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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
036 
FXUS66 KSGX 160319
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
819 PM PDT Wed Aug 15 2018

...Flash Flood Watch Issued for the San Bernardino, Riverside, 
and San Diego County Mountains Noon-8 PM Thursday...

.SYNOPSIS...
Monsoonal moisture will increase Thursday with an increasing risk
of flash-flood producing rainfall over the mountains. High 
pressure aloft will move westward and over Southern California for
Friday through Sunday, bringing drying and a warming trend. Minor
intrusions of monsoonal moisture may return next week as high 
pressure moves back to the east. Areas of night and morning low 
clouds and fog will continue for coastal areas.

&&

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

...Update - Flash-Flood Producing Thunderstorms Possible in the 
Mountains Thursday...

It was an active convective day across the SoCal mountains and
indications are that Thursday could be even more active. The
reason being that monsoonal moisture will increase further into
the area with a reservoir of higher PW off to the southeast that 
will be pulled further northwestward over Southern California. 
Wind flow will be light enough to allow for convergence over the 
mountains. Instability is shown to be higher in the mountains as
well on Thursday. Given these factors, there is high enough 
confidence that some flash flooding will occur over the mountains 
on Thursday to issue a Flash Flood Watch. However, low confidence 
on whether or not the heaviest rainfall will directly fall over 
recent burn scar locations, including the Cranston and Valley Fire
burn scars. It is a possibility though and, if it occurs, debris
flows would result. Localized 1-3" of rain is expected in the 
heaviest cells that develop Thursday afternoon. Tomorrow will 
again be a mountain focus, but the deserts could also see isolated
thunderstorms as well. 

...Previous Discussion (Issued at 127 PM PDT Wed Aug 15 2018)...

Coastal low clouds cleared later today and not completely from San
Diego. Thunderstorms are getting going, starting over the 
mountain crests. A few of these could get nasty before the day is
done, but will stay mostly confined to the mountains. A little
more moisture Thursday in easterly flow will combine with the 
high instability already present to bring more thunderstorms to 
the mountains and deserts, and even could help a couple 
thunderstorms drift west of the mountains into inland valleys. 
The tenacious nature of the coastal clouds and model guidance
suggest the marine layer to be largely unfazed by this latest 
monsoon push, and maintains night and morning coastal clouds 
getting into westernmost valleys each night. The monsoon flow 
backs off on Friday for fewer thunderstorms in the mountains and
deserts. High pressure aloft strengthens a bit this weekend while
the monsoon flow backs off. That will mean higher temperatures 
and a shallower marine layer, but no thunderstorms. Next week 
models show some increase in monsoonal moisture, but not very 
convincing to forecast much chance of thunderstorms until Tuesday
and Wednesday. The high breaks down slightly as well, so it 
doesn't appear as hot but probably more humid. Overall, minor ebbs
and flows in monsoon flow, slight variations in marine layer and 
coastal cloud extent, and minor fluctuations in temperatures. In 
other words, seasonal summer weather.

&&

.AVIATION... 
160315Z...Coasts/Valleys...Areas of BKN/OVC stratus will spread up 
to 20-25 miles inland overnight with bases mostly 1500-2000 ft MSL 
and tops to 2500 ft MSL. Most coastal airports that are still SKC-
SCT should have a cig by 06Z. Most surface vis will remain above 5 
miles overnight. Most areas will clear 16Z-19Z Thu, except for local 
BKN stratus lingering near the coast Thu afternoon.

Mountains/Deserts...Mostly SCT clouds at/above 10000 ft MSL will 
prevail through Thu morning. TSRA will develop again Thu afternoon 
and could be widespread over the mountains, with scattered TSRA 
likely in the deserts. With the TSRA, there will be strong 
up/downdrafts, surface gusts to 40 knots, hail and LLWS, with bases 
8000 ft MSL and tops to 50000 ft MSL.

&&

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

&&

.BEACHES...
Beach Hazard Statement will be in effect from 9 PM tonight through 
Friday for above average surf and strong rip currents. A Southern 
Hemisphere swell from 180 degree, with 4 ft heights and 17 second 
periods will arrive this evening and bring elevated surf and strong 
rip currents tonight through Friday. The surf will peak Friday at 3-
6 ft with sets to 8 ft on southwest facing beaches, then slowly 
subside Saturday and Sunday. 

&&

.SKYWARN... 
Skywarn activation may be needed Thursday afternoon.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch from Thursday afternoon through Thursday 
     evening for Riverside County Mountains-San Bernardino 
     County Mountains-San Diego County Mountains.

     Beach Hazards Statement through Friday evening for Orange County 
     Coastal Areas-San Diego County Coastal Areas.

PZ...NONE.
&&

$$

PUBLIC...Gregoria (Update)/MM (Prev Discussion)
AVIATION/MARINE...Maxwell