519
FXUS66 KSGX 172048
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
148 PM PDT Wed Sep 17 2025

.SYNOPSIS...
Tropical moisture will increase over the region through Thursday.
Showers and scattered thunderstorms will be expected, where some
may be locally heavy with localized flooding across the mountains
and the deserts. Moisture will decrease by Friday into the
weekend, with drier weather across the area with a lower chance
across the mountains and deserts. There will be a subtle warming
trend for this weekend into next week with drier weather.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

.SHORT TERM...

Increasing clouds and moisture have overspread the area this
afternoon as post tropical storm Mario continues to weaken. Radar
shows weak thunderstorms beginning to form over the mountains,
with greater coverage in rain across northern Baja. The chance for
showers and storms will increase from south to north through this
afternoon and evening across Southern California. Latest model
data shows storms dissipating overnight, where a brief lull in
activity is expected as an expansive mid level cloud deck moves
over the region. Another rather humid night will occur again
tonight with lows near 70 degrees west of the mountains into the
high desert. The humidity will stick around through Friday.

The tropical moisture will continue to move northward on Thursday,
bringing the greatest chance for the heaviest showers and
thunderstorms across the region. Hi-res models indicated heavier
downpours possible as daytime heating and higher instability will
foster an environment for growing thunderstorm activity. Some
models indicate hourly rain rates near 3/4" to 1" across these
regions, so a Flood Watch has been issued for early Thursday
morning through night, with heaviest rainfall expected on Thursday
afternoon and evening. Be on the lookout for flooded roadways and
areas near recent burn scars to impact communities and travel.
Storm activity will calm by Friday morning, with best chances for
showers to spring up across the mountains and deserts.

The weekend will look much drier as a trough offshore moves closer
to the area. There may be enough instability and moisture leftover
for a shower or two across the mountains and the deserts each
afternoon. Models indicate the trough forming into a closed low by
early next week, which makes for a fickle weather forecast. While
precip is not out of the question, most areas are expected to
remain dry with near average temperatures and a deeper marine
layer.

&&

.AVIATION...
172000Z...Radar imagery shows a wave of precipitation with a few
embedded in northwest Baja CA, moving northwestwards towards
southwest San Diego Co, arriving towards 21-22Z, though it will
likely weaken as it moves into CA. Meanwhile, ISO
-SHRA/TSRA are forming ahead of the wave along the mountain ridges
and west foothills. SHRA/TSRA continue to move northwestwards in a
wave through this evening, with ISO to SCT SHRA/TSRA likely
following behind it. Rain at the surface initially likely to be
light, but heavier pockets that pass over any terminals could bring
brief periods of lowered visibility 2-5 SM and gusty, erratic winds.
At the same time, some cloud bases to lower to around 5000 feet MSL
tonight into early Thursday, intermittently obscuring some of the
higher terrain.

A brief break in showers could occur late this evening into tonight,
but timing of this is uncertain, and showers resume early Thursday
morning. SHRA/TSRA start to shift eastwards toward inland
valleys/mountains/deserts, with lingering but decreasing chances for
ISO -SHRA/TSRA for coastal areas through Thursday evening.

&&

.MARINE...The remnants of Mario will bring a slight chance of
thunderstorms today through Friday morning, greatest chances
Thursday starting in the early morning. Any storm could briefly
produce lightning, gusty variable winds, and choppy seas.
Additionally, gusts around 20 kt possible in the vicinity of San
Clemente Island this evening through early Thursday morning.

No hazardous marine conditions are expected Friday through Monday.

&&

.BEACHES...There is a slight chance of lightning today through
Friday morning, greatest chances Thursday starting in the early
morning.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from late tonight through late Thursday night for
     Apple and Lucerne Valleys-Coachella Valley-Riverside County
     Mountains-San Bernardino County Mountains-San Diego County
     Deserts-San Diego County Mountains-San Gorgonio Pass near
     Banning.

PZ...None.

&&

$$

PUBLIC...APR
AVIATION/MARINE...CSP

NWS Tucson (SGX) Office