Technical NOAA Weather Discussion

FXUS66 KMTR 272027
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
127 PM PDT Thu Aug 27 2026

...New SHORT TERM, LONG TERM, FIRE WEATHER...

.KEY MESSAGES...
Updated at 125 PM PDT Thu Aug 27 2026

 - Slight chance of thunderstorms through Friday afternoon, 
   especially across the Central Coast

 - Cooling trend continues Friday into next week

&&

.SHORT TERM...
Issued at 125 PM PDT Thu Aug 27 2026
(This evening through Friday night)

Looking at satellite imagery, seeing more widespread mid-to-high 
level cloud cover spreading across the Central Coast and southern 
portion of the greater San Francisco Bay Area. The greatest 
potential for high-based convection will be across the Central Coast 
as the OAK Upper Air sounding reported MUCAPE (instability aloft) 
slightly greater than 400 J/kg. This is also where the CAMs and 
higher resolution deterministic models show the greatest MUCAPE 
values today through early Friday morning. Thus, have added isolated 
thunderstorms to the official forecast for the Central Coast. Further 
north across the Bay Area, seeing less threat for thunderstorms, yet 
remains a non-zero chance.  

The Precipitable Water (PWAT) values increase to around or slightly 
above 1.5" across the Central Coast by Friday morning with a greater 
potential for wetting rains. However, rainfall amounts are projected 
to largely be less than 0.10" throughout this event. Thus, we have 
issued a Red Flag Warning for much of the Central Coast including 
the Santa Cruz Mountains from 5 AM to 3 PM Friday. Fuels remain dry 
from the recent warm and very dry conditions above the marine 
influences, especially in areas above 1000 feet in elevation. This 
is also where the CAMs have the greatest instability on Friday. 
Given the Timber and Plaskett fires ongoing, any lightning strikes 
and/or erratic/gusty outflows would make for difficult firefighting 
efforts. Will closely monitor the latest forecast models and adjust 
accordingly.  

As the mid/upper level trough deepens off of the Pacific Northwest 
coast Friday evening into Saturday morning, expecting this deeper 
moisture plume to shift eastward resulting in PWAT values lowering 
to near 0.50". Thus, ending the threat for high-based convection. 
The mid/upper level low off of the Pacific Northwest will push 
inland Friday night into Saturday, leading to a tightening pressure 
gradient which will result in breezier winds Friday and into the 
weekend. Thus, wind gusts are forecast to reach 25 to 35 mph along 
the coast and through favored gaps, valleys, and passes. This 
pattern will also usher in cooler temperatures. Zonal flow develops 
over the weekend holding temperatures steadily below normal. 

&&

.LONG TERM...
Issued at 125 PM PDT Thu Aug 27 2026
(Saturday through next Wednesday)

A reinforcing trough deepens off of the Pacific Northwest coast by 
Monday, allowing afternoon temperatures to be some 5 to 15 degrees 
below seasonal averages. The current forecast has the potential for 
rain showers returning towards the middle of next week. However, the 
evolution of things across the tropical Eastern Pacific remains in 
question, mainly in the form of remnant tropical moisture to be 
pulled in from the south. Thus, forecast confidence remains low at 
this time.

&&

.AVIATION...
(18Z TAFS)
Issued at 1222 PM PDT Thu Aug 27 2026

Quite an interesting set up today. A moisture plume is moving across 
our region from the southwest and is producing lightning offshore as 
well as some light rain. Whether rain will reach the ground over the 
terminals is uncertain, but high confidence for virga at the very 
least. Overall, there is low confidence on the forecasted ceilings 
towards the end of the TAF period and exact timing of potential 
vicinity showers, but medium confidence for it to remain south of 
the Golden Gate. The potential for lightning to impact terminals 
this TAF period is low, around a 10-15% chance, and TAFs will be 
updated as needed.

Despite the current set up, VFR conditions prevail through the day 
with a parade of high level clouds associated with the high-based 
convective system. Moderate onshore breezes increase through the day 
and ease overnight. If thunderstorms do develop over land, we will 
monitor for gusty outflows and amend as needed.

Vicinity of SFO...VFR through the afternoon with onshore winds 
increasing to 15-20 kt and ease to around 10 kt overnight. SFO and 
OAK are currently on the edge of the previously mentioned convective 
system, so there is not an immediate concern for lightning or VCSH. 
However, this moisture plume is expected to become a bit more 
widespread early Friday morning. As of writing this, there is a 
great deal of discrepancy in model guidance, so the timing will be 
refined in following TAF issuances.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Moderate onshore winds prevail this 
afternoon and ease overnight. MRY and SNS have the highest chance of 
remaining under the influence of the moisture plume this TAF period. 
MRY may be close enough to the coast to still experience some marine 
influence, but medium confidence on the timing of IFR ceilings. 
Lightning is still being observed via satellite offshore and will 
continue to be monitored. 

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 1222 PM PDT Thu Aug 27 2026

Fresh northwest breezes with occasional near gale force gusts
persist through the weekend and ease by early next week. Wind
driven  seas remain moderate into next week as a long period
southerly swell  around 16 to 18 seconds approaches our coastal
waters by Friday  morning.

&&

.FIRE WEATHER...
Issued at 125 PM PDT Thu Aug 27 2026

One more day of near-critical fire weather conditions above 1000 
feet, especially across the Central Coast, as warm and dry 
conditions continue above the marine layer inversion and overnight 
humidity recoveries remain poor into Friday morning. There still is 
a non-zero chance for thunderstorms across the Central Coast today 
through tonight. By tomorrow, with the increasing potential for 
lightning, we have issued a Red Flag Warning for much of the Central 
Coast including the Santa Cruz Mountains from 5 AM to 3 PM Friday 
given the low confidence for wetting rains to develop. By Friday 
night and into the weekend, the cooling trend will continue and 
bring cooler temperatures and likely a deeper marine layer, bringing 
progressively more relief to some of the higher elevations. 

RGass

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 5 AM to 3 PM PDT Friday for Caz516-516-517-
528>530.

PZ...Small Craft Advisory until 2 PM PDT Friday for Pigeon Pt to Pt 
     Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt 
     Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt 
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 3 PM Friday to 9 AM PDT Saturday for 
     Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 
     NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 
     NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.

&&

$$

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

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Prepared by Boulder-Creek.com Weather at: Thu Aug 27 14:30:03 PDT 2026
From the National Weather Service