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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 202016

Area Forecast Discussion
National Weather Service San Diego CA
116 PM PDT Sat Apr 20 2019

A trough will move over Southern California this evening, followed 
by a low pressure center from the north late Sunday. These systems 
will ramp up the onshore flow and maintain a deep marine layer 
through Monday. Gusty west to northwest winds will prevail over the 
mountains and deserts, and a few light showers may develop over 
western San Diego County tonight, otherwise dry and cooler through 
Sunday. Warmer into next week as high pressure builds aloft and 
onshore winds weaken. By Wednesday, some inland valleys could be 
some ten to fifteen degrees above average. Cooling returns next 



At 1 PM PDT...Satellite imagery still showed considerable stratus 
covering most coastal areas, and extending through O.C and on into 
NW sections of the I.E. and up into and through the Cajon Pass. It 
was mostly clear over the higher mts/deserts, but a few wave clouds 
were forming at times in these areas. Onshore sfc pressure gradients 
are up considerably from yesterday, running around 11 MBS (KSAN to 
SW NV). Peak wind gusts of 50+ MPH have already been logged through 
the San Gorgonio Pass with several other sites in the mts/deserts 
reporting gusts between 35 and 45 MPH.

A shortwave trough is swinging across SoCal late today and will be 
followed-up by a digging low pressure system from the north on 
Sunday. This will keep a robust onshore flow across the region 
through Sunday, supporting gusty winds over the mts/deserts, and 
below average temperatures. The pattern supports coastal eddy 
development, which if it occurs, would prop up the marine layer 
further and possibly squeeze out some light precip west of the mts 
overnight into Sunday morning. Nothing significant is expected. A 
Wind Advisory remains in effect for our mts/deserts for local gusts 
45-55 MPH, along with some blowing dust and sand through the night. 

Operational models have the upper low centered over the Lower 
Colorado River Valley Mon morning, then over SE AZ on Tue morning. 
This sets up northern portions of our CWA for some northerly 
offshore flow Mon/Tue mornings which will scour out much of the 
marine layer over the I.E., but onshore flow will likely hold over 
coastal/valley areas each afternoon. Still, as heights/thickness 
rise and the onshore weakens, strong April sun will result in a 
warming trend that will push temps inland above average by some 10-
15F by Wed/Thu. The onshore will bring cooling to coastal areas 
again on Thursday, spreading farther inland next weekend as another 
trough pushes inland.

Late next week, there are significant operational model run 
differences between the ECMWF/GFS but both suggest some cooling. 
Once again, it is the 12Z ECMWF run that is more bullish, bringing a 
sharper trough onto the SoCal Coast Sunday with precip. It looks 
like an outlier at this point so will be discounted at this time.


202015Z...Coast/Valleys...SCT-BKN conditions with bases 2800-3800 ft 
MSL will continue until 03Z Sunday. After 03Z, BKN-OVC low clouds 
with bases 2000-3000 FT MSL will increase in coverage and spread 
inland. Bases expected to lower to 1200-2000 ft after 07Z Sunday, 
before lifting back to 2000-3000 ft MSL after 12Z. FEW-SCT 
conditions expected to return to TAF sites after 18Z Sunday.  

Mountains...Mostly clear with unrestricted vis through this evening. 
West winds 20-30 kt gusting to 40-45 kt over the mountains. MOD 
up/down drafts possible along desert mountain slopes. 

Deserts...Mostly clear skies. NW winds 15-25kt gusting to 30-35 kt 
at KPSP and KTRM after 21Z. Areas of reduced vis 3-5 miles in 
blowing dust possible. 


A Small Craft Advisory will be in effect over the Outer Waters from 
10 PM tonight to 9 AM Sunday for NW winds 10-15 kt gusting 20-25 kt 
and combined seas of 7-8 FT. Winds will weaken Sunday afternoon. 
Otherwise, no hazardous marine weather is expected for Monday 
through Thursday. 


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


CA...Wind Advisory from 2 PM this afternoon to 6 AM PDT Sunday for 
     Apple and Lucerne Valleys-Coachella Valley-Riverside County 
     Mountains-San Bernardino County Mountains-San Diego County 
     Deserts-San Diego County Mountains-San Gorgonio Pass Near 

PZ...Small Craft Advisory from 10 PM this evening to 9 AM PDT Sunday 
     for Waters from San Mateo point to the Mexican Border 
     Extending 30 to 60 nm out including San Clemente Island.