Area Forecast Discussion
Issued by NWS San Diego, CA

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128
FXUS66 KSGX 141030
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
230 AM PST Thu Dec 14 2017

.SYNOPSIS...
Several weak disturbances will dig south over the Great Basin into
early next week, squeezing the warm ridge aloft and resulting
in periods of Santa Ana winds over SoCal. Continued warm days
through Friday, then a bit cooler over the weekend as the marine
layer returns briefly. The ridge gradually weakens and shifts a bit
more to the west next week, allowing colder systems from the north
to enter the picture. They may bring a little rain to northern
portions of the state, with noticeable cooling farther to the south
as marine air returns.


&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

Some patchy marine clouds were visible in satellite imagery early
this morning over the CA Bight, otherwise it was clear. The offshore
sfc pressure gradient had relaxed quite a bit over the past 24
hours, but remained solidly offshore from the NE and weakly onshore
KSAN to the lower deserts. Winds were weak through 2 AM PST.

Water vapor imagery showed a wave diving south over the Great Basin.
This is the system that will rev up the offshore winds across SoCal
today. It will bring moderate upper-level support to enhance the
surface gradient, so expect some strong wind gusts below favored
wind-prone canyons and passes. It will also spike the wildfire
conditions due to the extreme dryness. Despite the offshore push, a
modest sea breeze should still aid in higher humidity along the
San Diego County coast, with a more muted effect in Orange County
where the offshore will be better established. Continued quite warm
inland with highs around 80 degrees F. Our local WRFEMS shows
strongest winds this evening and tonight in San Diego County where
mountain waves could roll off the slopes and onto the foothill
areas.

As the wave passes to the south, there is a brief rebuilding of the
ridge aloft on Fri, but not for long as another potent shortwave
attacks the ridge from the north. This one is a bit more problematic
with regard to model solutions. The 06Z NAM12 run has come on board
with the 00Z global model runs closing off a compact low somewhere
near/over the CA Bight Sat morning. The pattern suggests convective
development over the CA Bight Sat morning, but likely isolated and
fast-moving if at all. PWATS increase to around 0.75 inch from
less than 0.25 and MU Cape values exceed 200 J/K briefly. KSAN
stands the best chance at this point, but would like to see
confirmation in another run or two before introducing POPS to the
official forecast.

The pattern settles down early next week as the ridge broadens and
weakens, allowing gradual cooling over the region and less wind. The
ridging over the west transitions to troughing by the middle of next
week with the GFS favored in a retrogressive hemispheric pattern.
While still no definitive signal for any significant precipitation
emerging, cooling and onshore flow should help to ease wildfire
concerns.

&&

.AVIATION...
141000Z...Mostly clear today and tonight. Areas of low clouds will
continue over the coastal waters this morning. Low cloud or vis
restriction over coastal airports is unlikely, but a cig of 200-300
feet for a brief time is remotely possible through 15Z. Surface east
to northeast winds 15-25 kt with gusts 35-45 kt will develop this
morning mainly in the foothills and through mountain passes and
continue through afternoon. LLWS and weak/mod up/down drafts are
expected over areas just west of the mountains.

&&

.MARINE...
Patchy fog with visibility 1nm or less this morning. There is a
remote possibility of a thunderstorm on Saturday morning, but
confidence is too low to forecast at this time. Stay tuned to
forecast updates. Otherwise, no hazardous marine weather is expected
through Monday.

&&

.FIRE WEATHER...
Santa Ana winds will strengthen significantly early this morning,
first on the San Bernardino County coastal slopes, through and below
the Cajon Pass, and into the Santa Ana Mountains/foothills, and
then into the Riverside County coastal slopes/passes by 6 am and
finally into the San Diego County coastal slopes/foothills by 10 AM
PST. Northeast to east winds will peak at around 20-35 mph with
gusts 50 to 55 mph and isolated gusts 65 to 70 mph. Day-time
humidity once again will lower to the 5-8 percent range. The winds
will become less widespread Thursday evening but remain quite
strong on the San Diego County coastal slopes/foothills. Then,
gradually weaken early Friday morning. Overnight RH recovery will be
very poor in windy areas. A Red Flag Warning is in effect through
Friday morning.

Late Friday through Saturday afternoon, there will be a brief
period of onshore winds and higher humidity. Then another burst of
Santa Ana winds is likely Sunday into early Monday, with humidity
falling to 10-15 percent away from the coast. Thus, we could see
another round of critical fire weather conditions Sunday into Monday
morning.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 4 AM Thursday to 10 AM PST Friday for
     Orange County Inland Areas-Riverside County Mountains-
     Including The San Jacinto Ranger District Of The San
     Bernardino National Forest-San Bernardino County Mountains-
     Including The Mountain Top And Front Country Ranger
     Districts Of The San Bernardino National Forest-San
     Bernardino and Riverside County Valleys  -The Inland Empire-
     San Diego County Inland Valleys-San Diego County Mountains-
     Including The Palomar And Descanso Ranger Districts of the
     Cleveland National Forest-San Gorgonio Pass Near Banning-
     Santa Ana Mountains-Including The Trabuco Ranger District
     of the Cleveland National Forest.

PZ...NONE.
&&

$$

PUBLIC/FIRE WEATHER...jad
AVIATION/MARINE...mm



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