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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 180409 AAA

Area Forecast Discussion
National Weather Service San Diego CA
908 PM PDT Wed Jul 17 2019

Weak low pressure aloft over the West Coast will bring a cooling 
trend to most areas through Friday, along with greater coverage of 
night and morning low clouds west of the mountains. Some areas along 
the coast will have better clearing Thursday and Friday afternoons. 
A strong broad high pressure ridge will develop over the western 
United States early next week and bring hot weather inland. 
Monsoonal flow will also develop and could bring some afternoon and 
early evening thunderstorms beginning Sunday and increasing through 
Wednesday, mainly in the mountains and deserts. We may also see 
increasing humidity with fewer low clouds near the coast.




Forecast track remains relatively on point through the period. Only 
major changes are evening low clouds. Trends indicating delayed 
timing of Inland Empire low clouds this evening and Friday evening, 
so backed off cloud cover for those evenings until around 12Z each 
night. Otherwise, no other changes.

Previous Discussion: 

Areas of low clouds continued along the coast early this afternoon 
with sunny skies inland. Temperatures averaged 3 to 5 degrees lower 
versus 24 hours ago, though more in some valley locations due to the 
higher marine level inversion and in the lower deserts due to some 
increase in humidity from the southeast, albeit only at low 
elevations, below about 1000 feet. Overall, a weak trough will 
continue to develop over the West Coast through Friday and bring 
more cooling to the area along with a deeper marine layer for more 
nocturnal stratus in the valleys. The models, including our local 
WRF, are indicating little or no eddy Thursday and Friday, and that 
combined with cooling above the inversion could result in better 
clearing at the coast Thursday and Friday afternoons.

There will be a large-scale pattern change beginning this weekend as 
the upper high now over the eastern US retrogrades west to the 
central Rockies by Sunday/Monday and expanding Tuesday/Wednesday. 
The GFS ensemble runs are quite consistent with that as well as the 
southeast flow aloft which will develop and bring much larger 
amounts of mid and upper level moisture. The NBM 12-hour 
thunderstorm probabilities increase each day Sunday through 
Wednesday mainly over the mountains and deserts. Current forecast 
has thunderstorm chances beginning Sunday, but the last two 
(06Z/12Z) deterministic GFS runs show a wave from the southeast 
moving up towards southern California Sunday along with a sudden 
increase in moisture that date, though the ECMWF does not have this. 
In the afternoon forecast we are adding slight chances of 
thunderstorms in the mountains and deserts Sunday, and if the wave 
is associated with CAPE aloft, which is often the case, there could 
potentially be elevated convection west of the mountains. 
Regardless, thunderstorm chances should increase further Mon-Wed.


180300Z...Coast/Valleys...Low clouds with bases 800-1300 ft MSL 
currently impacting coastal TAF sites will move 25-35 miles inland 
overnight. Moderate to high confidence low clouds will impact KONT 
and KSBD from 11-16Z. Reduced vis of 3-5 miles on higher coastal 
terrain and coastal mesas through 17Z. Scatter out expected after 
16Z in the valleys and along the coast 17-19Z. Most of the immediate 
coast should clear out by early afternoon. 

Mountains/Deserts...Clear with unrestricted visibility through 


No hazardous marine weather expected through Monday.


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





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