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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 150522 AAA

Area Forecast Discussion
National Weather Service San Diego CA
1020 PM PDT Sat Sep 14 2019

High pressure will slowly lose it's grip on the region this weekend, 
resulting in a cooling trend Sunday into Monday. Some patchy, dense 
fog is possible again tonight into Sunday morning along the coast, 
spreading farther inland by Monday morning as the marine layer 
deepens ahead of a trough moving inland across the West. The trough 
will bring strong and gusty west winds over the mountains and 
deserts, and noticeable cooling. 



Evening Update:

No significant changes were necessary this evening. The 00Z sounding 
from Miramar shows a very weak marine inversion near the surface. 
There will be another threat of patchy dense fog late tonight and 
Sunday morning near the coast. An incoming trough will begin a slow 
cooldown Sunday, continuing through midweek. 

Previous Discussion, issued at 120 PM PDT: 

Satellite imagery at 1 PM PDT still showed some patchy low clouds/ 
fog persisting along the San Diego and O.C. County coasts, otherwise 
it was mostly clear over the CWA. The only near-term forecast 
concern is for further development of this area of fog/low clouds 
along the coast. Confidence is low due to lack of coverage at this 
time, but the potential is there, especially later tonight into 
Sunday morning. Surface pressure gradients were mixed and quite weak 
at midday keeping winds light.

High pressure over the SW today will shift east as a trough swings 
east over the Great Basin on Monday. This shift will mean cooler 
weather as the marine layer builds inland and gusty westerly winds 
develop over the mts/deserts. A second, deeper trough will follow 
over the West later in the week, reinforcing the cooling and gusty 
onshore flow. It will also keep the weather dry.

The main weather concern today continues to be elevated fire weather 
conditions due to the heat and low RH (See Fire Weather Discussion 
below). This will ease a bit on Sunday as the atmosphere begins to 
cool and the sea breeze carries more moisture inland. More 
noticeable cooling, and building of the marine layer occur on 
Monday. The onshore flow will increase late Monday/Monday night and 
may reach Advisory levels over the mts/deserts. 

There is high confidence in the medium term as most guidance 
indicates troughing over the West. This should keep the weather dry, 
with a healthy onshore flow and temperatures near-to-slightly below 
average. As of the latest 12Z model solutions...There is better 
agreement on a troughing pattern persisting through next weekend. 
Some warming is likley due to modest height/thickness recovery, but 
this would still leave daytime high temps (at present calculations) 
slightly below average. 



150400Z...Coast...Mostly clear skies with unrestricted vis will 
prevail through at least 08Z Sun. Patchy stratus/fog will likely 
develop near the immediate coast, though confidence is low for any 
cigs at SAN/CRQ/SNA. If cigs occur, they will most likely be between 
10Z and 15Z Sun. Cloud bases will be 300-500 ft MSL with tops to 600 
ft MSL. Areas of vis 1-3 miles with local vis below 1 mile could 
occur 10Z-16Z Sun. VFR conditions will prevail after 16Z Sun with 
high clouds above 20000 ft MSL increasing. Stratus/fog will likely 
be more extensive Sun night.

Valleys/Mountains/Deserts...Mostly clear skies will prevail through 
Sun evening with some SCT-BKN clouds possible in the 
mountains/deserts Sun and Sun evening with bases mostly 10000 ft MSL 
or higher. Vis will likely remain unrestricted through Sun evening.


Patchy late night and early morning low clouds and fog will likely 
reform late tonight through Sunday morning, with periods of 
visibility below 1 mile, locally down to 1/4 mile. 


Hot again today with many RH values near or below 15% more than 25 
miles inland at midday. Westerly winds with the afternoon sea breeze 
will gust 15 to locally 25 MPH at times, keeping an elevated fire 
weather threat into the evening. A cooling trend begins in Sunday, 
and is amplified into Monday as onshore flow strengthens and RH 

Strong west winds will develop over the mts/deserts by late Monday 
through Monday night at 25-30 MPH with peak gusts 40-50 MPH on the 
deserts slopes and near the passes. These winds will be accompanied 
by higher RH initially, but it will fall to around 15% or lower by 
Tuesday morning in some areas, followed by decreasing wind. So 
several hours of near critical fire weather conditions could develop 
Tuesday morning on the ridges and desert slopes before the winds 
drop off.  


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