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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
918 
FXUS66 KSGX 180515
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
915 PM PST Fri Nov 17 2017

.SYNOPSIS... 
Clearing skies will bring increased radiational cooling tonight,
with patchy frost possible over the upper deserts. Light offshore
flow tomorrow will fade as a subtropical ridge builds over the
Southwest through the middle of next week. This should bring 
unseasonably warm conditions to the region by Thanksgiving.

&&

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

At 845 PM...The upper trough that brought the first major 
snowfall of the year to the Sierra over the last 48 hours was 
shifting eastward. With this, upper level winds over Southern 
California were taking on a more prevalent westerly component, 
bringing upper level drying and a reduction in high cloud cover. 
Increased moisture below 700 mb, evident on the 00Z NKX sounding, 
was allowing a few low clouds to form along the coast. Surface 
gradients remained onshore with the majority of the region 
experiencing light winds. Signs of change were evident in the 
surface observations with a 1029 mb surface high building over 
Nevada.

For the remainder of tonight, surface high pressure currently over
northern Nevada will continue to build and propagate southeast.
This will result in weakly offshore surface pressure gradients,
and locally breezy northeast winds by Saturday Morning. Winds 
won't be particularly strong. More notable will be the falling 
dewpoints that will allow for significant radiational cooling over
Mojave Desert and Mountains overnight. Patchy frost is possible 
in low lying areas of Victorville, Hesperia, Apple Valley and 
Lucerne Valley as overnight lows fall briefly to 34-37 degrees 
prior to sunrise (colder at ground level). Will forgo a Frost 
Advisory for the upper deserts tonight, as light northeast winds 
and dewpoint temperatures above 32 F for most of the night should
limit the areal coverage of any frost that forms.

Another largely quiet weather day is on tap for tomorrow, with a
notable increase in sunshine compared to the prior two days and
afternoon highs within a couple degrees of seasonal averages.
Though limited, weak offshore surface pressure gradients will 
bring locally breezy winds and elevated fire weather conditions to
Fontana, Rialto and San Bernardino area. Cold overnight
temperatures are expected again saturday night in the upper
deserts.

Global models are in good agreement into early next week, proging
the subtropical ridge to build north over the Southwest. This will
bring a warming trend as 500 mb heights approach 590 dm by 
midweek. It should also make for a warm Thanksgiving with 
afternoon highs west of the mountains near 15 degrees above 
average. Certainly a change from last year, when Big Bear Lake 
reported 3.5 inches of snow over the Thanksgiving weekend.

&&

.AVIATION... 
180500Z...Coast and Valleys...SCT-BKN clouds of variable bases 
between 500 and 2000 ft MSL will prevail overnight. Timing of 
BKN cigs is uncertain as most areas will alternate between SCT and 
BKN overnight. Tops will be variable as well due to layered clouds, 
with local tops above 3000 ft. Areas of terrain will be obscured 
with local vis below 1 mile over higher terrain, including near the 
coast. Most areas will clear by 17Z Sat with VFR conditions 
prevailing after 17Z Sat. Local surface winds 15-25 KT with 
gusts to 35 KT from the northeast will occur in/below Cajon Pass
and Banning Pass 08Z-20Z Sat.

Mountains and Deserts...Local surface winds 15-25 KT with gusts to 
35 KT from the northeast will occur in the San Bernardino, San 
Jacinto and Santa Ana mountains and areas north of KPSP 08Z-20Z Sat.
Otherwise, clouds above 12000 ft MSL will gradually diminish the 
rest of tonight.
  
&&

.MARINE...
No hazardous marine conditions are expected through Wednesday.

&&

.FIRE WEATHER...
Light offshore winds will develop Saturday morning and continue
through the day. Elevated fire weather conditions will occur
south of the Cajon Pass, where wind gusts to 35 mph and minimum 
relative humidity values near 15% are expected. Winds will fade
Sunday as offshore flow dissipates. Warm weather and a possible 
return of offshore flow may bring locally elevated fire weather 
conditions Wednesday-Thursday.

&&

.SKYWARN... 
Skywarn activation will not be needed through next week.


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

PUBLIC/FIRE WEATHER...Albright
AVIATION/MARINE...Maxwell