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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 171646

Area Forecast Discussion
National Weather Service San Diego CA
930 AM PDT Wed Jul 17 2019

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. 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 mainly in the mountains and deserts 
plus increasing humidity but also fewer low clouds near the coast.



Low clouds covered coastal areas early this morning with most areas 
more than a few miles from the coast sunny by mid-morning. An eddy 
combined with a 10-deg C marine inversion should result in some 
coastal areas not fully clearing again this afternoon. It should be 
a little cooler today as well, and the NKX sounding has low-level 
cooling, including at 850 MB (2 deg C lower this morning versus the 
Tuesday morning sounding). 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 notable 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. Hence, in the afternoon forecast we 
will add 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.


171530Z...Coast/Valleys...BKN-0VC005-016 along the coast. Low clouds 
clearing in the valleys will continue to clear through 17Z. Clearing 
will make it close to the coast between 17Z and 19Z, but patchy and 
intermittent CIGS likely along immediate coast for most of the day. 
CIGS redeveloping after 01Z with bases 900-1500 ft MSL, extending 25-
35 mi inland. Local VIS 3-5 SM in BR on coastal mesas and higher 
coastal terrain.

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


No hazardous marine weather is expected through Sunday.


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