Area Forecast Discussion
Issued by NWS San Diego, CA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS66 KSGX 250433 AAA
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Sat Oct 24 2020

.SYNOPSIS...
A strong upper trough moving in from the north will produce
significant changes across Southern California through early next
week. Light showers will impact areas from the coast to the
mountains beginning late tonight and lasting through Sunday night.
Showers will be most numerous over San Diego County from late
Sunday afternoon through evening. Gusty west winds will impact
the mountains and parts of the deserts tonight through Sunday
evening, before Santa Ana winds rev up late Sunday night and last
through Tuesday. Strong north to northeast winds, potentially
damaging, along with extremely low humidity will create critical
fire weather conditions for the San Bernardino and Riverside
County mountains, the San Gorgonio Pass, the Inland Empire to
inland Orange County. High pressure aloft will build in from the
west during the middle to latter part of next week, returning
fair and warmer conditions to the region.

&&

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

-Forecast Highlights-

* Light showers coast to mountains late tonight-Sunday night
* Santa Ana Winds rev up Sunday night and last through Tuesday
* Damaging winds and critical fire weather concerns!

Evening Update:

The only change this evening was after examining the latest data,
the potential for strong Santa Ana winds reaching the coastal
sectors of Orange County is at least moderate, for wind gusts of 35
to 40 mph at times during the day Monday. Therefore, we decided to
add a wind advisory for that time frame for Coastal Orange County.
Otherwise, the forecast remains right now track. At this time there
has not been any rain reported yet, however some very weak returns
are starting to show up within the bands of mid level cloudiness
streaming across the SoCal Bight and also off the coast of Northern
Baja. Right now the forecast shows chances for showers beginning
after midnight tonight, and this seems on target, as we are seeing
that increasing mid level moisture now, with decks lowering also at
regional airport sites. For Sunday, scattered mainly light showers,
with the best late Sunday afternoon and evening.

Previous Forecaster Discussion, issued at 245 PM PDT:

A wild weather pattern developing across our region as a vigorous
upper trough is diving southward from the north. This trough will
phase with another weaker one over the Pacific and amplify.
Moisture convergence ahead of this trough will result in the first
measurable rainfall of the season from the coast to the mountains.

Rainfall amounts remain uncertain but the most likely amounts will
be in the 0.10-0.20 inch range with localized amounts to near 0.50
inch along the coastal mountain slopes. Highest amounts in San Diego
County with better and longer lasting moisture exposure. GFS and EC
ensembles still show quite a spread so this could go lower or
higher. Low impact. Moderate confidence.

Gusty west winds will impact the desert mountain slopes and parts of
the deserts ahead of the trough. Gusts mainly 30-40 mph but the
isolated wind-prone locales could see gusts to about 50 mph.

Once the trough axis slides east, winds will abruptly turn north-
northeast, commencing the Santa Ana wind event for Southern
California. This will occur just after midnight Sunday night with
winds strengthening through Monday morning. The strongest period of
winds, potentially damaging, will occur towards dawn Monday and
continue through the afternoon, impacting the San Bernardino and
Riverside County mountains, the Inland Empire, to inland parts of
Orange County, including the Santa Ana Mountains. This is where the
Red Flag Warning was just issued as extremely dry air will be
advected into the area, and even dry further on downslope
compression. See fire weather discussion for more details.

The wind component of this Santa Ana will be more northerly and this
raised uncertainty in just how strong winds may get for the San
Diego County mountains and inland valleys. The low level jet will
remain over the water and over SE California, and may just not be
optimal to produce strong winds over SD County. For this reason, the
main potential impacts of this event look to occur north of San
Diego County.

Note that the Santa Ana will drive colder and extremely dry air into
SoCal. This will really lower temperatures area wide through Monday
with cold temperatures in the mountains and high desert. Lows in the
lower to mid 30s are forecast for the High Desert Monday and Tuesday
morning. Tuesday looks the coldest as winds relax a tad.
Compressional warming though will lead to warming west of the
mountains by Tuesday as the Santa Ana winds continue.

