FXUS66 KSGX 242058
Area Forecast Discussion
National Weather Service San Diego CA
158 PM PDT Sat Jul 24 2021
.SYNOPSIS...High level cloudiness will continue to thicken from
the east associated with an upper level low positioned over
Arizona. This feature will move across northern Baja California on
Sunday, then off the coast Sunday night through Monday. While
moisture will be on the increase, thick cloud cover will limit
instability. Thus, only isolated showers, and perhaps a
thunderstorm, are forecast through Sunday afternoon. A slightly
better chance of showers, and perhaps a thunderstorm, is expected
Sunday evening through Monday afternoon along with a low risk of
flash flooding in the mountains and deserts. The cloudiness will
keep it cooler through Monday, before warming occurs through mid
week. Isolated thunderstorms will be possible in the mountains
each day next week as monsoonal flow continues, though moisture
will be less before increasing again late next week.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
* Thickening clouds with only isolated showers through Sunday
* Showers become more likely Sunday night-Monday
* Slight chance of thunderstorms with a low flash flood risk
Thickening mid/high level cloudiness continues this afternoon
ahead of a closed low positioned over Arizona. This feature will
move slowly west-southwest to over northern Baja California by
Sunday afternoon then off the coast Monday.
Moisture is high around this upper low and will be on the
increase through Sunday night with PW increasing into the 1.5-1.75
inch range, highest over the deserts. However, despite this
moisture increase, the thick cloud deck will limit instability
significantly with MUCAPE barely into the 100 J/KG range over the
mountains and deserts Sunday-Monday. Also, the low to mid level
wind fields will increase out of the east through Monday morning,
before becoming southwest. This will lead to less convergence
occurring over the mountains with cells on the move as well,
thus limiting rainfall amounts.
The best chance of showers, and possibly a thunderstorm, will be
Sunday evening through Monday when the trough axis moves across
Southern California, providing the best convergence and lift.
Hi-resolution models of the WRF, HRRR, and HREF are not bullish on
rainfall. This downward trend in rainfall amounts has continued
since last night. So now we are forecasting 0.10-0.25" over the
mountains with isolated amounts up to 1.00". Deserts 0.05-0.25".
Less than 0.10" west of the mountains.
Despite the limiting factors for more robust convection and
rainfall amounts, atmospheric moisture will be high, so there is
a low risk of flash flooding, particularly from Sunday evening
through Monday afternoon.
The trough will lift northwest away from the region on Tuesday.
However, monsoonal flow never really shuts off with southeast flow
continuing. Moisture will decrease but PW will remain at or above
1 inch, which is sufficient for a few showers or thunderstorms to
develop each day next week over the mountains. The risk of showers
and thunderstorms could expand into the deserts by late next week
as monsoonal moisture increases again.
Highs through Monday will be cooler than normal, especially over
the deserts before increasing through the middle part of next
week. However, lows will continue rather warm and on the muggy
side given the monsoonal moisture present.
241615Z...Coast/Valleys...Areas of low clouds with bases 1000-1500
ft MSL are currently clearing at the inland edges. Low clouds will
likely clear to the coast by 18z, although some patches will persist
at the beaches into the afternoon. Otherwise, SCT-BKN at or abv
15000 ft MSL through tonight. Patchy low cloud cover will move
inland after 03z Sun with bases 900-1300 FT MSL.
Mountains/Deserts...SCT-BKN high clouds at or abv 15000 ft,
increasing in coverage and lowering to 10000 FT early Sun. Isolated
TSRA will be possible 21z to 01z Sun, mainly across the mtns.
Lightning strikes and gusty and erratic winds will be possible vcnty
No hazardous marine conditions expected through Wednesday.
There is a low flash flood risk Sunday evening through Monday
afternoon when showers will be on the increase, along with a
slight chance of thunderstorms. However, a solid cloud deck will
limit instability and rather fast low/mid level wind flow will
keep showers and any thunderstorms that develop on the move. This
will limit rainfall amounts. Only a few showers, and an outside
risk of a thunderstorm, is expected tonight-Sunday afternoon.
Rainfall amounts have been lowered. Total rainfall through Monday
is forecast to be 0.10-0.25" with isolated amounts to 1.00" in
the mountains, 0.05-0.25" across the deserts, and less than 0.10"
west of the mountains.
Moisture will decrease by Tuesday, but enough moisture is expected
to be present each day next week to result in the potential of a
few showers or thunderstorms over the mountains each afternoon and
possibly including the deserts by late next week as moisture
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.