Winter Weather Advisory 2, NWS San Diego, CA

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

...Significant rainfall likely Monday and Monday night for
Southern San Diego County and Northern Baja California...

.SYNOPSIS...
A deepening low pressure trough in the upper atmosphere will draw
more moisture into the region with another round of precipitation,
first Monday morning, with another round Monday night. Greatest
precipiation will be across southern San Diego County. There will be
gusty westerly winds over the mountains with strongest gusts tonight
and Monday. Then becoming slightly warmer Tuesday. Then, fair and
warmer Wednesday through Friday with periods of weak offshore flow.

&&

.EVENING UPDATE...

Significant precipitation is now looking very likely Monday, with
the first round beginning early Monday morning, with a heavier surge
likely to arrive Monday evening and overnight. There will be the
potential for flooding in Southern San Diego County and within the
Tijuana River Basin. The latest high-res models are coming into much
better agreement on the track of the deep moisture plume across
Northern Baja and San Diego County. Rainfall amounts of 2+ inches
will be possible along the favorable upslope portion of the San
Diego County mountains, with around one inch in the coastal and
valley areas, mainly for the southern portion of San Diego County.
The ongoing forecast has this trend well covered and no significant
changes were made this evening.

&&

.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...

270530Z...Areas of BKN-OVC stratus with bases mostly 2000-3500 ft
MSL will continue through Monday night with coastal
foothill/mountain terrain obscured at times. Showers will occur and
gradually increase after 09Z Mon and peak late Monday
afternoon/evening, especially San Diego County, resulting in local
vis 2-4 miles and local cloud bases around 1000 ft MSL. In contrast,
desert areas will mostly just have SCT-BKN clouds with bases 4000-
8000 ft MSL and vis mostly unrestricted.

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

&&

.MARINE...
West-northwest winds 20-25 kt will likely occur in the outer waters
late Monday night and Tuesday morning, which will result in
conditions hazardous to small craft. A small craft advisory will be
in effect from 9 PM Mon-9 AM Tue for the outer waters (30-60 NM
out). Otherwise, no hazardous marine weather conditions are 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...JMB/JAD
AVIATION/MARINE...Maxwell