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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 150938

Area Forecast Discussion
National Weather Service San Diego CA
230 AM PDT Sun Sep 15 2019

After another exceptionally warm day today, Monday and Tuesday will 
turn cooler and less humid as a trough of low pressure moves across 
the region. This cooler weather pattern will persist through the end 
of the week, with temperatures a little below average Thursday and 
Friday. Gusty west winds will affect the mountains and deserts each 



Skies were generally clear this morning except for a few small 
pockets of low clouds and dense fog near the coast. Weak ridging 
aloft has smashed the marine layer down to about 200 feet, and this 
near surface-based marine layer means that it will be another very 
warm day in the coastal and valley zones. It won't be quite as hot 
as yesterday when highs reached 104 at Riverside, 100 at Yorba 
Linda, 97 at Anaheim, and 94 at Montgomery Field, but even with the 
5 degrees of expected cooling it will still be much warmer than 

The upper ridge drifts east tonight, and a trough moving in from the 
northwest will bring a cooler, less humid airmass into the area 
Monday and Tuesday. Monday will be a noticeably cooler day, and 
further cooling on Tuesday will bring high temperatures right in 
line with the normals for this time of year. A coastal eddy will 
spin up tonight and that will deepen the marine layer for more 
coastal clouds tomorrow morning. 

A reinforcing shot of shortwave energy over the Gulf of Alaska will 
dig a second deep trough down the West Coast Wednesday through 
Saturday for cooler than average weather, strong onshore flow and 
gusty mountain and desert winds. 


150840Z...Coast...Patchy fog developing near the immediate coast 
this morning, though confidence is low for any cigs/vis restrictions 
at SAN/CRQ/SNA. If clouds/fog do reach the airports, it will most 
likely be between 11Z and 15Z Sun. Cloud bases will be 300-500 ft 
MSL with of vis 1-3 SM with local vis below 1 SM. VFR after 16Z with 
increasing clouds AOA 15000 ft MSL. Stratus/fog will likely be more 
extensive late tonight into Monday morning with bases near 500-800 
ft MSL.

Valleys/Mountains/Deserts...Increasing clouds AOA 15000 ft MSL and 
unrestricted vis through Monday morning.


Patchy fog developing this morning with periods of visibility below 
1 mile, locally down to 1/4 mile. Conditions will improve after 8 
AM. Fog is possible again tonight into Monday morning, but 
visibility should mostly be above 1 mile. 


Today will be another very warm, dry day with RH values near or 
below 15% in inland valleys and in the deserts. A late afternoon sea 
breeze will bring gusts of 15 to 20 MPH at times through early 

On Monday the onshore flow strengthens and RH increases as an upper 
trough moves in from the north. A coastal eddy spins up and the RH's 
in the coastal and western valley areas will slowly increase.

Strong southwest winds will develop over the mts/deserts Monday 
afternoon through Monday night with gusts of 30-50 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. Critical fire weather conditions may 
develop for a few hours on the ridges and desert slopes before the 
winds drop off.  


Skywarn activation will not be needed this week.