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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 201025

Area Forecast Discussion
National Weather Service San Diego CA
325 AM PDT Mon Aug 20 2018

Southwestern California will be on the western fringe of a more
substantial influx of monsoonal moisture into southern Arizona on
Tuesday. This will bring a slight chance of thunderstorms this 
afternoon and early evening and again Tuesday afternoon, mainly 
along the desert slopes of the mountains of Riverside and San 
Diego Counties. Onshore lower level flow and drier southwest flow 
aloft will then prevail for much of the week. Areas of night and
morning coastal low clouds and fog will spread into the western
valleys at times. There will be gusty southwest to west winds in
the afternoon and evening in the mountains and deserts for the
latter half of the week.



.SHORT TERM (Today through Wednesday)...
Early this morning, high pressure aloft is centered over Arizona 
with a low pressure system moving into the Pacific Northwest. The
high pressure will move eastward, moving over Texas on Tuesday as
the trough of low pressure over the Pacific Northwest expands
towards the southwest. The NAM is alone in taking more substantial
moisture farther towards the west than the other models across
northern Mexico into the southwestern states. The current forecast
favors the global models in taking the more substantial moisture
into Arizona on Tuesday with southwestern California on the
western edge of that moisture. A slight chance of thunderstorms is
retained for this afternoon and early evening and again Tuesday
afternoon, mainly along the desert slopes of the mountains of
Riverside and San Diego Counties. 

The marine layer and weak onshore flow will bring some slow
deepening of the marine layer into Tuesday with areas of night and
morning coastal low clouds and fog spreading a little farther into
the western valleys for late tonight in to Wednesday morning. This
will also spread slow cooling inland through Tuesday.

For late Tuesday and Wednesday, drier southwest flow aloft will
develop bringing a more substantial decrease in monsoonal 
moisture. On Wednesday, high pressure aloft over Texas will begin
to expand westward with the ridge axis to the south of Southern 
California. This will bring some warming to the valleys as the 
marine layer becomes a little shallower.


.LONG TERM (Thursday through Sunday)...
Subtropical high pressure will remain centered to the south of
Southern California with a trough of low pressure near the Pacific
Northwest coast. This will maintain onshore lower level flow
across Southern California with dry southwest to west flow aloft.
There will be minor day to day temperature changes with high
temperatures remaining a few degrees above average.


201030Z...Coasts/Valleys...Stratus will continue through 16-18Z 
within 25-30 miles of the coast. Bases will be 1200-1500 ft MSL with 
tops to 1800 ft MSL. Local vis 2-4 miles will occur in the valleys 
through 16Z. After 18Z, mostly clear skies will prevail until 
stratus develops again tonight between 03Z and 06Z and spreads up to 
25 miles inland by 12Z Tue with similar bases compared to this 

Mountains/Deserts...There is a slight chance of thunderstorms this
afternoon over mainly the mountains, with CB bases 8000-9000 ft MSL 
and tops to 40000 ft MSL, along with strong up/downdrafts and local 
surface gusts to 30 kt. Otherwise, mostly SCT clouds above 10000 ft 
MSL will prevail with mostly unrestricted vis.


No hazardous marine weather is expected through Friday.


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