Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 182050

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

Strengthening high pressure aloft across California will continue
the warming trend this afternoon with the warming continuing in the
deserts Friday. 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.



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.


182010Z...Coast/Valleys...SCT-BKN high clouds expected today with
bases AOA 20000 FT MSL. High confidence in low clouds returning to
coastal areas tonight. Low confidence in exact timing but cigs most
likely to form after 06Z Fri, with bases 500-1000 ft MSL. Reduced
vis of 3 miles or less will be possible due to fog over higher
coastal terrain. SCT conditions expected by 15Z Fri at coastal TAF

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
expected Friday afternoon-Friday night. NW winds 10-15 kt gusting 20-
25 over the outer waters 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.





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