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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
000
FXUS66 KSGX 142022
AFDSGX

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

.SYNOPSIS...
High pressure will slowly loose 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. 

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

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. 

&&

.AVIATION...
142000Z...Coast...High confidence in VFR conditions prevailing at 
coastal terminals through midnight tonight. Patchy low stratus will 
redevelop along the coast after 07Z, but there is moderate 
confidence that skies will remain prevailing SCT at TAF sites 
through approximately 10-11Z. CIGS becoming more likely after 11Z at 
all coastal terminals, with similar bases of 100-300 FT MSL. That 
said, there is uncertainty in how widespread the CIGS will be, with 
potential for certain locations on the coast to remain prevailing 
SCT overnight, especially in San Diego County. In areas where low 
CIGS develop, expect significant VIS restrictions of 1-4 SM, with 
periods of VIS 1/4 SM in FG possible. Terminals are expected to 
clear near 16Z. 

Valleys/Mountains/Deserts...VFR conditions with CLR skies prevailing 
through 09Z. SCT-BKN clouds with variable bases 5000-15000 FT MSL 
after 09Z in the mountains and deserts. 

&&

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

&&

.FIRE WEATHER...
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 
increases. 

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

PUBLIC/FIRE WEATHER...10
AVIATION/MARINE...Rodriguez