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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 190437

Area Forecast Discussion
National Weather Service San Diego CA
937 PM PDT Thu Apr 18 2019

The marine layer will return late tonight with low clouds and 
fog, with local dense fog possible, in coastal areas. The marine 
layer will deepen Friday through Sunday with low clouds spreading 
to the coastal mountain slopes Saturday night. A low pressure 
system will move across California late Saturday and early Sunday 
and will bring substantial cooling over the weekend and a slight 
chance of light showers. Stronger onshore flow will bring gusty 
west winds to the mountains and deserts with strongest winds 
Saturday afternoon and night. High pressure will rebuild across 
California next Monday through Thursday and bring a return to fair
warm weather. 



Low clouds developing over the coastal waters this evening are
beginning to be hidden from satellites by high clouds. These high
clouds will likely limit much more development of low clouds over
the next few hours before they push into the coast and coastal 
valleys overnight. Areas of fog along the coast may be dense at 
times, especially for early Friday morning. No changes were needed
to the forecast this evening.


High pressure aloft is peaking today and bringing warm weather to 
the region, with Thermal Airport up to 96 at 1 PM, and quite a few 
places approaching 90 in the Inland Empire (92 at Lake Elsinore). 
Models indicate a shallow marine layer developing tonight, with some 
stratus and fog near the immediate coast. Local dense fog could form 
over higher coastal terrain, such as the mesas. Latest observation 
at San Clemente Island (NUC) has broken clouds at 500 feet above 
ground level (around 700 feet above sea level), and aircraft 
soundings have shown similar inversion heights, so elevations below 
about 700 feet could get fog tonight. Due to the developing marine 
layer, Friday should be a little cooler in coastal areas, though 
with 850 MB temps peaking in the deserts then despite the height 
falls, the mountains and deserts should have their warmest day of 
this week then.

A weak shortwave will move through the region Friday night and help 
to deepen the marine layer, then a longwave trough will move through 
Saturday night and Sunday. NAM and local WRF both have some light 
precipitation, mainly from the valleys to the west mountain slopes, 
and given the nocturnal passage of the main vorticity maximum with 
the system, I added slight chances of showers to the forecast in the 
afternoon package. Temperatures will be lower, mostly about 5 
degrees below seasonal normals over the weekend. As is typical for 
low pressure troughs during the spring, strong gusty winds will 
develop over the mountains and deserts, with gusts likely over 50 
MPH in the usual windy spots, especially Saturday afternoon and 
evening. High pressure will rebuild next week and bring fair warmer 
weather again, with the possibility of 100 degree temps in the 
hottest parts of the lower deserts.


190405Z...Coast/Valleys...SCT-BKN low clouds over the coastal waters 
moving inland, staying west of KONT and KRNM, with bases 500-800 
feet MSL with tops around 1500 feet MSL. Local visibilities 1-2SM in 
BR but locally 1SM or less higher coastal terrain. Risk of low 
clouds at the coastal airfields is moderate-high. The confidence in 
temporary BKN-OVC low cloud based as low as 500 feet MSL with 
visibilities around 2SM in BR at KSAN overnight is low-moderate. 
Skies should become SCT at about 1500 feet MSL around 15Z-16Z 
coastal TAF sites. 

Mountains/Deserts...Mostly clear skies and unrestricted vis expected 
through Friday afternoon.  


A brief period of NW winds 12-18 kt over the outer coastal waters is 
possible Friday afternoon-Friday night, with NW winds 10-15 kt 
gusting 20-25 over the outer waters possible late Saturday night 
into Sunday. These conditions may become hazardous to small craft.


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