Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 192042

Area Forecast Discussion
National Weather Service San Diego CA
142 PM PDT Tue Sep 19 2017

Amplified onshore flow will contribute to continued below average
temperatures through the weekend, with the coolest days occurring
on Thursday and Friday. A deep marine layer will allow for
persistent night and morning low clouds west of the mountains,
with slow to limited clearing at times. Isolated light showers are
possible west of the mountains on Thursday, with little to no
accumulation. Gusty west winds and patchy blowing dust will impact
the deserts slopes and deserts Thursday into Friday. A transition
to offshore flow will produce a slow warming trend early next



At 130 PM, A broad longwave trough remained in place over the
Western US. Water vapor imagery showed a shortwave trough embedded
within the longwave pattern off the coast of British Colombia.
Locally, dry west northwest flow above 800 mb was keeping skies
clear in the mountains and deserts, while a marine layer stratus
was rapidly burning off. Surface winds were light across the
region apart from a few isolated gusts to 50 mph through the San
Gorgonio Pass.

Through tonight, increased onshore flow will continue to bring
local wind gusts of 40-50 mph to the passes and along the desert
slopes. Clear skies will prevail in the mountains and deserts,
while marine layer stratus returns over the valleys and coast. The
marine layer stratus should rebuild quickly after sunset,
reaching the coastal slopes overnight with a depth of around 4,000
ft MSL. Elevations between 3,000 ft and 4,000 ft may experience
patchy fog Wednesday morning.

The shortwave currently off the coast of British Colombia will
dig along the coast, amplifying the longwave trough over the
Western US Thursday and Friday. With this, heights will fall,
onshore flow will increase and afternoon high temperatures will
dip a few degrees further into the sub average category. Thursday
looks to be the coolest day, with the most persistent low clouds
and inland temperatures running 10-20 degrees below average. A few
light showers are also possible Thursday west of the mountain
ridges. Winds winds will become gusty in the passes, along the
desert slopes and over the desert floor Thursday, and a Wind
Advisory may be needed for the desert slopes and deserts of
Riverside, San Diego, and San Bernardino Counties. Local wind
gusts may exceed 60 mph during this period along the desert slopes
in Riverside and San Diego Counties.

Global models show some agreement on the overall synoptic pattern
for next week. Suggesting that the longwave trough will weaken,
transitioning to ridging with a cutoff upper low off the coast.
This pattern would support the development of a weak surface high
in the Great Basin and offshore flow early next week. At the
moment this looks like it will be sufficient for a warming trend,
and breezy winds below the Cajon Pass. This may result in locally
elevated fire weather conditions (especially in the northern
Inland Empire), but a significant offshore wind event is unlikely
given current model projections.


192000Z...Coast/Valleys...SCT to locally BKN stratus with bases 3500-
4500 ft continuing over parts of Orange/San Bernardino counties this
afternoon. Stratus becoming BKN-OVC and spreading into the coastal
areas 02-05Z before moving into the inland valleys overnight. Bases
and tops will remain around 2000-3000 ft MSL with tops to 4000 ft
MSL. Areas of vis 3-5SM in BR/HZ developing in the valleys as
stratus moves inland.

Mountain/Deserts...Clear with unrestricted vis through Wednesday
afternoon. Local west wind gusts 25-35 knots will occur on desert
slopes and through San Gorgonio Pass late this afternoon through the


No hazardous marine weather is expected through Wednesday.
Northwest winds 20-25 kt Thursday afternoon through Friday will
result in hazardous boating conditions, especially in the outer
coastal waters. Lighter winds will likely return Saturday.


Skywarn activation will not be needed this week.




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