIFR stratus lingers at the immediate coast with breezy onshore 
flow continuing through the evening hours. An overall lower 
confidence forecast for stratus development for this TAF period, as 
the models did not capture the maintenance of the marine layer very 
well. The marine layer depth remains around 1500 feet, but drier 
northwest winds are lurking offshore and this is evidenced in the 
clearing off the coast of the North Bay. Opted to go for a more 
persistent forecast for the stratus timing tonight and dissipation 
times Sunday morning, for an overall moderate confidence forecast. 
Low confidence that LVK gets a brief ceiling sometime early Sunday 
morning. Any stratus that forms will dissipate through Sunday 
morning, and breezy onshore flow will resume in the afternoon. 
Beyond that TAF period, the high resolution models do show some 
compression of the marine layer resulting in much less stratus 
development, but this is low confidence so far out in the forecast.

Vicinity of SFO... VFR through the evening with moderate northwest 
winds. Moderate confidence of IFR ceilings developing after midnight 
with dissipation expected late Sunday morning. Breezy west winds 
develop Sunday afternoon and evening, with low confidence for 
stratus impacts Sunday night.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals... VFR conditions with a moderate northwest 
breeze through the next couple of hours, with moderate confidence 
that IFR-LIFR stratus returning around sunset as the winds gradually 
diminish. Stratus will dissipate through Sunday morning with breezy 
onshore winds resuming in the afternoon.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 408 PM PDT Sat Aug 8 2026

Northwest winds become fresh to strong this evening and persist
into  early next week. In response, moderate seas build to rough
(9 to 12  feet) by Sunday, lasting through the middle of next
week. Northern  coastal waters will be hazardous for small craft
with occasional  near gale force gusts possible through the
weekend. The San  Francisco Bay north of the Bay Bridge into the
Delta continue to  experience gusty onshore winds through Sunday
night. Long period  southerly swells around 15 to 17 seconds are
still present in our  coastal waters.

&&

.FIRE WEATHER...
Issued at 1206 AM PDT Sat Aug 8 2026

Near-critical fire weather conditions persist through the weekend 
across the interior due to very hot and dry conditions. Strong 
thermal belting, extended burn periods, elevated PIG, and near or 
record BI/ERC values are expected. Temperatures will be 10 to 15 
degrees above normal with a compressed marine layer. Poor minimum 
daytime relative humidity as low as 10% in the afternoons with 
little-to-no recovery overnight is expected with nocturnal drying in 
the highest terrain. Even with critically dry conditions, wind is 
the limiting factor preventing this from being a Fire Weather 
Watch/Red Flag Warning event. Outside of the diurnal sea breeze that 
will bring breezy conditions in the afternoons and evenings through 
gaps and passes, onshore flow will be light. Additionally, critical 
conditions are not expected to meet coverage or duration 
requirements to warrant a product. Elevated fire weather conditions 
are expected into next week as onshore flow increases on the heels 
of the near-critical hot and dry event this weekend. The upper-level 
pattern along with a resurgence in monsoonal moisture may be enough 
to bring elevated rain showers/thunderstorms to the forecast as 
early as Thursday and into next weekend.

Sarment

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 2 AM PDT Thursday for CAZ006-506-
     508-529-530.

     Heat Advisory until 10 PM PDT Sunday for CAZ503-504-510-512>518.

PZ...Small Craft Advisory until 9 PM PDT this evening for SF Bay N of 
     Bay Bridge.

     Small Craft Advisory from 9 AM to 9 PM PDT Sunday for Pt Arena 
     to Pt Reyes 0-10 nm-SF Bay N of Bay Bridge.

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

&&

$$

SHORT TERM...RGass
LONG TERM....RGass
AVIATION...DialH
MARINE...Navarrete

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Prepared by Boulder-Creek.com Weather at: Sat Aug 8 20:30:02 PDT 2026