The air will become extremely dry with dewpoints falling even below
zero from the mountains and deserts and even parts of the inland
valleys, especially the Inland Empire.

Upper level high pressure will build in from the west during the
middle to latter part of next week and this will lead to a return of
fair and warmer conditions returning to the area.

&&

.AVIATION...
250330Z...Coast/Valleys/Mountains...BKN/OVC low clouds with bases
2500-3500 feet MSL and tops to 4500 ft MSL will continue through
tonight and Sunday morning, with scattered -RA developing after 08Z.
Most vis will be above 6 miles below the cloud base through Sun
morning, but areas of higher terrain will be obscured in clouds at
times.

Deserts...SCT-BKN high clouds at/above 20000 feet MSL, slowly
clearing tonight. Areas of west winds will continue on the desert
mountain slopes and north/east of mountain passes, becoming stronger
Sunday. Expect mdt up/downdrafts over and east of the mtns through
Sunday.

&&

.MARINE...
No hazardous marine conditions are expected through Sunday.
Northeast winds occasionally gusting 25 to 30 knots Monday and
Monday night could create conditions hazardous to small craft.

&&

.FIRE WEATHER...

...Red Flag Warning 2 AM Monday Through 6 PM Tuesday for Strong
Winds and Low Relative Humidity for the San Bernardino and Riverside
County Mountains, the Inland Empire, San Gorgonio Pass, and Inland
Orange County...

Santa Ana winds will begin after midnight Sunday night,
strengthening and lasting through Tuesday. The strongest north-
northeast winds will occur from dawn Monday through Monday
afternoon. Northeast winds of 20 to 40 MPH with isolated gusts 60 to
70 MPH will drive extremely dry air southwestward with relative
humidity tumbling into the 5-10% range for all but the coast. The
strongest winds will impact areas below the Cajon Pass stretching
into inland Orange County. The winds are expected to ease a bit on
Tuesday, but will still be strong and gusty with little overnight
humidity recovery.

The strength and areal coverage of wind is highly uncertain for San
Diego County. Highest winds could be stuck on the ridges and very
localized. Right now, strong winds are not anticipated over San
Diego County, but we will continue to assess wind potential across
this area. Still locally gusty winds and low relative humidity will
lead to enhanced fire weather conditions Monday through Tuesday.

&&

.HYDROLOGY...
Light showers will develop late tonight and last through Sunday
night, impacting areas from the coast to the mountains. Rainfall
amounts remain uncertain but the most likely amounts will be in the
0.10-0.20 inch range with localized amounts to near 0.50 inch along
the coastal mountain slopes. Highest amounts in San Diego County
with better and longer lasting moisture exposure. GFS and EC
ensembles still show quite a spread so this could go lower or
higher. Max hourly rainfall rate forecast to be 0.10/hr. Low impact.
Moderate confidence.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 2 AM Monday to 6 PM PDT Tuesday for Orange
     County Inland Areas-Riverside County Mountains-Including
     The San Jacinto Ranger District Of The San Bernardino
     National Forest-San Bernardino County Mountains-Including
     The Mountain Top And Front Country Ranger Districts Of The
     San Bernardino National Forest-San Bernardino and Riverside
     County Valleys  -The Inland Empire-San Gorgonio Pass Near
     Banning-Santa Ana Mountains-Including The Trabuco Ranger
     District of the Cleveland National Forest.

     High Wind Warning from 2 AM to 5 PM PDT Monday for Orange County
     Inland Areas-San Bernardino County Mountains-San Bernardino
     and Riverside County Valleys-The Inland Empire-Santa Ana
     Mountains and Foothills.

     Wind Advisory from 2 AM to 11 PM PDT Monday for Coachella Valley.

     Wind Advisory from 2 AM to 5 PM PDT Monday for Orange County
     Coastal Areas.

PZ...Small Craft Advisory from 5 AM Monday to 5 AM PDT Tuesday 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...Gregoria/JMB
AVIATION/MARINE...PG



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.