Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 131805 AAA
AFDSGX

Area Forecast Discussion...UPDATED
National Weather Service San Diego CA
1005 AM PST Wed Dec 13 2017

.SYNOPSIS...
High pressure aloft and offshore flow at the surface will continue
warm and dry weather into the middle of next week. Stronger Santa
Ana winds are likely on Thursday and again on Sunday over the
mountains, foothills and passes. Skies will remain fair with very
low relative humidity except near the immediate coast and locally
inland at times with the sea breeze.

&&

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

Satellite imagery indicated that skies were mostly clear across
the region this morning, with wind observations showing some weak
Santa Ana winds continuing (gusts of 20-30 mph and locally 40 mph
in the windiest coastal foothill/pass locations). The dry air and
clear skies across the region resulted in another morning of
highly varying temperatures. One example: Lower Moro Campground in
Aliso Viejo (elevation 876 ft) was 72 at 7 am, while just nearby
in a cold, valley location in Laguna Beach (elevation 49 ft) the
temperature was 42. An upper level low remained over Baja and
Sonora, while an upper level ridge extended into California and
Nevada. The continued offshore flow (Santa Ana winds) will
continue to bring much warmer than normal weather this afternoon,
with highs 5-15 degrees above normal in most locations. Latest
hi-res model guidance actually shows a coastal eddy spinning up
tonight with the possibility of fog occurring along the coast
tonight into Thursday morning. However, confidence is low as this
might be overly aggressive given the very dry air and offshore
pressure gradients that continues across the region.

Currently the offshore pressure gradients from San Diego to Las
Vegas were 4.2 mb, whereas they were 6.4 mb at this time
yesterday. This will change however when a short-wave trough moves
through the Great Basin area tonight and down into Sonora Mexico
tomorrow evening. This will result in strengthening offshore
pressure gradients (peaking around 8-9 mb from San Diego to Las
Vegas Thursday morning) and upper level support in the form of 850
mb winds NE 25-35 kt. Thus, we will get a burst of Santa Ana winds
Thursday morning and afternoon, then gradually weakening Thursday
night into Friday morning. NE to E winds of 20-30 mph with gusts
of 50 mph and locally 60 mph can be expected in the coastal
foothills/slopes and through/below the canyons/passes.

Winds then turn briefly onshore late Friday through Saturday
afternoon as a trough moves through the Pacific Northwest. Thus,
expect some cooling to take place Saturday. This trough will then
move into the Great Basin late Saturday, once again creating a
strong surface high in that area and bringing a period of moderate
Santa Ana winds Sunday through early Monday. The offshore flow
weakens later Monday but may not transition to onshore until maybe
next Wednesday. Unfortunately, none of the models indicate much
in the way of a potential rain-maker headed toward Southern
California next week. Thus, expect warmer than normal and fair
weather to continue.

&&

.AVIATION...
131530Z...Mostly clear with unrestricted VIS through tonight.
Surface northeast winds 15-25 kt with gusts near 45 kt will develop
late tonight mainly in the foothills and through mountain passes,
and peak Thursday morning. LLWS and weak/mod up/down drafts are
expected over areas just west of the mountains through the period.

&&

.MARINE...
No hazardous marine weather is expected through Sunday.

&&

.FIRE WEATHER...
Weak Santa Ana winds will continue today with humidity falling
into the 5-8 percent range away from the coast. The Santa Ana
winds will strengthen significantly early Thursday morning, first
in the San Bernardino county coastal slopes, through and below the
Cajon Pass, and into the Santa Ana mountains/foothills around 4 am
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. NE to E winds will peak at around 20-30 mph with gusts to
50 mph and locally 60 mph in those areas, with day-time humidity
once again lowering to the 5-8 percent range. The winds will
become less widespread Thursday evening and gradually weaken
through early Friday morning. Overnight recovery will be very
poor. A Red Flag Warning is now in effect for early Thursday
morning through Friday morning for gusty winds and very low
humidity.

Late Friday through Saturday afternoon, there will be a brief
period of onshore winds and slightly 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-Early Monday.

&&

.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...Harrison
AVIATION/MARINE...JJT



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