Area Forecast Discussion
Issued by NWS San Diego, CA

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793
FXUS66 KSGX 262115
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
114 PM PST Sun Feb 26 2017

.SYNOPSIS...
Unsettled weather, will continue into Monday morning, with periods
of light rain and mountain snow. On Monday, a deepening trough in
the upper atmosphere will draw more moisture into the region with
another round of precipitation. Greatest across southern San Diego
County. There will be gusty westerly winds over the mountains with
strongest gusts tonight and Monday. Slightly warmer Tuesday. Then,
fair and warmer Wednesday through Friday with periods of weak
offshore flow.

&&

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

Skies were mostly cloudy across SoCal early this afternoon, along
with some scattered shower activity. NWS radar indicated it was
light and moving rapidly east. The greatest amounts through noon PST
were aligned along the coastal slopes/foothills of the San
Bernardino/Riverside County Mts, where up to 1/3 inch has been
recorded. Most reporting sites farther south and west had one-tenth
of an inch or less. Some light snow fell this morning in the San
Bernardino and Riverside County mts as well.

Surface pressure gradients were trending onshore at 4-7 MBS KSAN to
the deserts, with peak wind gusts of 35 to locally 50 MPH through
wind-prone mtn areas. A Wind Advisory is in effect for the higher
mtn and desert slope areas through Monday evening.

A weakening low pressure area is exiting the region this afternoon,
while a larger trough drops SE along the West Coast and interacts
with a weak upper-level low and moisture plume over the EastPac.
These systems will team up to bring enhanced precip to extreme SW
CA, and especially extreme NW Baja CA Mon/Mon night. Please see
QPF/Hydrology discussion below.

Continued cool on Mon, then slightly warmer with drying on Tue as
the upper-level trough slides east. Fair, dry, and warmer for the
balance of the work week as heights/thickness build across the West
and weak offshore flow develops.

&&

.QPF/HYDROLOGY...
Rainfall amounts over the past 12 hours have been largely under one-
quarter inch except for a small area along the coastal slopes/
foothills of the northern mtn areas, so little run-off is likely.
Additional rainfall overnight looks to be similar with little impact
expected.

The greatest potential for higher accumulations comes Monday.
Confidence in this event is still impaired by a complex interaction
of low pressure offshore and a deepening trough to the north. This
has resulted in variations in model QPF. The ECMWF model has
consistently kept southern San Diego County in an area of enhanced
precip, while other models have kept this area largely south of the
International Border. The 12Z GFS/NAM 12 have moved the bullseye
farther to the north, while still keeping the heaviest precip south
of the border.

Using a blend of QPF from the 00Z/12Z runs of the CMC/NAM12/GFS/
ECMWF/WRFEMS...it still appears from one to three inches of rain
will soak much of the Tijuana watershed from 06Z Mon-06Z Tue. This
amount of rain would have impacts downstream across SW San Diego
County Mon night through Tuesday. Moderate flows could develop by
late Mon or Tue in the Tijuana river, with possible flooding in low-
lying flood-prone areas near the International Border if this rain
materializes.

For San Diego County, current projections are from 1/3 inch for far
NW sections, to nearly 2 inches southern mountains. This may create
some urban and small stream flooding issues by Monday evening. Given
the substantial gradient of QPF just south of the border, any
northward shift of this expected rainfall could result in more rain
and additional runoff and flooding issues for southern border
rivers, streams and creeks including the San Diego, Cottonwood, and
Tecate.

&&

.AVIATION...
262053Z...Multiple BKN-OVC cloud decks within the 2500-8000 ft msl
layer (with scattered showers) over and west of the mountains, SCT-
BKN clouds decks within the 5000-8000 ft msl layer in the deserts,
local vis/cigs 3-5 sm/1500 ft agl, and mountain obscuration of
coastal slopes in clouds/showers, will continue through 27/0000 UTC
with showers gradually decreasing. 27/0000-1500 UTC, multiple BKN-
OVC cloud decks within the 1500-7000 ft msl layer over and west of
the mountains with mountain obscuration of coastal slopes
continuing, and areas of vis 3-5 sm. There is another burst of
shower activity possible for San Diego county after 27/0600 UTC,
which could bring local vis/cigs 1-2 sm/800-1000 ft agl. Meanwhile,
SCT clouds 4000-7000 ft msl and P6SM vis in the deserts.

W winds 15-25 kt with gusts 35-45 kt will continue over mountain
ridges, through/below passes/canyons, and along desert slopes,
resulting in mod-stg uddfs/llws over and E of the mountains, with
strongest winds and associated uddfs expected after 27/0300 UTC.

&&

.MARINE...
West-northwest winds 20-25 kt possible mainly in the outer waters
late Monday night and Tuesday morning, which could result in
conditions hazardous to small craft. Otherwise, no hazardous marine
weather conditions expected through Friday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 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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