Wildfire and Heavy Rain in California, August 1 to 8, 2024

Area Forecast Discussion
National Weather Service San Diego CA
940 PM PDT Wed Jul 31 2024

.SYNOPSIS...
Areas of showers and thunderstorms are possible Thursday for the
mountains; deserts; portions of the Orange County coast; and the
valleys. Best chances will be in the late morning and afternoon
hours, potentially continuing into the early evening. Chances of
showers and thunderstorms continue most afternoons and evenings
Friday through the middle of next week for the mountains and at
times the deserts. Hot and humid conditions are expected for the
weekend into early next week. The marine layer will remain
shallow, leaving coastal clouds and fog each night and morning
near the coast and into portions of the western valleys.

&&

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

The 00Z sounding from KNKX was showing an increase in precipitable
water, 0.86 inches vs the 0.46 inches on the 12Z sounding Wednesday
morning. Moisture is expected to continue to increase Thursday and
Friday. The increased moisture combined with an inverted trough
feature moving across the area will bring chances of showers and
thunderstorms Thursday from the deserts to portions of the coast.
The timing of the potential precipitation has been tricky to pin
down. Local high resolution WRF and the HRRR are now showing a band
of shower/potential thunderstorm activity moving east to west across
the forecast area late Thursday morning into the afternoon, which is
earlier than previous runs suggested. Guidance is indicating that
little rainfall accumulation will occur in lower elevation areas but
the environment could support the development of a few thunderstorms
west of the mountains. The mountains are the most likely place to
see shower and thunderstorm activity Thursday, with the potential
for locally heavy rainfall. Shower and thunderstorm chances will
continue, mainly in the mountains and deserts Friday afternoon, with
a greater potential for areas of heavy rain.

From previous discussion issued at 2 PM July 31, 2024...

A weak inverted trough over northern Baja will creep its way
northward by Thursday into Friday. This feature will make things a
little different from what have been seeing as of late. Elevated
moisture levels will begin to enter the area by early Thursday with
increased PWAT expected to ramp up near 1.25" and CAPE values over
300 J/kg. This, along with the movement of the trough by later in
the day, will lead to enough instability to provide the chance for
showers and thunderstorms across parts of the Inland Empire and
Orange County. The timing looks primarily to be on Thursday evening
into Friday morning. Whatever storms that occur look to be fast
moving per latest WRF model guidance. This would bring brief heavy
rain (mainly in the mountains) and gusty winds, along with cloud to
ground lightning. If no rain, high clouds will be aplenty during
this period which will help limit the intense heat at times. Chance
for storms will then continue in the mountains and deserts on Friday
afternoon.

High temperatures will begin to increase as the area of high
pressure expands westward across our region. Through Friday, expect
temperatures to warm each day with highs and lows near to slightly
above average. The marine layer will shrink slightly, with low
clouds and fog most nights through the period. This may be disrupted
if high clouds from the inverted trough cover the coastal regions.

&&

.LONG TERM (This Weekend into Early Next Week and Beyond)...
The ridge will amplify across the desert southwest into the southern
Rockies. This will bring continued moist southerly flow into the
region, with chances for showers and thunderstorms each afternoon
across the mountains and deserts. Moisture decreases slightly over
the weekend, so storm chances will be best across the mountains
during this time. PWAT ensembles show a slight peak in moisture
occurring again on Tuesday and Wednesday, which will lead to greater
chances for storm activity into early next week for mountains and
deserts.

As the high strengthens, 850 mb temperatures will push near 30C
across the region per latest GFS/ECMWF by this weekend. NBM shows
50th percentile temperatures near 115 across the lower deserts.
Temperatures were increased slightly due to drier air expected this
weekend before lower level moisture comes into the region again
early next week. There is now (again) Excessive Heat alerts across
the lower deserts as areas of high heat risk are anticipated. No
matter which way you slice it, warm and more humid conditions is
expected for all areas of SoCal this weekend into early next week
with dew points into the 60s. Taking a look at the middle and latter
parts of next week, there may be a hint of the ridge breaking down
per latest model ensemble clusters. If this occurs, the chance for
daily afternoon storms and daytime highs and humidity would begin to
lower.

&&

.AVIATION...
010430Z...Coast/Valleys...Areas of low clouds will gradually develop
within 10 miles of the coast through 10Z Thu with bases 600-1000 ft
MSL and tops to 1200 ft MSL, spreading locally to 20 miles inland
12Z-15Z Thu, with clearing in most locations 15Z-17Z Thu. SAN and
CRQ will likely have a CIG by 07Z, though SNA could be delayed until
10Z. Local VIS 0-3 miles in FG/BR will occur over higher coastal
terrain 10Z-15Z Thu. Otherwise, SCT-BKN clouds at/above 12000 ft MSL
will increase after 10Z Thu with around 10% chance of TSRA spreading
west into the valleys from the mountains Thu afternoon/evening.

Mountains/Deserts...SCT-BKN clouds at/above 12000 ft MSL will
increase overnight and continue through Thu evening. ISOLD TSRA will
develop over the mountains and high deserts Thu afternoon/evening
with CB bases 10000 ft MSL and tops to 35000 ft MSL and strong UDDFS
and local surface gusts to 40 kt. Most VIS will remain unrestricted,
with a small chance of VIS restrictions from BLDU in gusty winds
from TS.

&&

.MARINE...
No hazardous marine conditions are expected through Monday.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Watch from Saturday morning through Sunday
     evening for Coachella Valley-San Diego County Deserts-San
     Gorgonio Pass Near Banning.

PZ...None.

&&

PUBLIC...CO/APR
AVIATION/MARINE...Maxwell