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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 191603

Area Forecast Discussion
National Weather Service San Diego CA
902 AM PDT Sun Aug 19 2018

High pressure aloft over Southern California today 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. 



Except for some patchy marine clouds over coastal areas this 
morning, skies were clear at 9 AM PDT. The Miramar sounding had a 
stable profile and lower PW at around 1.27 inches. The marine 
inversion was based near 1900 FT MSL and was about 7.5C. Moderate 
easterly flow at around 15KTs was becoming reestablished in the 9-
12K Ft layer with weak NW flow below, to the top of the inversion. 
Surface pressure gradients were fairly weak but a few peak westerly 
wind gusts over 30 MPH were still reported in the passes. 

Looks like another warm and sunny day most areas today as marine 
clouds evaporate. There will be some cumulus clouds developing over 
the mts though. Based on dew points at mtn top, weakening high 
pressure aloft, and the forecast for easterly flow and increasing 
instability/moisture in the mid-levels, there is some chance for 
afternoon cumulus over the southern mountains to pop into isolated 
thunderstorms. Not enough evidence for this yet to quantify adding 
POPS to the forecast. This is based on current hires model guidance 
and lack of satellite confirmation of the moisture advection. So no 
forecast changes at this time. 

From previous discussion...

High pressure aloft currently resides over Southern California.
Global models are in agreement in showing this high transitioning
to the east to be placed over Arizona on Monday then all the way
to Texas during the middle to later part of the week as a trough
moves into NorCal extending southwest into the Pacific. 

There now only looks to be about a two day window where monsoonal
moisture returns to SoCal for afternoon mountain and desert 
thunderstorms. That looks to be Monday and Tuesday afternoon. By 
Wednesday, both GFS and EC now show the local area between a
trough lying to the northwest and ridging centered well off to 
the east with a dry west-southwest flow aloft, shoving the 
monsoonal moisture back to the east. This has been a change from 
model runs 24-48 hours ago, but there now has been some run-to- 
run consistency on this scenario. As such, the forecast was 
trended to reflect isolated-scattered mountain and desert 
thunderstorms Monday-Tuesday with dry weather from Wednesday
onward. Even on Monday-Tuesday, the moisture is not nearly as 
high as our recent monsoonal tstorm episodes, so only isolated- 
scattered tstorms are forecast with a low risk of flash flooding.

Temperatures will continue to run above average through the week
(trough too far away/heights remain rather high locally) and it 
will continue humid near the coast. Gusty west- southwest winds 
will occur especially mid week through passes and from mountain 
ridges to the desert slopes, but doesn't look to reach even 
advisory levels at this time.

191530Z...Coasts/Valleys...Variable low clouds with bases 1000-1800 
feet MSL will scatter out by 17Z. Low clouds will gather this 
evening after 06Z with similar bases and coverage. Scatter out by 16-
17Z Monday.

Mountains/Deserts...Cumulus clouds based at around 10000 feet MSL 
between 18-01Z today. Otherwise, mostly clear through Monday 


No hazardous marine weather is expected through Thursday.


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