Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 232122
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
122 PM PST Fri Feb 23 2018

.SYNOPSIS...
Cold tonight due to a dry, cold air mass and clear skies. Dry and
warmer for the weekend with locally gusty northeast winds near
the coastal foothills in the mornings and strong and gusty west
winds in the mountains and deserts Saturday evening. Another low
pressure system from the north will bring more showers with low
snow levels and strong and gusty west winds in the mountains and
deserts for late Monday night into Tuesday. Smaller chances for
showers for Wednesday and Thursday with increasing chances for
showers for Friday and Saturday.

&&

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

A trough is indicated over the Great Basin area this afternoon
according to water vapor satellite imagery. Radar imagery shows
that showers have ended over the region. With the continued cold
air mass over the region, highs today will likely be 10-20 degrees
below normal, with highs only in the 50s in the coast/valleys,
low 60s in the lower deserts, 40s in the high deserts, 40s/30s in
the lower mountain elevations, and 20s/10s in the high mountain
elevations.

Offshore flow will develop tonight into Saturday morning as
surface high pressure builds in the Great Basin in the wake of the
departing trough, then turn briefly onshore Saturday evening
before turning back offshore Sunday. The offshore winds
tonight/Saturday morning will be pretty weak, with local N to NE
gusts of 35 mph, but the turn to onshore winds Saturday evening
could be strong in the typical mountain ridges/desert
slopes/mountain passes and also the High Deserts, and so a Wind
Advisory may be needed for those areas. The offshore winds for
early Sunday look mostly weak once again, with local gusts of
35-40 mph through/below the passes/canyons and into the coastal
foothills. Tonight should be pretty cold due to the cold, dry air
mass, clear skies and mostly weak winds. Temperatures are likely
to reach below freezing over much of the Inland Empire (except
below the Cajon Pass where gusty offshore winds surface), so a
Freeze Warning has been issued. Meanwhile, it may not get quite as
cold in the San Diego county valleys and San Gorgonio Pass, so a
Frost Advisory was issued for those areas. The offshore winds and
weak ridging aloft building into the area will bring drier
weather and slight warming Saturday through Sunday. Highs may
reach near normal on Sunday west of the mountains, but still
colder than normal elsewhere.

Flow turns back onshore late Sunday night into Monday as a trough
digs down along the West coast. Models are in pretty good
agreement with bringing this trough Southern California Monday
night through Tuesday, likely resulting in widespread showers over
and west of the mountains and a few showers in the deserts.
Precipitation totals with this system do not appear to be too
heavy at the moment, with potential amounts of 0.10-0.50 inches in
the coast/valleys, up to 1 inch in the mountains, and a few
hundredths in the deserts. This will be another cold system, with
snow levels falling to around 3500 feet and a few inches of snow
possible above that level. With the cold air aloft, instability
will increase, with the potential for some thunderstorms. This may
need to be added to the forecast soon. Onshore flow will
strengthen as well, with another round of strong and gusty west
winds in the mountains and deserts possible Tuesday.

The GFS and ECMWF have come into a little better agreement with
the 12Z runs regarding another system for later next week, as they
both show a trough axis moving through northern California
Thursday with a cold front potentially bringing showers on late
Thursday and Friday.

&&

.AVIATION...
232100Z...Coast/Valleys...Mostly clear tonight and Saturday. Through
02Z...west to northwest winds 10-20 kt with gusts to 25 kt.

Mountain/Deserts...West winds 15-25 kt with gusts to 30 kt through
02Z with moderate up/downdrafts and LLWS on the desert slopes.
Decreasing winds overnight. Mostly clear tonight and Saturday.

&&

.MARINE...
A Small Craft Advisory remains in effect through 8 PM for west to
northwest winds 15-25 kt with gusts to 30 kt and combined seas of 7-
10 ft. The winds will decrease overnight, but it will remain breezy
over the weekend with northwest winds of 10-20 kt, highest over the
Outer Waters. Early next week another cold storm will bring more
strong winds and high seas Monday and Tuesday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Freeze Warning from 10 PM this evening to 8 AM PST Saturday for
     San Bernardino and Riverside County Valleys-The Inland
     Empire.

     Frost Advisory from 10 PM this evening to 8 AM PST Saturday for
     San Diego County Valleys-San Gorgonio Pass Near Banning.

PZ...Small Craft Advisory until 8 PM PST this evening 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...Harrison
AVIATION/MARINE...Moede


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