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NWS Technical
Weather Discussion |
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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
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From the National Weather
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Revised:
29 Mar 2008 09:53 -0800 GMT
(Pacific) |
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