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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
269 
FXUS66 KSGX 181547
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
847 AM PDT Sat Aug 18 2018

.SYNOPSIS...
High pressure will strengthen over the area today. There is still 
enough moisture present for the possibility of a few showers to 
develop over the mountains this afternoon. Moisture will decrease 
further Sunday through Monday before monsoonal moisture returns 
during the middle part of next week, rekindling the chance for 
mountain and desert thunderstorms. 

&&

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

Satellite imagery this morning had a continuous marine stratus layer 
covering all coastal areas. The layer extended up to 20 miles inland 
across San Diego County. Elsewhere across SoCal, the skies were 
clear. The 00Z Miramar sounding showed a sharp shift in winds below 
8K FT from east to NW. Some drying accompanied the shift, but PW was 
still 1.75 inch. There was a 5C inversion based near 2K FT MSL. The 
sounding had stabilized considerably from yesterday. Except for the 
desert passes where westerly winds were gusting to around 25 MPH, 
winds were light.

Given the high PW values still present, a small chance of showers 
seems warranted, but they will have to overcome continued drying and 
stabilizing of the environment. No forecast changes.

From previous discussion...

Water vapor imagery shows a shortwave departing through the 
intermountain west with upper level ridging building in overhead 
SoCal. 

Hi-res models are dry today with the WRF showing no cell 
development anywhere today. HRRR shows a small light cells over
the Riverside mountains. Moisture levels are still sufficient 
under the upper ridge. In fact, satellite-derived Precipitable 
Water shows values ranging from 1.6 inches near the border to 
1.2-1.4 inches across the mountains, highest south. Some 
instability develops along the mountain crests this afternoon 
with MUCAPE values of 500+ J/KG. These parameters suggest some 
cumulus will develop over the mountains this afternoon with even 
the potential of isolated showers. However, the suppressing ridge 
should inhibit thunderstorm development. No flash flooding is 
expected.

Tranquil days are expected Monday-Tuesday. Kept the forecast dry,
but there still could be just enough moisture present for cumulus
if not a few showers developing in the mountains. We can see the
latest hi-res runs and see how activity fares today to adjust the
forecast as needed. Either way, no significant impacts are
expected with no flash flooding anticipated through Monday.

Models show monsoonal moisture increasing Tuesday-Thursday, as the
upper high shifts east. This will bring a return to afternoon 
mountain and desert thunderstorm activity then. How active it gets
though is uncertain and will assess in the days ahead. 

Nighttime and morning low cloudiness will dominate at the coast
each day with sunshine each afternoon. It will continue humid as
well. Temps will continue to run above average through next week,
but not to excessive levels. 

&&

.AVIATION... 
181530Z...Coasts/Valleys...Low clouds with bases 1200-1500 feet MSL 
and tops to 2200 feet will scatter out by 18Z. Spots along immediate 
coast may not clear fully or for very long. Clouds will redevelop 
and spread inland after 01Z with bases 1000-1400 feet MSL and spread 
not as far inland overnight. Local vis restrictions along inland 
edge of cloud deck. Scatter out Sunday by 17Z.

Mountains/Deserts...Cumulus developing over the mountains this 
afternoon with bases near 10000 feet MSL. There is a slight chance 
of showers over mountain crests. Otherwise mostly clear with 
unrestricted vis through Sunday morning.

&&

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

&&

.BEACHES...
Beach Hazard Statement will continue through this evening for 
elevated surf and strong rip currents. A 3-4 ft/15-16 second period 
swell from 180 degrees will bring surf of 3-6 ft, with isolated sets 
to 9 ft, mainly along the south facing beaches of Northern Orange 
County, and strong rip currents. Surf will lower tonight through 
Sunday. 

&&


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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through this evening for Orange County 
     Coastal Areas-San Diego County Coastal Areas.

PZ...NONE.
&&

$$

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