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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 210415

Area Forecast Discussion
National Weather Service San Diego CA
900 PM PDT Fri Sep 20 2019

Fair and warmer weather is expected over the weekend, with limited 
night and morning low clouds and fog near the coast. A low pressure 
system will bring partly cloudy skies and slight cooling Monday and 
could bring enough moisture from the south to bring a few showers or 
thunderstorms Tuesday through Thursday. A broad trough of low 
pressure moving into the West Coast will bring additional chances of 
precipitation late next week.



Skies were clear this evening with mostly light winds. High 
temperatures recovered to just slightly below seasonal normals 
overall, and the 00Z NKX sounding had generally 1-3 deg C warming 
below 800 MB. That warming trend will continue Saturday as the upper 
level ridge moves over, along with further warming aloft. Stratus 
will be very limited tonight along the coast as a combination of the 
subsidence plus some northwest flow along the immediate coast this 
evening makes the marine layer more shallow, and stratus off the 
northern Baja coast appears to be drifting a little bit south.

The forecast for next week is a bit low confidence right now as GFS, 
ECWMF and Canadian/GEM ensemble solutions show substantial 
variability with the position of the closed low after it moves 
southeast over the far western US Monday, and the remnants of 
Hurricane Lorena move north over or near Baja California. Most 
likely, the main moisture will stay just to our east, but 1/3 to 1/2 
of the solutions are far enough west that some showers or possibly 
thunderstorms could form over our forecast area of extreme 
southwestern California. Some moisture from Lorena will likely 
interact with the low in some way, and instability of the cold core 
low certainly could enhance any convection. Chances of any heavy 
rain is fairly low right now, but I did add slight chances of 
showers or thunderstorms to parts of the area Tue-Thu, mainly in the 
afternoons/evenings. The mountains would be the most favorable 
location for convection, but lower elevations are not exempt. If the 
upper low ends up to our east, some offshore flow could develop, 
though models generally do not have much wind as there is no cold 
air mass over the Great Basin to advect in and help enhance any 
mountain waves. Temperatures will depend on the upper low position 
but will likely be near or just slightly below normal. Regardless, 
the marine layer stratus should be limited, either if there is an 
offshore flow solution or upper low/convective solution. Ensembles 
are mostly showing a deep trough over the West Coast the following 
weekend (28-29 Sep) but with differences in timing by a day or two. 
This would likely bring cooler weather, some showers and locally 
strong westerly winds starting as early as Friday.


210200Z...Coast...Low confidence forecast tonight. Low clouds are 
expected to develop first in San Diego County, starting as early as 
07Z, but most likely after 09Z. CIGS should arrive quite a bit later 
at KSNA, and there's a chance they may not even appear. Moderate-
high confidence in at least some periods of prevailing BKN-OVC at 
KSAN/KCRQ 10Z-16Z. Bases will be lower tonight, around 800-1200 FT 
MSL, and locally below at KCRQ. CIGS will mostly be confined to the 
immediate coast, with VIS restrictions of 2-4 SM likely in areas a 
few hundred feet MSL 10-16Z. High confidence in terminals clearing 
by 16Z Saturday. 

Valleys/Mountains/Deserts...VFR conditions will prevail through 
Sunday, with CLR skies and unrestricted VIS. 


No hazardous marine weather expected through Wednesday.


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