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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
000
FXUS66 KSGX 172002
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
100 PM PDT Wed Jul 17 2019

.SYNOPSIS...
Weak low pressure aloft over the West Coast will bring a cooling 
trend to most areas through Friday, along with greater coverage of 
night and morning low clouds west of the mountains. Some areas along 
the coast will have lingering clouds this afternoon, but better 
clearing is expected Thursday and Friday afternoons. A strong 
broad high pressure ridge will develop over the western United 
States early next week and bring hot weather inland. Monsoonal flow 
will also develop and could bring some afternoon and early evening 
thunderstorms beginning Sunday and increasing through Wednesday 
mainly in the mountains and deserts plus increasing humidity but 
also fewer low clouds near the coast.

&&

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

Areas of low clouds continued along the coast early this afternoon 
with sunny skies inland. Temperatures averaged 3 to 5 degrees lower 
versus 24 hours ago, though more in some valley locations due to the 
higher marine level inversion and in the lower deserts due to some 
increase in humidity from the southeast, albeit only at low 
elevations, below about 1000 feet. Overall, a weak trough will 
continue to develop over the West Coast through Friday and bring 
more cooling to the area along with a deeper marine layer for more 
nocturnal stratus in the valleys. The models, including our local 
WRF, are indicating little or no eddy Thursday and Friday, and that 
combined with cooling above the inversion could result in better 
clearing at the coast Thursday and Friday afternoons.

There will be a large-scale pattern change beginning this weekend as 
the upper high now over the eastern US retrogrades west to the 
central Rockies by Sunday/Monday and expanding Tuesday/Wednesday. 
The GFS ensemble runs are quite consistent with that as well as the 
southeast flow aloft which will develop and bring much larger 
amounts of mid and upper level moisture. The NBM 12-hour 
thunderstorm probabilities increase each day Sunday through 
Wednesday mainly over the mountains and deserts. Current forecast 
has thunderstorm chances beginning Sunday, but the last two 
(06Z/12Z) deterministic GFS runs show a wave from the southeast 
moving up towards southern California Sunday along with a sudden 
increase in moisture that date, though the ECMWF does not have this. 
In the afternoon forecast we are adding slight chances of 
thunderstorms in the mountains and deserts Sunday, and if the wave 
is associated with CAPE aloft, which is often the case, there could 
potentially be elevated convection west of the mountains. 
Regardless, thunderstorm chances should increase further Mon-Wed.

&&

.AVIATION... 
172000Z...Coast/Valleys...Low clouds have cleared the valleys but 
continue to linger along the immediate coast with bases 1000-1500 ft 
MSL. Low clouds become more widespread after 01Z and will move 
inland 25-35 mi with bases 900-1500 ft MSL. Local VIS 3-5 SM in 
BR/HZ on coastal mesas and higher coastal terrain. SCT conditions 
expected to return to the valleys after 16Z, and to the coast 17-
19Z. 

Mountains/Deserts...CLR with unrestricted VIS through Thursday.

&&

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


&&

.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...Maxwell
AVIATION/MARINE...CO