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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 231718

Area Forecast Discussion
National Weather Service San Diego CA
1015 AM PDT Mon Sep 23 2019

A trough of low pressure moving south across the Great Basin today 
will stall over northern Baja Tuesday and Wednesday. Tuesday will be 
warm and dry with gusty offshore winds in the mountains and valleys. 
Rain is possible Wednesday and Thursday. A deep trough will develop 
over the west this weekend bringing cooler weather and a deeper 
marine layer.   



The marine layer deepened was a few hundred feet overnight and the 
low clouds near the coast were hanging on longer today than 
yesterday. Elsewhere the skies were clear and it is shaping up 
to be a sunny warm day.  

A 576 dm trough over central California near Bishop this morning 
will dig south down the California interior today, then evolve into 
a closed low that will stall over northern Baja Tuesday and 
Wednesday. This potent trough is producing strong storms with heavy 
rain over Arizona today as it interacts with remnant moisture from 
Lorena. The deep layer moisture will be too far east to impact our 
area today. 

Today's weak onshore flow ahead of the trough will shift offshore on 
Tuesday as the aforementioned upper low moves south and stalls over 
northern Baja. The models have been consistent in forecasting a 
period of gusty east to northeast Santa Ana winds Tuesday afternoon. 
The strongest winds will surface in the San Bernardino Mountains and 
foothills, the IE and the Santa Ana Mountains. It will be a hot dry 
day in the Inland Empire with downslope adiabatic warming pushing 
high temperatures well into the 90s and close to 100. 

The ensemble runs showing the evolution of the closed upper low over 
northern Baja Tuesday and Wednesday are becoming better clustered. 
Tropical moisture starts advecting into SoCal Tuesday, eventually 
reaching near saturation Wednesday and Thursday when the GEFS and 
EPS ensembles and the operational GFS20 and ECMWF show rain over the 
area. Rain is most likely in the mountains and deserts, but recent 
model trends show stronger easterly flow pushing rain into the 
coastal and valley zones. This is reflected in the current forecast. 
Forecast confidence is low to moderate at this time due to the 
potential track and position changes of closed upper lows cut off 
from the main steering flow.  

On Friday the upper low fills and lifts northeast into Arizona ahead 
of an upstream shortwave that will deepen into a longwave trough 
over the west this weekend. This trough will bring significantly 
cooler weather Sunday and Monday along with gusty westerly winds to 
the mountains and deserts.   


231520Z...Coast/Valleys...Stratus with bases 800-1500 ft MSL will 
continue along the coast and about 15 mi inland this morning. VIS 
locally reduced to 2-4 SM where clouds near higher terrain in the 
valleys. Clouds and vis restrictions will clear 16-18Z, then spread 
back inland after 02Z with bases 800-1500 ft MSL. Otherwise, a few 
high clouds and unrestricted vis through Tuesday morning.

Mountains/Deserts...A few high clouds and unrestricted vis through 
Tuesday morning. 

No hazardous marine weather is expected through Friday.


Greatest fire weather impacts will be Tuesday afternoon in the San 
Bernardino Mountains, the Inland Empire and the Santa Ana Mountains. 
That's when the offshore Santa Ana winds reach their peak and the RH 
is the lowest, with northeast winds from 15-30 mph and minimum RH 
from 7-12%. High temperatures will be in the 90s to near 100. 

The winds will decrease Tuesday evening and the RH will increase 
Wednesday and Thursday as wrap around moisture from a low pressure 
system over northern Baja moves into SoCal from the southeast. 

Onshore flow returns Friday, becoming stronger Saturday and Sunday 
with a trough of low pressure bringing cooler weather, a deeper 
marine and gusty westerly winds to the mountains and deserts.


Skywarn activation is not requested.