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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 212046

Area Forecast Discussion
National Weather Service San Diego CA
146 PM PDT Sat Sep 21 2019

Dry and warm weather will continue through Monday, with limited 
night and morning low clouds and fog near the coast. An upper
level low pressure system will move south across southern
California and Arizona Monday, and linger through Wednesday. This
system will combine with remnant moisture from the eastern Pacific
tropical systems to result in a few showers or thunderstorms,
mainly over the mountains, Tuesday and Wednesday. Temperatures
will be cooler during the first half of the week as well. A much 
larger upper level low pressure area will develop over the western
states late in the week. This will result in a deepening marine
layer, cooler temperatures, and an increasing chance of light
showers late in the week into next weekend. 



Expect dry conditions through Sunday night with only limited 
intrusion of the marine layer. Sunday will be the warmest day of
the entire upcoming week with highs reaching the 90s most inland
areas and around 100 in the lower deserts. An upper trough off the
Pacific northwest coast will dive southward with an upper low
setting up over southern California and Arizona Monday night. This
low is expected to stall or slowly meander over the region through
Wednesday before lifting eastward on Thursday. This system will
help to draw moisture from the two eastern Pacific tropical
systems Lorena and Mario northward. Models continue to indicate
the bulk of the moisture remaining east of the area. Have a slight
chance of showers/thunderstorms over the eastern portions of the
lower deserts beginning overnight Monday night. Moisture begins 
to wrap around the upper low into the area Tuesday/Wednesday with
PWs reaching 1.1-1.3 inches. Have kept 20-30 POPs in the forecast
for showers/thunderstorms mainly over the mountains and deserts.
It is possible a few showers could develop west of the mountains
as well. Temperatures will be slightly cooler across the area 
Monday through Wednesday given the proximity of the upper low. 

A much larger, deeper longwave trough is forecast to develop
across the West Coast late this week which will kick the upper low
off to the east on Thursday. While there are run to run model
differences with the developing trough, ensemble runs all agree on
the general pattern of deep troughing along the West Coast by
Friday. Expect a deepening marine layer with more extensive cloud
cover and scattered showers mainly over and west of the mountains,
and much cooler temperatures area wide by Friday/Saturday.
Generally went with NBM guidance for temps/pops late in the week
and coordinated forecast with LOX. 


212020Z...Coast...Mostly clear with unrestricted vis through this 
evening. Patchy low clouds may redevelop after 06Z Sunday with bases 
near 1000 ft MSL and will be confined to the immediate coast. Cigs 
could impact coastal TAF sites for a short period, likely between 12-
15Z Sunday, but confidence is low. 

Valleys/Mountains/Deserts...Clear with unrestricted vis through 


No hazardous marine weather expected through Wednesday.