Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 270429
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
929 PM PDT Mon Jul 26 2021

.SYNOPSIS...
Warmer and drier weather will prevail through Thursday, though
there is still a slight chance of thunderstorms in the mountains
Tuesday afternoon. Monsoon moisture will increase again late in
the week for a better chance of afternoon thunderstorms, mainly
in the mountains and deserts Friday through the weekend.

&&

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

Update:

Thunderstorms did not materialize this afternoon, despite good
moisture (PW 1.5-1.8 inches) and instability (MUCAPE of 2500
J/KG). This was one tough forecast. It looks as though the
primary mitigating factor for tstorm development over the
mountains was subsidence behind the upper trough axis. The Flash
Flood Watch was cancelled early given the no go on thunderstorm
developement.

It is looking like June out there with a deep marine layer well
into the inland valleys. In fact, drizzle was reported at Del Mar
and there is light mist at the office currently. Have added patchy
drizzle to the coast and much of the valleys for the overnight
hours. Only trace to a hundredth of an inch or so of rainfall is
expected overnight. Fog will occur over parts of the valleys,
mainly where the marine layer clouds intersect terrain.

Residual moisture will be present to allow for isolated showers
and perhaps isolated thunderstorms to develop over the mountains
Tuesday afternoon, before further drying occurs Wed-Thu, likely
fully shutting off monsoonal convection for a couple of days.
However, southeasterly monsoonal flow will bring increasing
moisture into the area, increasing the risk of showers and
thunderstorms to the mountains and deserts Friday and through next
weekend.

Previous Discussion (Issued at 132 PM PDT Mon Jul 26 2021):

The widespread showers from this morning finished up and now we`re
beginning to see some thunderstorms pop up in the deserts and
mountains. One was observed east of Lucerne Valley and one in the
mountains south of the border near Imperial County. Latest
guidance is trimming back the chances or numbers of afternoon
thunderstorms anywhere in the inland valleys, mountains and
deserts (check Hydrology section below). Best chances continue in
San Bernardino and Riverside Counties. The Inland Empire`s best
chance would be along the Elsinore convergence zone between Lake
Elsinore and Beaumont. A few storms this afternoon could be
intense, producing brief severe weather in addition to the intense
rain. By sunset, storms should be finishing up. The upper low
(easterly wave) responsible for all this activity will rotate
northward tonight and Tuesday, sweeping away much of the moisture.
There is only a small chance of thunderstorms in the mountains
and high desert Tuesday afternoon. Meanwhile, the marine layer
thinks it`s June. Most of the coast, including the valleys in San
Diego County have yet to see the sun today. The wetting rain of
this morning put a literal damper on the sun`s ability to
evaporate the low clouds today. Drier air starts to move in
tonight into Tuesday, with just a small chance of afternoon
thunderstorms mostly in the mountains Tuesday. It appears too dry
Wednesday and Thursday for any thunderstorms. A modest increase in
moisture comes in Friday and Saturday for increasing chances of
thunderstorms those afternoons. A spread in model guidance for
next week paints a dry or a wet picture, with a growing consensus
for the dry solution. For next week our current forecast features
a small chance of afternoon/mountain thunderstorms Sunday, but not
Monday or Tuesday. Marine layer low cloudiness during nights and
mornings will continue a little more confined to coastal areas
Wednesday and beyond. That is because the high pressure aloft
strengthens in the next few days. That will also bring a warming
trend with more sunshine and drier air allowing temperatures
inland to climb back to around normal by Wednesday and above
normal Thursday. The increase in humidity this weekend will trim
those high temps a few degrees Friday through Sunday. A dry
southwest flow appears the most likely solution next week with
seasonable temperatures.

&&

.AVIATION...
270400Z...Coast/Valleys...BKN-OVC low clouds along the coast, Inland
Orange County, and into the valleys of San Diego County. Bases will
continue to range from 500-1000 ft MSL with tops to 2500 ft MSL.
Moderate confidence that cigs will prevail through midday Tuesday,
perhaps into the afternoon near the immediate coast. Otherwise, low
clouds are expected to clear inland areas 16-18Z Tue, with SCT-BKN
conditions lingering through the afternoon for coastal areas.

Mountains/Deserts...VFR conditions are likely to continue through
Tuesday afternoon.

.MARINE...
No hazardous marine weather is expected through Saturday.

&&

.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...Gregoria (Update)/MM (Previous Discussion)
AVIATION/MARINE...Brotherton


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