919
FXUS66 KMTR 262142
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
242 PM PDT Sat Apr 26 2025

...New SYNOPSIS, SHORT TERM, LONG TERM, MARINE...

.SYNOPSIS...
Issued at 240 PM PDT Sat Apr 26 2025

Scattered showers and low chance of thunderstorms continues
through this evening before diminishing overnight into tomorrow
morning. Warmer, seasonal temperatures prevail this upcoming week
before the pattern becomes unsettled again next weekend.

&&

.SHORT TERM...
(This evening through Sunday)
Issued at 240 PM PDT Sat Apr 26 2025

Scattered thunderstorms remain possible across the interior Central
Coast through late this afternoon as a cut-off low enters Southern
CA with thunderstorm chances to diminish overnight into tomorrow
morning. The 12Z OAK sounding (5AM) showed fairly moist low and mid
levels (65-74%) with no real temperature inversion at the lower
levels which is beneficial for convective development. When a
temperature inversion is present, it acts as a cap for
instability/upwards motion. As a refresher, a low level temperature
inversion means temperatures are increasing as you go up with height
in the atmosphere rather than decreasing. Now assume we have a
positively buoyant air parcel at the surface, that air parcel will
want to rise as long as it is warmer (i.e. less dense) than the air
parcels surrounding it. When an inversion is present, that air
parcel will effectively rise until it hits the temperature inversion
(where the ambient air temperature is warmer than that of the
parcel) causing it to sink back to the surface. The inversion
therefore serves to cap the upwards transport of air from the
surface and limits convective development. The lack of a capping
inversion means that surface based air parcels will be able to rise
more freely, with convective activity able to develop.

Shower activity increased late this morning with showers currently
becoming more widespread over the South Bay and Central Coast. Mid-
level overcast skies over the North Bay have kept the atmosphere
more stable there resulting in light stratiform precipitation rather
than more convective cells developing. Farther south, skies were
able to clear for much of the morning (South Bay, Central Coast)
resulting in increased surface warming and low-level instability. As
seen on both satellite imagery and on KMUX, convective cells have
started to develop where clearing was able to occur (generally south
of the SF Bay) with light rain showers being observed. So far no
lightning strikes have been reported within our CWA but thunderstorm
potential will remain elevated through this evening. Thunderstorms
are most likely across the interior Central Coast but that depends
on mid to upper level clouds continuing to clear across that region.
Another limiting factor is the lack of low level wind shear which
may reduce our overall thunderstorm risk. Thunderstorm chances drop
off this evening (less than 10% everywhere) with scattered showers
and light rain continuing through Sunday morning. High temperatures
will peak in the upper 50s to mid 60s today and tomorrow. While not
anticipating winds to be an issue with this system, if a
thunderstorm does develop locally breezy to gusty winds may occur
with it. As always, remember, when thunder roars go indoors.

&&

.LONG TERM...
(Sunday night through next Friday)
Issued at 240 PM PDT Sat Apr 26 2025

Dry conditions will prevail for most of next week with surface high
pressure and upper level ridging building back in. The return of
upper level ridging will kick off a warming trend with temperatures
becoming more seasonal to slightly above average across the region.
For interior regions, highs will be in the mid to upper 70s for much
of next week while coastal areas stay in the low to mid 60s. Light
offshore flow is possible Monday to Tuesday next week as a the cut-
off low moving through CA today is expected to deepen and become
positively tilted as it passes through the Four Corners region.
Minor HeatRisk is forecast Tuesday through Thursday with impacts
mainly confined to those who are extremely sensitive to heat. If you
are outdoors this week and are sensitive to heat, remember to listen
to your body and take breaks/drink plenty of water. The pattern
looks to become unsettled again Friday into next weekend with long
range guidance showing another deep upper level trough moving into
the West Coast. Temperatures are expected to drop back into the mid
to upper 60s across the interior and upper 50s to low 60s along the
coast starting Friday. Initial ensemble guidance shows this system
may bring light rain (similar totals to today`s event) to the region.
Given that we are almost a week out the specifics may change as we
get closer to this event but, for now, be aware that the pattern is
likely to become unsettled again with cooler temperatures and some
potential for light rain next weekend.

&&

.AVIATION...
(18Z TAFS)
Issued at 1106 AM PDT Sat Apr 26 2025

Convective clouds starting to rapidly develop over higher terrain
with upper low nearby. Most terminals are forecast to remain
mainly dry but tafs will show vicinity showers. Most cigs should
remain mvfr outside of heavier downpours. Cant rule out isolated
lightning chances but best chances remain away from terminals
over higher terrain and out towards the Central Valley. Best
chance of showers will be afternoon hours, roughly 21-00z and then
a final shot 02-06z as final shortwave rotates down the coast
from North Bay to Central coast with most likely terminals to see
showers being Half Moon Bay and Monterey under post frontal
favorable NW wind flow pattern. Skies to quickly trend VFR after
sunrise Sunday with drying trend building in behind upper trof.

Vicinity of SFO...WNW winds mainly under 15 kt with cigs mainly
MVFR. Nearby showers this afternoon and into this evening. Drying
trend returns by 12z Sunday.

SFO Bridge Approach...Similar to SFO with showers passing over the
Bay possibly impacting visuals of the runways at times this
afternoon and evening.

Monterey Bay Terminals...Building cumulus over the higher terrain
around MRY and SNS with prevailing NW winds this afternoon and
evening. Nearby showers this afternoon then better chances for
light rain and lowering cigs for KMRY between 02-07z as final
disturbances pushes through. Drying trend with VFR skies
developing 13-17z Sunday.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 240 PM PDT Sat Apr 26 2025

Lingering showers over the coastal waters though tonight as an
upper low passes over the region. Gusty northwest winds will
develop behind the low overnight through Sunday. Persistent
springtime northwest flow will continue through early next week as
dry high pressure builds with moderate nw seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 PM this evening to 9 AM PDT Sunday
     for Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-10
     nm.

     Small Craft Advisory until 9 PM PDT Sunday for Pt Arena to Pt
     Reyes 10-60 NM.

     Small Craft Advisory from 9 PM this evening to 9 PM PDT Sunday
     for Pigeon Pt to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...RW
MARINE...RW

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