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000 
FXUS66 KSGX 300428
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
928 PM PDT Wed Mar 29 2017

...Strong to Locally Damaging Winds Mountains-Deserts Thursday
Afternoon and Evening...

.SYNOPSIS...
High pressure aloft will drift east tonight. On Thursday a deep 
trough moving across the Great Basin will bring strong and locally
damaging west winds to the mountains and deserts. Cooler Friday 
behind the trough with winds shift to the northeast. High pressure
rebuilding over the region Saturday will bring sunny and warm 
this weekend. Cooler early next week as another trough moves in 
from the northwest. 

&&

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

...Update - High Wind Warning Issued for Mountains/Deserts...

Upgraded to a High Wind Warning for the mountains to the desert
floor. The pressure gradient will really tighten up and result in
strong to locally damaging winds across the mountains to the
deserts Thursday afternoon and evening. 

The strongest winds are expected to occur across the high desert 
and the mountain tops/desert slopes. High sustained winds are
possible across the Apple and Lucerne Valleys and points north.
GFS MOS has sustained wind to 34 knots at Victorville Thu
afternoon. Hazardous travel from strong cross-winds and localized
severe visibility restrictions is expected along Interstate 15 in
the high desert.

Localized damaging wind is possible across the Coachella Valley 
and the San Diego County deserts, so included these areas in the 
High Wind Warning. Cross sections show mountain waves developing 
with surfacing and rotoring. Localized gusts to 60 mph will occur
as mountain waves surface. 

Timing of strongest winds is forecast to be 2-6 PM Thursday.

/Gregoria

...Previous Discussion (Issued at 229 PM PDT Wed Mar 29 2017)...

High pressure aloft brought sunny, warm weather Wednesday. Highs 
were in the 80s in most areas west of the mountains. Mountain highs 
in the 60s and 70s, and the lower deserts were in the lower 90s. 
Records are a little out of reach today. 

The ridge axis shifts east tonight and we will transition to a 
progressively stronger onshore flow pattern Thursday. A deep upper 
trough will move into NorCal tonight and then across the Great 
Basin/California interior on Thursday. The core of the strongest 
winds in SoCal will be Thursday afternoon through Thursday evening, 
a relatively short duration onshore wind event. Pressure gradients 
ramp up Thursday afternoon and peak Thursday evening with a 994 mb 
surface low over southern Utah. Strong gradient winds over the high 
deserts from 20-30 mph with gust to 50 mph+. Mountain waves are a 
near certainty with an inversion near 700 mb. West winds will 
surface down the eastern slopes and into the lower deserts, but the 
magnitude of the wind is still in question. The WRF is strongest 
with the winds, as usual, and support a low end Wind Warning. But 
the Cansac and NAM3 are weaker and support a Wind Advisory. Either 
way, there will be a 6-12 hour period of gusty west winds between 
Noon-10 pm Thursday. Surface winds of 20-30 mph are likely with peak 
gusts of 50-60 mph on the slopes and some parts of the desert floor. 
Rotors are likely in the Coachella Valley. A High Wind Watch remain 
in effect for most of the mountain and desert areas Thursday 
afternoon in Thursday night.

The winds shift north and weaken Thursday night as the trough moves 
over northern AZ, setting up a brief period of offshore flow through 
midday Friday. The north-south alignment favors gusty winds in the 
Cajon Pass, parts of the IE and the Santa Ana Mountains. The 
northern part of the Coachella Valley will also see gust north 
winds. The remainder of Riverside County and most of San Diego 
County will not be experience strong winds.

An sizable coastal eddy will spin up over the coastal waters tonight 
which could generate low clouds or fog in the coastal zones tomorrow 
morning. 

Weekend outlook: High pressure aloft fills in behind the trough with 
fair warm weather Saturday and Sunday. Another trough will move 
across the Great Basin Monday and Tuesday for minor cooling. This is 
a weaker inside slider that the upcoming Thursday event, and the 
winds won't be as strong in the mtns and deserts. 

&&

.AVIATION... 
300415Z...Coast/Valleys...Mostly clear skies and unrestricted 
vis will prevail through at least 11Z Thu. Low stratus/fog will 
develop along the coast after 11Z Thu with the possibility of a cig 
at KSAN, KCRQ and KSNA between around 12Z and 16Z. Clouds bases 
could start out as low as around 400 ft MSL but rapidly increase to 
700-1000 ft MSL around 16Z with local vis below 1 mile in fog. Vis 
of 1 mile or less could briefly affect KSAN, KCRQ and KSNA though 
confidence is low. SCT-local BKN conditions will occur after 16Z Thu 
near the coast, with stratus spreading into some valleys Thu 
evening.
 
Mountains and Deserts...Mostly SCT clouds above 20000 ft MSL will 
continue through Thu evening. Gusty west winds will develop Thu, 
with areas of winds 25-35 KT gusts 40-50 KT after 20Z Thu near 
mountain crests, desert mountain slopes and some lower elevation 
desert locations. Some strong winds could occur at KPSP and KTRM, 
with peak winds likely to be 23Z Thu to 04Z Fri, though KPSP could 
have a rotor at times with lighter easterly winds. LLWS and strong 
up/downdrafts will also occur after 20Z Thu, and surface vis will 
locally be below 1 mile in blowing dust/sand in the deserts.

&&

.MARINE...
No hazardous marine weather is expected through Thursday mid-
afternoon. NW winds will increase to 15-25 kt late Thursday 
afternoon through Friday. Strongest winds will likely be found 10 
miles offshore and beyond Thursday evening. Highest combined seas 
are anticipated overnight Thursday into Friday, with 4-7 ft seas 
inside of 30 miles and 7-11 ft near and south of San Clemente 
Island. Winds and swell should decrease over the weekend, before 
increasing again early next week.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Warning from noon to 11 PM PDT Thursday for Apple and 
     Lucerne Valleys-Coachella Valley-Riverside County Mountains-
     San Bernardino County Mountains-San Diego County Deserts-
     San Diego County Mountains-San Gorgonio Pass Near Banning.

PZ...None.
&&

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