Special Weather Statement, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
1250 PM PDT Sun Jul 18 2021

.SYNOPSIS...
Monsoonal moisture and daytime heating will increase the potential
for strong thunderstorm development over the mountains this
afternoon. Local flash flooding is possible where these storms
develop. Thunderstorms are expected over the mountains again on
Monday, decreasing into midweek, before increasing again later in
the week as a monsoonal flow prevails around strong high pressure
over the Rocky Mountains. Mild nights and seasonally warm to hot
days are expected all week, along with higher humidity.

&&

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

Cumulus began development mid morning over the mountains with full
heating underway and ample moisture. Any subsidence in the wake of
the wave that passed this morning should become less of a factor
this afternoon, especially over the southern mountains where the
hires models have repeatedly shown strong convective development in
isolated cells. Due to the high available moisture and slow steering
currents along the peninsular mountains, the convective potential
supported the issuance of a Flash Flood Watch that will be in effect
through early evening for the mountains of Riverside an San Diego
County. The potential decreases a bit tomorrow, but the chance of
isolated thunderstorms remains over the mountains in the afternoon.

Despite the weak monsoonal regime over SoCal, the marine layer and
onshore flow will continue to moderate areas closer to the coast,
resulting in seasonal mid-summer weather. Dewpoints, however are
somewhat anomalous in the lower 70s over western San Diego County
today so you will feel the humidity in the air.

Strong high pressure aloft near the Four corners area is steering
monsoonal moisture into SoCal today. The high is forecast to build
northward through Monday, with a 598DM center over Colorado. The
ridge axis is expected to change from N-S to NE-SW through midweek
(back over SoCal), which tends to decrease the available monsoonal
moisture over us, and stabilize the atmosphere a bit more,
especially after the passage of another weak easterly wave on
Monday. A slight chance of thunderstorms will continue after Monday,
but only over the mts and high desert areas. A further reorienting
of the ridge axis to more E-W late in the week, may open the door to
a better monsoonal flow and weak disturbances to move over SoCal.
This is supported by some of the various global model ensemble
members, but many are not on board, so forecast uncertainty with
regard to convective rainfall, increases after Monday.

Outside of the monsoonal convection, seasonal daytime temperatures
are forecast to continue, which means warm to hot days inland. But
even the nighttime temperatures will be elevated in areas where
humidity is high. At this time it looks like overnight minimums
could be tracking from 5-10F above average, especially over the
mts/high deserts much of this week. Also, the marine layer clouds
may remain limited for the next night or two before filling back in
again. Some of this is due to the warmer sea surface over the CA
Bight, but also the more humid airmass aloft, which will begin to
decrease after today. The ensembles/mean show a clear trend downward
in temperatures starting next weekend. This is due to the proximity
of a weak trough near the West coast and a weaker inland ridge
aloft.

&&

.AVIATION...
181930Z...Coast/Valleys...FEW-SCT clouds at/above 10,000 feet MSL
will prevail through tonight. SCT SHRA and TSRA will be possible in
the eastern portions of the valleys through 02Z Monday. Lightning
and gusty, erratic winds will be possible near TSRA.

Patchy low clouds with bases 1000-1500 feet MSL may redevelop after
04Z Monday along the coast, but confidence is low in ceilings at
coastal terminals. Any clouds that develop will clear 16-17Z Monday.

Mountains/Deserts...FEW-SCT clouds at/above 10,000 feet MSL will
prevail through tonight. Scattered TSRA expected this afternoon and
evening, with the greatest coverage over the Riverside and San Diego
County mountains. Gusty and erratic winds will be possible vcty TSRA
as well as lightning strikes. TSRA will begin to diminish after 02z
Monday.

&&

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

&&

.SKYWARN...
Skywarn activation is requested for the afternoon and early evening.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch from noon today to 7 PM PDT this evening for
     Riverside County Mountains-San Diego County Mountains.

PZ...NONE.

&&

PUBLIC...10
AVIATION/MARINE...Connolly

Special Weather Statement
National Weather Service San Diego CA
131 PM PDT Sun Jul 18 2021

CAZ050-058-182115-
San Diego County Mountains CA-San Diego County Inland Valleys CA-
131 PM PDT Sun Jul 18 2021

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN SAN DIEGO COUNTY
UNTIL 215 PM PDT...

At 130 PM PDT, Doppler radar was tracking a strong thunderstorm near
Lake Morena, or near I-8 Between Pine Valley And Boulevard, moving
northwest at 15 mph.

Half inch hail and wind gusts up to 50 mph will be possible with this
storm.

Locations impacted include...
Alpine, Mount Laguna, I-8 Between Pine Valley And Boulevard, Cuyamaca
Rancho State Park, Pine Valley, southwestern Boulevard, Lake Morena,
Descanso, Campo, Hwy S1 Between Lake Cuyamaca And Mount Laguna, La
Posta Indian Reservation, Tierra Del Sol, Campo Indian Reservation,
Live Oak Springs, Guatay, Viejas Indian Reservation, Manzanita Indian
Reservation, Japatul Valley and Captain Grande Indian Reservation.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 3288 11670 3295 11648 3261 11627 3260 11632
      3259 11646
TIME...MOT...LOC 2030Z 144DEG 11KT 3268 11643

SS