Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 252116
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
116 PM PST Sat Feb 25 2017

.SYNOPSIS...
Dry this evening with increasing high clouds ahead of a couple of
weak low pressure systems that will bring more clouds, cooling, and
scattered, mostly light showers at times through Monday night.
Heavier rainfall is possible for San Diego County later Monday. Some
light snow accumulations are possible at mountain resorts as well.
Dry, but quite cool again on Tuesday. Then fair and warmer into next
weekend with periods of offshore flow.

&&

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

Skies were sunny early this afternoon, but high clouds were
increasing. At Noon PST, temperatures were a few degrees over values
observed at midday yesterday. Surface pressure gradients to the
deserts were still very weak onshore. Only the windiest mtn areas
had gusts of 25 MPH or more.

High clouds will continue to thicken this afternoon/evening, which
will help keep overnight lows above values observed last night.
Westerly winds will be increasing over the mountains and through the
San Gorgonio Pass. Strongest winds are expected Sunday night through
Monday, when peak gusts on the ridges and desert slopes could reach
65 MPH or more. A Wind Advisory has been posted. Please see
LAXNPWSGX for details.

A compact low pressure trough off the NoCal Coast is dropping SE
this afternoon and forecast to swing through SOCal on Sunday. The
system is forecast to weaken as it moves rapidly inland, but should
hold enough of its identity to manage some light showers as it moves
rapidly east through the day. It will be quite cool as well, with
daytime high temperatures forecast to be 5 to 10 degrees below
average from the coast to the valleys, and even further below normal
in the mts.

The forecast becomes more challenging as a second trough drops SE
just inland on Monday and interacts with a deeper moisture stream
and remnant low offshore. Several model runs are now advertising a
heavier precip potential for, mainly far NW Baja CA and San Diego
County. This one is a close call, but for now, confidence is high
enough to increase POPS for later Mon/Mon night. PW should approach
one inch, so rainfall amounts could be substantial over southern
San Diego County for a time late Mon. Please see QPF/Hydrology
section below.

The trough swings east on Tue, allowing heights/thickness to recover
as sfc winds turn offshore. This should provide an extended fair,
dry, and warmer period beginning Wed, continuing through the end of
the week.

&&

.QPF/HYDROLOGY...
Light rain showers should begin to break out later tonight on the
San Bernardino Mtn coastal slopes, become more widespread across
northern portions of the forecast area by early morning and spread
south through midday Sunday. Models show only modest amounts with
little runoff expected.

Some light showers are possible then at times through Monday
morning, but again, rainfall totals should be very light, ranging
from a few hundredths of an inch, to one-quarter inch in the wettest
spots.

The greatest potential for higher accumulations comes later Mon. The
ECMWF model has consistently been the wettest, but the CMC has a
large rainfall bullseye near Tijuana by Mon afternoon as well, and
the 12Z/18Z WRFEMS/NAM runs are wetter across San Diego County. The
brunt of this rainfall has always looked to occur over NW Baja as
higher PW air over the EastPac is tapped, but trends are edging this
area northward. Initial QPF is now for max amounts of around one
inch southern San Diego County late Mon, decreasing northward, with
more across the Mexican portion of the Tijuana Basin. If this
remains on track, larger flows and some flooding issues will be
possible throughout the Tijuana Basin. We will be particularly
watching model trends in this area over the next 24 hours to gauge
possible impacts.

&&

.AVIATION...
252051Z...P6SM vis and increasing clouds AOA 15000 ft msl through
26/0000 UTC, with some SCT-FEW clouds 2000-3000 ft msl possibly
developing in the coastal areas. 26/0000-0900 UTC, BKN cloud decks
developing in the 2000-6000 ft msl layer over and west of the
mountains and also AOA 10000 ft msl all areas, with primarily P6SM
vis continuing except for mountain obscuration of coastal slopes due
to clouds. After 26/0900 UTC, showers moving into the region (mainly
over and west of the mountains) with multiple OVC-BKN cloud decks in
the 2000-15000 ft msl layer over and west of the mountains and BKN-
SCT cloud decks in the 5000-15000 ft msl layer in the deserts. Due
to showers, expect areas of vis 3-5 sm with local vis 1-2 sm, and
cigs locally dropping to 1000-1500 ft agl, with mountain obscuration
continuing.

After 26/0000 UTC, W winds strengthening, becoming 15-25 kt with
gusts 35-45 kt by 26/0900 UTC over mountain ridges, through/below
passes/canyons, and along desert slopes, resulting in mod-stg
uddfs/llws over and E of the mountains.

&&

.MARINE...
Latest model guidance suggests that a trough passing through the
region could bring 20-25 kt winds mainly to the outer waters Monday
night into Tuesday morning, which could result in conditions
hazardous to small craft.


&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 4 PM this afternoon to 10 PM PST Monday for
     Riverside County Mountains-San Bernardino County Mountains-
     San Diego County Mountains-San Gorgonio Pass Near Banning.

PZ...None.

&&

$$

PUBLIC...JAD
AVIATION/MARINE...Harrison


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