Walnut Creek, California Weather Forecast Discussion

259
FXUS66 KMTR 131150
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
450 AM PDT Sat Sep 13 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 120 AM PDT Sat Sep 13 2025
- Temperatures around or slightly below normal today and Sunday
- Much warmer temperatures for the first half of the upcoming
   work week
- Moderate HeatRisk possible inland Monday through Wednesday
&&
.SHORT TERM...
Issued at 120 AM PDT Sat Sep 13 2025
(Today and tonight)
Coverage of low stratus is increasing overnight. The marine layer
has built up to around 2000ft allowing for a farther intrusion
inland. Low cloud deck should erode inland by late morning, likely
by early afternoon closer to the coast. Portions of the immediate
coast from the Bay Area down to Monterey Bay may have low clouds
linger throughout the entire day, even as the marine layer
retreats back to the coastline. Despite the passage of a
progressive, amplified ridge across the west coast today, highs
will remain near or slightly below normal. Overnights lows into
Sunday morning will generally be in the mid 50s to low 60s.
&&
.LONG TERM...
Issued at 120 AM PDT Sat Sep 13 2025
(Sunday through Friday)
An upper trough in the wake of the progressive ridging today will
push a weak cold front through our area Sunday, with little to no
effect on temperatures. The primary concern for the extended
forecast will be HeatRisk for the first half of the upcoming work
week. A broad upper level ridge begins to develop over the eastern
Pacific on Monday. A period of weak offshore flow Monday morning
through Tuesday morning will lift temperatures above normal, with
biggest impacts being felt closer to the coast as the marine layer
erodes. The East Bay Hills, portions of the South Bay and
southern Monterey County may see highs in the upper 90s
approaching 100 Monday through Wednesday. Some uncertainty remains
in the upper level pattern. The ECMWF has an upper level low
undercutting the base of the ridge closer to our latitude with the
GFS much farther south late Tuesday into Wednesday. Will wait to
start messaging more impactful heat until better consensus is
reached in guidance. NBM seems reasonable at this point, but did
still bump temperatures up Mon-Wed. Confidence in the pattern and
resulting weather is even lower into late next week and weekend.
&&
.AVIATION...
(12Z TAFS)
Issued at 437 AM PDT Sat Sep 13 2025
Deep and well-formed marine layer this morning allowing for
ample inland cloud cover. Coastal DZ this morning will very slowly
give way to clearing skies, except for areas along the immediate
coast, which may hold on to MVFR cigs all day. Fairly early return
of MVFR/IFR cigs this evening from the Bay Area southward.
However, terminals north of the SF Bay are more than likely to
remain clear through most of the overnight hours into Sunday
morning thanks to some dry air mixing in aloft and marine
layer compression.
Vicinity of SFO...Marine stratus fulling taking over the Bay this
morning, anticipated to slowly erode through the late morning.
Some uncertainty with clearing time given that it may be a more
gradual mix out to VFR. The same is true regarding stratus return
this evening. CIGS through the Golden Gate area may be abrupt, but
expecting a slower spread southward over the terminal.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...MVFR/IFR through much of this morning.
Occasional coastal -DZ around and just after sunrise. High
confidence in cigs remaining in place through this morning and
gradually mixing back to the coastline. Lower confidence in
coverage overnight tonight as some drier air mixes in from aloft.
Still expecting prevailing MVFR/IFR at this point, but coverage
may be patchy.
&&
.MARINE...
(Today through Thursday)
Issued at 437 AM PDT Sat Sep 13 2025
Diurnal winds ease overnight before increasing to fresh breezes
again Saturday afternoon along prominent coastal jet areas.
Moderate seas persist, especially offshore. More widespread fresh
breezes build across the rest of the waters into Sunday afternoon
before subsiding into the beginning of next week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
     evening for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory from 3 PM to 9 PM PDT Sunday for Mry Bay-
     Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Sunday
     for Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory from 3 PM Sunday to 3 AM PDT Monday for Pt
     Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Monday
     for Pt Pinos to Pt Piedras Blancas 0-10 nm.
&&
$$
SHORT TERM...JM
LONG TERM....JM
AVIATION...Behringer
MARINE...Behringer
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NWS MTR Office Area Forecast Discussion