Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 210400
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
800 PM PST Tue Feb 20 2018

.SYNOPSIS...
Cold again tonight, by Southern California standards, with
widespread freezing to near freezing temperatures inland.
Certainly another night to protect sensitive plants and bring pets
inside. Increasing cloud cover and below average high
temperatures again Wednesday. A few light very showers are
possible, especially in San Diego County, Wednesday afternoon and
evening. An active pattern will continue into next week, with
occasional chances for light shower activity and below average
temperatures

&&

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

At 800 PM...A deep longwave trough was entrenched over the
Western U.S., with several shortwave troughs rotating through
their large cousin. The airmass over SoCal had dried out
considerably since this time yesterday, especially below 700 mb
(per the 00Z NKX sounding). Surface dewpoints ranged from the
single digits in the mountains/deserts to the low 30s at the
beaches. Skies were crystal clear and surface winds were light and
variable.

Tonight will be another chilly one, with the clear skies and low
dewpoints allowing for strong radiational cooling. This will
likely bring 4-6 hours of freezing to near freezing temperatures
to portions of the Inland Empire and eastern San Diego County
Valleys. The near freezing temperatures should extend into inland
Orange County, the Coachella Valley and Borrego area as well. If
you arn`t into the cold in the valleys, it could be worse, as the
Big Bear area will see widespread lows in the single digits
Wednesday morning.

Temperatures should rebound after daybreak, with all but the
mountains returning quickly to values above freezing. A weak
upper level short wave (over San Francisco this evening) should
rotate around the base of the longwave trough during the day. This
will bring increasing clouds through the day and even a chance
for some very light showers (mainly in SD County) by Tuesday
afternoon/evening. Any accumulations should be very light, just a
couple hundredths of an inch. Light snow showers/flurries will
also be possible in the mountains above 3,500 ft. Only trace
amounts of snow possible. Increased cloud cover and more moisture
should temper low temperatures Wednesday night, limiting
frost/freeze potential in the valleys.

The longwave trough should hang around over the Western U.S. into
next week. As such, temperatures are expected to remain below
seasonal averages along the West Coast, with periodic weak
shortwaves bringing the potential for rain/mountain snow showers.
No major storms are in the pipeline at the moment, but at least it
is an active "ridge free" pattern.

&&

.AVIATION...
210315Z...Mostly clear skies with unrestricted visibility through
early Wednesday morning. Increasing clouds over/west of the
mountains with bases 2500-3500 ft MSL and tops varying between 5000-
8000 ft MSL after 12Z. ISO-SCT light showers developing Wednesday
afternoon and evening. Coastal mountain slopes will become obscured
in clouds/fog/precip.

&&

.MARINE...
No hazardous marine conditions are expected through early Thursday,
but hazardous winds and seas are likely again Thursday afternoon
through Friday. Winds and seas will diminish through the weekend.

&&

.SKYWARN...
Skywarn activation will not be needed today.

&&

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

     Frost Advisory from 10 PM this evening to 8 AM PST Wednesday for
     Coachella Valley-Orange County Inland Areas-San Diego
     County Deserts-San Diego County Valleys-San Gorgonio Pass
     Near Banning.

PZ...NONE.
&&

$$

PUBLIC...Albright
AVIATION/MARINE...SS



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