FXUS66 KSGX 252142

Area Forecast Discussion
National Weather Service San Diego CA
240 PM PDT Tue Sep 25 2018

High pressure building aloft will bring warmer days Wednesday
through Friday, followed by cooler weather over the weekend. The
marine layer will turn shallower Wednesday through Friday, then
deepen Saturday and Sunday as a trough of low pressure develops off
the California coast. There is some potential for this trough to
draw tropical moisture into the Southwestern States early next week.



Marine layer clouds retreated back to the coast this afternoon with
clearing all the way to the beaches. The clouds will return tonight,
then burn off to the coast again Wednesday. Warmer days are ahead as
high pressure builds over the Southwest. 500 mb heights of 586 dm
today increase to 593 dm Wednesday and Thursday. The marine layer
will turn shallower and the low clouds will not spread nearly as far
inland Thursday and Friday mornings.

High temperatures were raised in most areas Wednesday through
Friday, with greatest increases made to the valley temperatures due
to the drier airmass and weak offshore downslope flow. Inland Empire
highs will like reach triple digits in some areas Thursday. Highs
Thursday will range from the 80s in the coastal zones, in the 90s in
the San Diego Valleys, and 97-105 in the Inland Empire. Mountain
highs will be in the 80s, High Desert highs in the mid 90s, and
Lower Desert highs from 106-111. Temperatures for Friday have also
been raised with the upper ridge slowing its eastward movement.

The ridge will break down Friday night and Saturday when a trough of
low pressure over the eastern Pacific moves into northern
California. This will bring cooling and a deeper marine layer
through the upcoming weekend.

The weather gets quite interesting next week. Long range model
guidance is hinting at a potential influx of tropical moisture from
what is now Tropical Storm Rosa off the west coast of Mexico. The
trough position will be a key player into just how much moisture
reaches our area. The GFS and most of it's ensembles are much more
bullish than the ECMWF in bringing deeper moisture into the
Southwest, particularity Arizona. The ECMWF and far fewer ensembles
show this, and are much drier Monday. But the ECMWF holds onto the
deep trough over the Pacific through the middle of next week for
possible rain midweek. The ECMWF usually performs better in the long
range than the GFS, but there is a wide range of possible solutions
with very low confidence in any one of them at this time.


252030Z...Coast/Valleys...Low clouds have cleared from the valleys
and coastlines this afternoon. Clear skies are expected through 02z.
Low clouds will move back ashore after 02Z and move inland about 20-
25 miles. Bases/tops will be trending lower going into Wednesday,
with bases 800-1100 ft MSL and tops to 1400 ft MSL. Most areas
should clear 16z-18z Wednesday.

Mountains/Deserts...Mostly clear with unrestricted vis through


No hazardous marine weather is expected through Monday. Wave heights
could increase slightly going into Monday due to enhanced swell from
Tropical Storm Rosa. 3-4 ft swells are possible, but could be
substantially higher depending on the track.


Weak offshore flow Wednesday and Thursday, possibly into Friday
morning, with locally gusty east winds in the mountains, foothills,
and eastern valleys. Winds are expected to gust to around 25 mph at
times. Lower humidity will expand into the mountains today, and into
the inland valleys on Wednesday. Low inland humidity will continue
through Friday, but with weaker winds. Onshore flow will strengthen
for this weekend with a recovery in humidity for the valleys into
the mountains.


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





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