Area Forecast Discussion
Issued by NWS San Diego, CA

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240
FXUS66 KSGX 022140
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
140 PM PST Fri Dec 2 2016

...High Impact Santa Ana Wind Event Through Saturday...

.SYNOPSIS...
Strong and gusty Santa Ana winds will continue through Saturday.
The greatest impacts will be across Orange County to the Inland
Empire and adjacent mountain areas. Warmer with lighter winds
Sunday. Cooler early next week with gusty onshore winds in the
mountains and deserts and night and morning marine layer clouds in
the coast and valleys. Another round of offshore winds is
possible late in the week, in addition to a gradual warming trend.

&&

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

This afternoon`s water vapor satellite imagery shows a deepening
trough over the California/Arizona border. This trough will
continue to deepen during today and then develop into a cut-off
upper low over Sonora by Saturday morning. Meanwhile, a strong
surface high (of around 1034-1036 mb) will move into the Great
Basin. Currently the offshore pressure gradient from San Diego to
Tonopah is up 10.4 mb, and this gradient will continue to
strengthen through the day today (reaching up to 15 mb by
tonight). With the expected strong offshore surface pressure
gradients and north-northeast winds of 35-45 kt at 850 mb, this
is an ideal set-up for a widespread strong offshore wind event.
The northeast winds are holding steady this afternoon with
widespread 25-35 mph gusts and locally 50-60 mph. Fremont Canyon
had the strongest wind gust of 70 mph this morning, while Ontario
International Airport reached 51 mph. However, the WRF cross-
section through the Santa Ana Mountains and up through the Cajon
Pass shows another burst of wind due to mountain wave activity
this evening and late tonight, so widespread 50-60 mph gusts and
locally 70+ mph could occur at that time. With the more northerly
gradient, the winds will continue to be stronger over Orange
county, the Santa Ana mountains/foothills, Inland Empire, and the
coastal foothills of San Bernardino and Riverside counties, and a
High Wind Warning continues through early Saturday afternoon.
There have already been many reports of downed trees and power
lines, and these kinds of impacts, along with possible structural
damage, could continue, as the strength and widespread nature of
the winds will result in strong winds impacting places that
normally do not get strong Santa Ana winds.

Meanwhile for San Diego county, there will likely be some strong
and gusty northeast to east winds as well, but not quite as strong
as areas farther north, so a Wind Advisory continues for the San
Diego county valleys and mountains where gusts of 50 mph and local
60 mph could occur. The Coachella Valley and San Diego county
deserts may see some downsloping strong winds from the north to
northeast as well, so a Wind Advisory is in effect for those
areas.

With the shorter days and very cold origins of the northeast
winds, there will not be as much, if any, warming today from the
offshore winds. However, it should get a little warmer on Saturday
with the continued offshore flow and upper level ridging from the
west nudging into the region, resulting in highs reaching a few
degrees above normal in the coast and valleys, while elsewhere
will still be colder than normal. The upper ridge strengthens a
little more over us on Sunday with weak offshore flow, resulting
in additional warming as all areas see highs 3-10 degrees above
normal.

Another cooling trend commences on Monday and continues through
Tuesday as the ridge weakens, onshore flow increases and northwest
flow aloft moves into the region. The ECMWF and Canadian models
continue to show the a weak short-wave trough moving into the
Great Basin on Tuesday, bringing just enough moisture for a few
light showers, while the GFS keeps Southern California mostly dry.
Nevertheless, with the marine layer should return and deepen
significantly, with night and morning clouds in the coast and
valleys, in addition to windy conditions in the mountains and
deserts.

Offshore flow may return Wednesday through the end of next work
week, resulting in the marine layer clouds clearing out and
gradual warming. The 12Z ECMWF and 12Z Canadian do not look as
strong with the offshore as the previous runs showed.

&&

.AVIATION...
022130Z...Windy conditions will continue through Saturday morning,
especially over and west of the mountains, including VCNTY KONT,
KRAL, KCNO, KSNA, KBNG. Areas of northeast surface winds 20-30G50+
knots will occur, though winds in San Diego County will be somewhat
lighter, with NE 15-25G40 knots through/below passes/canyons. LLWS
will occur at times, especially where surface winds are light, and
KSAN should have LLWS overnight. Except for some lingering clouds
with bases around 6000 ft MSL over San Diego County through early
evening, skies will be mostly clear with mostly unrestricted vis,
though very local 3-5 mile vis could occur in blowing dust. Winds
will decrease somewhat Saturday afternoon.

&&

.MARINE...
Areas of strong northeasterly winds will continue this afternoon,
with winds gradually decreasing later tonight and Saturday. Northeast
winds of 15-25 kt with gusts 30-35 kt are expected, mainly from San
Clemente Island northeast to San Mateo Point. A Small Craft Advisory
is in effect through 10 AM Saturday. NW wind gusts near 20 kt are
possible on Monday as an upper-level disturbance moves across the
region.

&&

.FIRE WEATHER...
Strong north to northeast winds will continue through Saturday.
Strongest winds today will affect the San Bernardino and Santa Ana
mountains and passes as well as the Inland Empire and Orange
County, and to a lesser extent the Riverside County Mountains.
Winds 20-40 mph with gusts of 50-70 mph are expected in these
areas, with locally higher gusts possible. Winds will not be quite
as strong in San Diego County during this time, with gusts 35-50
MPH.

Afternoon humidity will fall to around 10-15 percent in the
valleys and coastal areas today with poor recovery tonight. Some
residual moisture in the mid-levels will help moderate humidity in
the mountains. Widespread daytime humidity in the upper single
digits and low teens is expected on Saturday.

Red Flag Warnings remain in effect for Inland Orange County, the
Santa Ana mountains/foothills, and Inland Empire and a Red Flag
Warning goes into effect just for Saturday for the San Bernardino
county mountains. Conditions will improve on Sunday as offshore
winds weaken.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 2 PM PST Saturday for Coachella Valley-San
     Diego County Deserts-San Diego County Mountains-San Diego
     County Valleys-San Gorgonio Pass Near Banning.

     High Wind Warning until 2 PM PST Saturday for Orange County
     Coastal Areas-Orange County Inland Areas-Riverside County
     Mountains-San Bernardino County Mountains-San Bernardino
     and Riverside County Valleys-The Inland Empire-Santa Ana
     Mountains and Foothills.

     Red Flag Warning until 6 PM PST Saturday for Orange County
     Inland Areas-San Bernardino and Riverside County Valleys  -
     The Inland Empire-Santa Ana Mountains-Including The Trabuco
     Ranger District of the Cleveland National Forest.

     Red Flag Warning from 6 AM to 6 PM PST Saturday for San
     Bernardino County Mountains-Including The Mountain Top And
     Front Country Ranger Districts Of The San Bernardino
     National Forest.

PZ...Small Craft Advisory until 10 AM PST Saturday for Coastal Waters
     from San Mateo Point to the Mexican Border and out to 30 nm-
     Waters from San Mateo point to the Mexican Border Extending
     30 to 60 nm out including San Clemente Island.

&&

$$

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



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