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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
982 
FXUS66 KSGX 192053
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
150 PM PDT Sun Aug 19 2018

.SYNOPSIS...
High pressure aloft over Southern California will shift southeast to 
the Rio Grande Valley by Tuesday. This will allow some monsoonal 
moisture to return, possibly sparking more thunderstorm activity 
over the mountains and deserts.  At the same time, a trough will 
develop over the Pacific Northwest and slowly broaden over the West 
Coast through the week, bringing a drying trend, along with slightly 
cooler weather later in the week. The night and morning low clouds 
along the coast are expected to decrease a bit into Wednesday, then 
become more widespread during the latter half of the week. 

&&

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

Some patchy marine clouds near the Coast and cumulus over the 
mountain crests were being detected in satellite imagery early this 
afternoon, otherwise skies were clear. Winds were generally light 
with temps close to values observed yesterday. 

High pressure aloft over SoCal today is headed SE and will be 
centered over West TX by Tue. This will briefly throw parts of 
southeastern CA into a monsoonal flow. Based on blended TPW 
imagery...Moisture is already beginning to increase over eastern 
portions of the CWA and will likley continue into Tue morning as an 
easterly wave rides north into the Lower Colorado River Valley. The 
latest WRFEMS shows some convective showers firing late tomorrow 
over the lower deserts, with much more activity creeping north along 
the mountains and desert areas on Tuesday. We are already forecast 
to be on the edge of this activity, so any shift to the east would 
leave the entire area dry. For now, low POPS are in place for mts/ 
lower deserts.

Once the easterly wave passes, there is a shift in the upper-level 
flow to predominantly westerly through the week. This becomes even 
more pronounced early next week as both the GFS/ECMWF models show a 
marked shift in the pattern over the Western Hemisphere. If the 
models are correct...This shift will allow the strong subtropical 
high to set up over the Lower Mississippi Valley, while a longwave 
trough develops over the West to end August. Besides driving any 
Monsoonal activity completely out of the region, it would bring 
widespread and substantial cooling. Lets see if a few more model 
runs keep this trend.

In the meantime...Don't look for any meaningful changes in the 
weather across SoCal this week. Humidity will likely remain elevated 
with onshore flow and a warmer than average sea surface. Daytime 
temps will be within a few degrees of average, while overnight lows 
continue 3 to 7 degrees above average. 

Despite the low POPS, PW values of near two inches will be available 
if thunderstorms develop on our desert slopes on Tuesday. Even on 
Monday, the threat for any thunderstorms looks to be in the lower 
deserts. The passage of a wave on Tuesday increases forcing in this 
potentially rich CAPE environment. The only wild card that could 
enhance updrafts even more would be enough afternoon sun (which 
could be in short supply). The main threat area for severe 
convection and Flash Flooding at this time appears to be the Borrego 
Desert areas, the Coachella Valley, and adjacent mountains. 

&&

.AVIATION... 
192030Z...Coasts/Valleys...Coastal low clouds will gather tonight 
after 06Z with bases around 1200 feet MSL and moving into western 
valleys overnight. Scatter out by 16-17Z Monday.

Mountains/Deserts...Cumulus clouds based at around 10000 feet MSL 
through 01Z today. Cumulus clouds over mountains and moving into 
deserts after 19Z Monday, growing into isolated thunderstorms. Bases 
around 9000 feet growing to 35000 feet.

&&

.MARINE...
No hazardous marine weather is expected through Friday.

&&


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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

PUBLIC...10
AVIATION/MARINE...MM