West Portal Station San Francisco Post Office, California Weather Forecast Discussion

339
FXUS66 KMTR 280455
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
955 PM PDT Sat Sep 27 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 209 PM PDT Sat Sep 27 2025
- Rain and thunderstorm chances return Monday
- Rain lingers through Wednesday
&&
.UPDATE...
Issued at 900 PM PDT Sat Sep 27 2025
Northerly winds (carrying wildfire smoke southward from the Moon
complex in southwestern Oregon) over the outer waters were
juxtaposed to southerly winds and onshore winds along the
coastline today. These combined winds ushered some wildfire smoke
into at least the northern San Francisco Peninsula, the East Bay
and the North Bay today. Recent high resolution rapid refresh model
(HRRR) shows near surface smoke extending north-south along our
coast and gradually mixing out if not tonight then through the day
Sunday.
Southerly winds along the coastline also carried higher humidity
in the boundary layer (from all recent showers/t`storms) back
north from Central CA/SoCal. Upslope winds developed over the
coastal mountains and helped generate light drizzle and/or
sprinkles mainly earlier today, with not much indication of
measurable rain i.e. bucket tips. Satellite imagery shows stratus
quickly redeveloping along the coast where it had earlier mixed
out. Checking on recent model output from the HRRR as well as the
12z ECMWF, GFS and recent NAM, we may need to add coastal drizzle
to our forecast grids for late tonight and Sunday. This is good
that we`re seeing high humidity from sea level all the way up to
hilltops and mountain tops, it`s much better than hot/dry N-NE
offshore winds and dangerous fire conditions Sep-Oct can produce.
With the long wave trough setting up over the northeast Pacific,
early 00z model output for this evening continue to show a trend
toward wet weather for our area, best chances north Central
Coastline and areas northward across the Bay Area Mon-early Thu
this upcoming week. With the first cold frontal system approaching
the Bay Area Monday and Monday night, today`s Sep 27th 12z ECMWF
run (and hinting at it also is the NAM) shows potential secondary
surface low pressure development inside of 40N/130W. Recent GFS
has not shown secondary development (GFS shows less rain
potential). A plume of water vapor timing with secondary or no
secondary low pressure development will be key as to potential
rain amounts and winds too here in our forecast area. It`ll be
interesting to see what both the 00z Sep 28th ECMWF and GFS show.
For additional info on the forecast please see the previous
discussions.
&&
.SHORT TERM...
Issued at 209 PM PDT Sat Sep 27 2025
(This evening through Sunday)
We have a southerly surge underway with low stratus along the
Pacific Coast and even continuing to push inland through the Golden
Gate Gap and Monterey Bay - as such, I don`t think the sun will be
seen along the Pacific Coast today. The cooling and moistening trend
will continue through the weekend due to an approaching upper-level
longwave trough. This aforementioned feature will allow for the
marine layer to deepen and perhaps even provide enough lift to
squeeze out some drizzle along the coast and orographically enhanced
locations. Temperatures will be near to slightly below normal today
with widespread well below normal temperatures by tomorrow.
&&
.LONG TERM...
Issued at 209 PM PDT Sat Sep 27 2025
(Monday through next Friday)
The first of two systems will approach the region on Monday. Warm
sector (light, stratiform) rain is expected Monday morning ahead of
the cold front that will bring relatively intense rainfall and the
chance for thunderstorms Monday afternoon into Monday evening. The
moisture is sourced from the subtropics with the GEFS and ECMWF
Ensemble IVT Forecast ensemble mean showing it peaking around 350
kg/ms Monday afternoon and dropping below 250 kg/ms by Monday night.
Thunderstorm chances remain, but have decreased. The essential
ingredients for a thunderstorm are lift, instability, and moisture.
The lifting mechanisms and moisture have already been discussed, but
the instability is looking more and more like it will be the
limiting factor. Lapse rates have decreased to become stable and
MUCAPE has trended down to 100-150 J/kg and even this was difficult
to find as coverage has shrunk. Post-frontal rain showers are to be
expected on Tuesday between system one and two. System two will get
it`s moisture from Typhoon Neoguri in the Northwestern Pacific Ocean
as it overtops a subtropical ridge in the Central Pacific Ocean and
feeds into a developing gale force low pressure system off the
Washington Coast. There is uncertainty in the exact timing of this
second system, but as with the first system, the timing has been
pushed back. It is now looking like the arrival time of this second
system will be early Wednesday morning. This system is expected to
have even less favorable ingredients for thunderstorms. As is
typical with Gulf of Alaska systems, there will be a stark northwest
to southeast rainfall gradient. When all is said and done, the North
Bay can expect to see up to 1.25" with the Interior Central Coast
becoming less likely to receive any rainfall. We will continue to
monitor the potential for any stalling/stationary nature of these
systems. Fortunately, right now rainfall rates and totals are not
worrisome. It is also worth noting that as the track has shifted
farther north, rainfall totals have decreased.
&&
.AVIATION...
(06Z TAFS)
Issued at 955 PM PDT Sat Sep 27 2025
The SBA-SFO and SMX-SFO pressure gradients are 0.9 mb and 1.6 mb
respectively, while the SFO-SAC pressure gradient is 1.9 mb, these
are stronger than the ACV-SFO pressure gradient which is flat. The
surface pressure gradients are resulting in a mix of southerly and
onshore winds, combining with deepening marine layer stratus, fog
and a few areas of light drizzle/sprinkles in the coastal mountains.
The NAM and HREF show an increase in stratus /IFR-MVFR/ tonight and
Sunday. 90-100% probability of IFR in the HREF suggests increasing
drizzle potential in the coastal mountains, some of which may also
reach into the North Bay tonight and Sunday morning.
Vicinity of SFO...VFR-MVFR. Southwest wind 10 knots tonight and
Sunday morning with a few gusts to 20 knots Sunday afternoon and
early evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...MVFR-IFR in stratus tonight and Sunday
morning. Patchy drizzle possible tonight and Sunday. Southerly
winds along the coast are resulting in a mix of variable and
southwesterly wind at KMRY while it`s northwesterly at KSNS. Winds
generally becoming light and variable tonight and Sunday morning
then onshore Sunday afternoon and early evening.
&&
.MARINE...
(Tonight through next Thursday)
Issued at 851 PM PDT Sat Sep 27 2025
Gentle to moderate southerly breezes will allow for seas to
subside into early Sunday morning. An approaching upper level
trough and surface cold front will bring rain, isolated embedded
thunderstorms, and building seas to the coastal waters Monday into
early Tuesday. A second system will arrive Tuesday into Wednesday
and continue rain chances across the coastal waters.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Canepa
MARINE...Canepa
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NWS MTR Office Area Forecast Discussion