Millbrae, California Weather Forecast Discussion

309
FXUS66 KMTR 211547
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
847 AM PDT Sun Sep 21 2025
...New UPDATE, MARINE...
.KEY MESSAGES...
Updated at 326 AM PDT Sun Sep 21 2025
- Warming trend through Monday and into Tuesday
- Weak offshore flow on Monday and Monday night in the higher
   terrain
- Rain showers and isolated thunderstorm chances return early to
   mid week
&&
.UPDATE...
Issued at 846 AM PDT Sun Sep 21 2025
Low stratus and patchy fog persist this morning across the Pacific
and coastal adjacent valleys. However, these clouds are forecast to
retreat to the coast by late morning and more so by early afternoon.
Thus will result in mostly sunny conditions across much of the
region this afternoon. Thus, temperatures are forecast to be around
5-10 degrees above seasonal averages. No updates anticipated this
morning with the current forecast remaining on track.
RGass
&&
.SHORT TERM...
Issued at 326 AM PDT Sun Sep 21 2025
(Today and tonight)
Satellite water vapor imagery has blessed with another beautiful
picture from space. To our north, over the PacNW, a trough and front
passes overhead. To our southwest, an upper low churns off the
coast. Night fog products show stratus has started to  blanket our
coastal waters and intruding on coastal communities. Nonetheless,
any inland expansion is expected to be short lived with a sunny
skies expected by late morning or early afternoon. The warming trend
continues with highs peaking in the 80s and low 90s for interior
locations and in the 60s and 70s.
Tonight, upper level ridging noses in with winds coming in from the
north. This might keep stratus at bay, especially if the marine
compresses to 500-1000ft.
&&
.LONG TERM...
Issued at 326 AM PDT Sun Sep 21 2025
(Monday through Saturday)
Upper level ridging will lead to one more day of warmer temperatures.
Weak onshore flow is expected Monday night into Tuesday, as the
ridge flattens and the upper level lifts towards southern or central
CA. The pattern continues to favor showers mid week, with
thunderstorms remaining possible. Convective ingredients look semi
favorable, with a couple hundred J/kg of MUCAPE looking likely for
southern Monterey and San Benito counties. Some ensemble members
take this up the Diablo Range and to a lesser extent the interior
North Bay; however, this is likely due to the models determining
where the low will go. Moisture looks to be prevalent, with PWATs
valued between 1.0-1.3 inches. Shear looks to be minimal, but
perhaps just enough to keep storms going if the they`re elevated.
Drier weather should return Thursday and into the weekend. Cooler
weather should be on tap as well, as the upper low exits the
region.
&&
.AVIATION...
(12Z TAFS)
Issued at 502 AM PDT Sun Sep 21 2025
Low CIGs have filled along the coast and the bays with CIGs
approaching the North Bay terminals. FOG and LIFR CIGS affect STS
through the mid morning. CIGs begin to erode in the late morning
leading to widespread VFR for the Afternoon as winds build and
become moderate to breezy. Winds reduce into the mid evening and
into the night. CIGs return to the coast and Monterey Bay into the
late night.
Vicinity of SFO...CIGS last into the late morning, but erode into as
moderate northwest winds return. Expect breezier winds for the
afternoon with gusts peaking around 25 kts. These winds reduce into
the night, becoming moderate.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...IFR/LIFR CIGs with light winds last into
the late morning.Expect VFR and moderate west winds for the
afternoon. Winds reduce the evening and early night, with IFR CIGs
returning in the late night.
&&
.MARINE...
(Today through Friday)
Issued at 846 AM PDT Sun Sep 21 2025
Northerly flow with light to moderate winds will persist through
early Sunday. Northerly winds increase Sunday afternoon as a weak
cold front approaches from the north. Wave heights build across
the coastal waters late Sunday through early next week as a cutoff
low moves into California. There is a low chance for isolated
showers and embedded thunderstorms Tuesday into Wednesday across
the southernmost waters. Winds decrease and seas abate across the
coastal waters Tuesday through late week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Monday
     for Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory from 3 PM this afternoon to 3 PM PDT Monday
     for Pt Arena to Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...KR
LONG TERM....KR
AVIATION...Murdock
MARINE...MM
Visit us at www.weather.gov/sanfrancisco
Follow us on Facebook, X, and YouTube at:
www.facebook.com/nwsbayarea
x.com/nwsbayarea
www.youtube.com/nwsbayarea

NWS MTR Office Area Forecast Discussion