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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
000
FXUS66 KSGX 191808
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 AM PDT Mon Aug 19 2019

.SYNOPSIS...
Building high pressure over the Southwest will bring warmer days 
through Wednesday. An Excessive Heat Warning is in effect for the 
deserts on Wednesday. A cooler marine airmass will keep temperatures 
in the coastal and western valley zones near their seasonal 
averages. The weather will turn a little cooler Thursday and Friday, 
and a surge of monsoon moisture is possible this weekend. 

&&

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

The forecast challenges today are determining just how hot it 
will get on Wednesday, and the chances of monsoon moisture reaching 
Southern California this weekend or early next week. 

Regarding the heat, broad upper level ridging over Texas this 
morning will expand westward Tuesday and Wednesday. This looks like 
it will be a brief heat episode, and won't be as hot as it could be 
because the ridge amplification over the Southwest will be muted by 
an anomalous upper level trough moving into the Pacific Northwest. 
Wednesday will be the hottest day, and an Excessive Heat Warning 
will be in effect for the deserts. Record highs are within reach at 
a few sites in the lower deserts including Palm Springs (117), 
Thermal (118), Indio (115) and Borrego (116). High temperatures in 
upper deserts east of Apple Valley will range from 103 to 109 on 
Wednesday. 

Temperatures west of the mountains will be kept in check by the 
cooler marine air, especially areas within 10 miles of the coast. By 
Wednesday morning the marine layer will mostly be confined to areas 
near the coast, and that will allow the hotter airmass to spill into 
the valleys. High temperatures in the Inland Empire will likely 
eclipse 100 degrees in a few areas on Wednesday. In San Diego 
County, the eastern valleys and foothills will see highs from 95-
100. 

On Thursday and Friday the upper ridge gets flattened by the trough 
of low pressure moving in from the northwest. Thursday will not be 
quite as hot as Wednesday, and further cooling by a degree or two 
will continue on Friday. The marine layer will slowly deepen with 
low clouds returning to the far western valleys on Friday morning.

*From Previous Discussion*

Less certain are the details of the forecast for the weekend into
early next week. The current forecast shows a modest warming trend
west of the mountains with the marine layer becoming shallower.
For the mountains and deserts, the current forecast shows a slow 
return of monsoonal moisture with afternoon clouds over the 
mountains and a slight chance of thunderstorms near the mountains 
on Monday. High temperatures wouldn't change much from those on
Friday, remaining around average. Among the range of possibilities is
a greater return of moisture some time during this period which 
could both lower high temperatures in the lower deserts and 
increase the potential for afternoon thunderstorms near the 
mountains.

&&

.AVIATION...
191530Z...Coast/Valleys...Low clouds with bases 1000-1800 ft MSL and 
tops to 2300 ft MSL beginning to scatter out of the valleys at this 
hour, and should be fully scattered out of all areas by 17Z. Reduced 
vis of 4-6 miles in portions of the valleys through 17Z. Low clouds 
redeveloping and moving ashore after 05Z Tuesday, moving 15-20 miles 
inland overnight.

Mountains/Deserts...Mostly clear with unrestricted vis through 
Tuesday. 

&&

.MARINE... 
No hazardous marine conditions are expected through Friday. 

&&

.SKYWARN... 
Skywarn activation will not be needed today.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning from 11 AM to 8 PM PDT Wednesday for 
     Apple and Lucerne Valleys-Coachella Valley-San Diego County 
     Deserts-San Gorgonio Pass Near Banning.

PZ...NONE.

&&

$$

PUBLIC...Moede/17
AVIATION/MARINE...Connolly