Area Forecast Discussion
Issued by NWS San Diego, CA

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210
FXUS66 KSGX 172127
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
127 PM PST Sun Dec 17 2017

.SYNOPSIS...
High pressure will bring fair skies, lighter winds and mild weather
Monday and Tuesday with seasonal temperatures. Strong onshore flow
will develop Wednesday as low pressure swings into the Great Basin,
pulling in the marine layer briefly. As the low passes, high
pressure will build inland, forcing flow back offshore across
Southern California with another round of gusty northeast winds
below passes and canyons. Cooler, with more seasonal temperatures
into the weekend.


&&

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

Satellite imagery early this afternoon showed mostly clear skies
over the region. Gusty NE wind reports continued at some spots in
the vicinity of the Santa Ana and San Bernardino Mts at Noon.
Fremont Canyon still had a gust of 61 MPH, but most other sites
reported peak gusts in the 30 to 45 MPH range. Farther to the south
and east, winds were also gusty through and below passes and
canyons, but lighter. A wind Advisory remains in effect for areas
below the San Gorgonio and Cajon Passes, and in the SAnta Ana
Mountains and Orange County until 8 PM PST this evening. Please see
the PNSSGX statement headlined on our website for peak gusts
recorded today. A Red Flag Warning continues as well over northern
portions of the CWA. Please see the fire weather discussion below
for details.

The NE winds will be decreasing later today and overnight, setting
us up for a couple of nice days. It will still be quite dry though
with low RH values.

A strong ridge remains parked just off the West Coast this
afternoon. The ridge will expand across the west on Monday, then
flatten as a strong shortwave trough slides SE from higher
latitudes. The trough will dig SE into NV on Wednesday as the ridge
off the coast builds northward. Surface pressure falls over the
Great Basin will set the stage for strong onshore flow over our
mts/deserts later Wed/Wed night, before winds turn back offshore on
Thursday as the wave passes.

Current indications are for westerly winds gusts of 50 to 60 MPH
over the mtn crests/desert slopes late Wednesday before swinging
northerly and then NE on Thursday. The NE winds could be quite
strong as well, especially through the passes and canyons and along
the coastal slopes of the Mts. This event looks to be short-lived
though as models show the trough moving east rather quickly.

The weekend looks like a transition period with higher uncertainty
as the hemispheric pattern begins to undergo a rather large shift.
Both the 12Z GFS/ECMWF runs appear to be hinting at establishing the
massive EastPac ridge farther to the north, allowing shortwave
energy to undercut. How this energy manifests itself remains to be
worked out, but it will at least keep us cooler and tamp down the
persistent dry offshore flow we have been experiencing this month.
The shift in the ridge, if it does occur as seems to be indicated,
will also open the door to Pacific systems that could eventually
bring much needed precipitation.

&&

.AVIATION...
172116Z...P6SM vis and FEW clouds AOA 10000 ft msl will continue
through Monday morning. N to NE winds and associated UDDFS/LLWS will
continue to gradually dissipate through Monday morning.

&&

.MARINE...
West-northwest swell will continue to dissipate today, with seas
lowering to 3-6 ft in the outer waters and 2-4 ft in the inner
waters.

A trough of low pressure moving through the region will bring
strengthening northwesterly winds Wednesday night through Thursday
morning, with the possibility of Small Craft Advisory conditions.
Wind gusts could reach 25-30 kt in the outer waters.

&&

.FIRE WEATHER...
A Red Flag Warning remains in effect for portions of the Inland
Empire, Santa Ana Mts, Orange County coastal plain, and lower
coastal slope and foothill areas of Riverside and San Bernardino
Counties through 8 PM PST for strong northeast winds/gusts 15 to 30
MPH with gusts 45 to locally 55 MPH, and several hours of marginal
RH values near 15%. Northeast Winds will be weaker across San Diego
County, but with some local gusts 40 MPH, mainly on the offshore,
wind-prone slopes. RH levels are expected to remain above 15%.

Low pressure over far northwest Mexico will move eastward tonight
and Monday, with residual mid-level moisture over the mountains and
deserts of San Bernardino...Riverside...and San Diego Counties
decreasing. Winds will subside overnight, with moderate RH recovery.
Drier most areas on Monday as daytime RH falls into the teens over
the mtns/valleys, but winds will be light.

Onshore flow will be increasing and become strong and gusty over the
mtns/desert slopes by Wednesday afternoon and evening as low
pressure passes to the northeast. Gusty offshore flow will follow on
Thursday and could be quite strong into Thursday night.

&&

.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 8 AM this morning to 8 PM PST this evening
     for Orange County Coastal Areas-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 Gorgonio Pass Near Banning-Santa
     Ana Mountains-Including The Trabuco Ranger District of the
     Cleveland National Forest.

     Wind Advisory until 8 PM PST this evening for Orange County
     Coastal Areas-Orange County Inland Areas-San Bernardino
     County Mountains-San Bernardino and Riverside County
     Valleys-The Inland Empire-San Gorgonio Pass Near Banning-
     Santa Ana Mountains and Foothills.

PZ...NONE.

&&

$$

PUBLIC/FIRE WEATHER...jad/17
AVIATION/MARINE...Harrison



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