Flash Flood Watch 6, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
944 PM PDT Sun Sep 1 2019

.SYNOPSIS...
High pressure aloft will continue very warm conditions most of this
week. Monsoonal flow and warm coastal waters will result in
generally humid conditions as well, and the monsoonal flow will
bring chances of mainly afternoon and evening thunderstorms over the
mountains and deserts, especially Monday through Wednesday when a
few storms could drift west of the mountains. Night and morning low
clouds will mainly be less extensive most of the rest of the week. A
trough of low pressure off the coast will likely bring drier air and
some cooling though also an increase in the marine layer low clouds
next weekend.

&&

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

Evening Update:

Thunderstorm complex over the Colorado River Valley could bring a
dusty gust front into the Coachella Valley late tonight along with a
few thunderstorms. Have updated the forecast to adjust accordingly.
Have also added pops into the mountains as the thunderstorms could
redevelop along the desert slopes late tonight. Activity should
diminish by 12z with thunderstorms likely redeveloping in the
deserts tomorrow late morning in a moist monsoonal flow regime. No
other changes to the forecast at this time.

Previous Discussion from 100 PM PDT:

Stratus was still widespread over the coastal waters and beaches but
have cleared over land. Some mid-high clouds from the southeast were
drifting over land as well. Cumulus was forming over the mountain
crests, but no thunderstorms had formed yet. Models do not have much
explicit precipitation for this afternoon, with some HRRR runs even
keeping us dry, but a few thunderstorms should form this afternoon,
mainly over the mountains and desert slopes in San Diego and
Riverside County where SPC mesoanalysis shows some CAPE over 1000
J/kg.

The upper high will continue roughly over the Four-Corners region
through Thu or Fri, with a weak inverted trough moving west to our
south Monday. Most ensemble solutions show Tropical Storm Juliette
staying well to the south, except for a few GEFS solutions showing
remnants approaching from the south next weekend when a Pacific
trough moves into California from the west. NHC guidance keeps it
south of the Tropic of Cancer through Friday, and this should have
little or no impact here. What we will have is high amounts of
moisture (RH > 70%) in the 700-500 MB layer especially Mon/Tue and
moderate amounts of moisture (RH > 50% at times) even below that,
with some decreases Wed, especially below 700 MB. Total precipitable
water could exceed 2 inches both along the coast and in the lower
deserts. Instability will be sufficient for thunderstorm
development, with surface-based CAPE over 1000 J/kg at times. With
all the moisture and instability, there is a concern of local flash
flooding from heavier rains either Monday or Tuesday. Mid-level
winds will be moderate, 10-20 knots, which won't be enough to
inhibit heavy rains. A few storms could drift west of the mountains
Monday or Tuesday, but this mostly will be confined to the Elsinore
Convergence Zone, especially since any upper level trigger would
appear to weak to spark the moderate amounts of elevated CAPE in the
700-500 MB layer. Some thunderstorm chances will remain through
Friday but should diminish by Saturday as the trough moving into
California will bring drier southwest flow. The marine layer
stratus, after its stronghold this morning, should be less notable
Monday and Tuesday due to the moisture aloft increasing the long
wave radiation reflected back to the surface. Temperatures should
decrease slightly Monday, though only by a few degrees, and the heat
advisory for the Inland Empire and excessive heat warning for the
high deserts will be allowed to expire tonight. Some renewed warming
west of the mountains Tue/Wed may be enough to require an excessive
heat advisory again in the Inland Empire as a lot of areas will
likely be over 100.

&&

.AVIATION...

020445Z...Coast/Valleys...SCT-BKN high clouds AOA 15,000 FT MSL will
prevail through the midnight hour, though will watch for patchy
stratus on the beaches which may generate brief periods of BKN CIGS
at the coastal terminals. Confidence increasing toward predominant
VFR cigs through the night. Model guidance has been trending with
higher bases and less coverage time, so brief periods of BKN CIGS
with bases 500-900 FT MSL cannot be ruled out mainly occurring after
06Z with VIS 5 sm or above. Scatter out of low clouds after 16Z
tomorrow with CIGS AOA 15,000 FT MSL through the afternoon. Chance
for -TSRA, especially in the RIV/SD County valleys, after 18Z Monday
as well.

Mountains/Deserts...Mostly clear tonight with SCT-BKN clouds AOA
15,000 FT MSL starting late Monday morning. Outflow winds from
storms in southeast California may bring areas of BLDU and reduced
VIS 3-5 sm for KPSP/KTRM 08-11Z Monday. MOD confidence for
-TSRA in both mountains and deserts after 18Z Monday, most likely
across the mountain chains. Areas of BKN050-070CB possible through a
majority of the afternoon.

&&

.MARINE...
No hazardous marine conditions expected through Friday.

&&

.MARINE...
No hazardous marine conditions expected through Friday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.

PZ...NONE.

&&

PUBLIC...Maxwell/JMB
AVIATION/MARINE...APR