NWS San Diego, CA wrote its last area forecast discussion before the first event at Jan 4, 9:30 PM PST, 1 hour 30 minutes earlier.
Area forecast discussion, Jan 4, 9:30 PM PST, 1 hour 30 minutes before
Area Forecast Discussion
National Weather Service San Diego CA
930 PM PST Wed Jan 4 2023
.SYNOPSIS...
Scattered, mainly light showers will continue overnight. A strong
Pacific storm will bring widespread and heavier precipitation,
beginning just before daybreak Thursday from the northwest,
spreading south and east throughout the day Thursday. Strong and
gusty south to southeast winds will occur ahead of the cold front
overnight and into Thursday. Mostly dry conditions are expected
Friday into Saturday, with increasing chances for more precipitation
early next week.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
As of 9 pm this evening, heavy rain associated with a strong Pacific
front had reached as far south as Point Conception. Scattered light
showers were noted over the LA Basin. The forecast remains on track
this evening, and no significant updates were needed. Scattered,
mainly light showers will move into our northern forecast areas,
including Orange, San Bernardino, and Western Riverside Counties,
with heavier and more consistent rain not likely til very late
tonight, just before daybreak Thursday really, and slowly spreading
south and east during the day Thursday. South to southwest winds
will also increase late tonight and become quite strong Thursday
morning across much of the area. No changes were needed to the
weather headlines this evening, either, including a flood watch for
coastal, valley, and mountain areas of Orange, San Bernardino, and
Western Riverside Counties, where heavy rainfall could trigger
flooding of low-lying and poor drainage areas. Please read for more
details below.
Remainder of the Wednesday Afternoon Discussion:
As we get into late tonight into Thursday morning, rain will become
more widespread across Southern California as the Pacific storm,
currently situated about 500 miles northwest of San Francisco,
continues to advance inland. The cold front associated with this
compact low will push into northern California later today, then
quickly sweep south into Southern California late tonight through
Thursday afternoon. Ample instability behind the cold frontal
passage could result in a few thunderstorms over and west of the
mountains Thursday. The system will weaken as it approaches Southern
California, but will still bring us widespread moderate to locally
heavy rainfall Thursday. The hydrology section below goes into more
detail on timing and forecasted rain amounts.
Gusty south winds will occur ahead of the cold front tonight into
Thursday. Winds will be strongest over the coastal waters, along the
coast, and on the desert slopes of the mountains. At the coast,
winds will be strongest in Orange County, with peak gusts to 40 mph
expected. Winds along the San Diego County coast will peak 30 to 35
mph. The strongest winds will occur over the desert slopes of the
San Bernardino Mountains, with gusts to to 60 mph expected. After
the cold front passes Thursday afternoon, winds will turn
west/southwest and weaken Thursday evening.
Though the rain and the winds are the main weather story for
Thursday, high elevation mountain snow will occur as well. With the
low pressure system tapping into warm atmospheric river moisture,
snow levels are expected to be quite high with this storm, above
7,000 feet during the heaviest precipitation. After the cold front
passes through Thursday afternoon, snow levels will drop to around
6,000 feet. So an inch or two could fall between 6,000 and 7,000
feet, but the higher snow totals will occur above 7,000 feet during
the brunt of the storm. Snow accumulations of 6 to 16 inches will
occur above 7,000 feet. Up to 24 inches of snow could fall on the
highest peaks (above 8,500 feet) of the San Bernardino Mountains.
Precipitation will gradually decrease from northwest to southeast
Thursday evening and night. Weak ridging builds in from the south as
the storm as it exits east, which will bring dry conditions and
warming for Friday and Saturday.
Looking into the extended forecast period, models and ensembles
continue to keep California in a longwave troughing pattern early
next week. Differences exist in the ensembles on the timing and
placement of low pressure systems, so generally sticking with NBM in
the extended period, which has gradually increasing chances for
precipitation Sunday through Tuesday.
&&
.AVIATION...
0530Z...BKN-OVC clouds with varying bases 1000-3500 feet MSL layered
to over 10000 feet will continue through tonight. Mod/hi confidence
in -SHRA through much of the day areas north of SD County with VIS 2-
4SM near higher terrain obscured. Mod confidence on the heavy rain
band with the next storm approaching OC/IE 11-13Z Thu. This will
push south and east throughout the day on Thursday arriving at KSAN
14-16Z. VIS 2-4sm in RA expected with cigs locally below 1000 ft MSL.
Desert airports will remain dry with BKN-OVC clouds at/above 5000
feet MSL with -SHRA occurring during Tuesday morning hours.
Southerly winds in mountains and deserts will increase to 35-50 kts
after 04Z into Thursday morning, producing strong up/downdrafts over
and in lee of ridges across the SBD mts into the high desert.
Southerly winds increasing for coastal areas Thursday morning as
heavy rain band approaches. Mod confidence gusts 25-35 kts VCTY
KSNA, KCRQ, and KSAN.
&&
.MARINE...
A storm Thursday will bring gales and hazardous seas. A Gale Warning
is in effect from 7 PM today through 4 PM Thursday. A west swell of
10-13 feet with a period of 16-19 seconds, will arrive Thursday
afternoon and continue through Friday. Additional high swells,
though not quite as strong, will occur over the weekend and early
next week.
&&
.BEACHES...
A potent west swell of 10-13 feet, with a long period of 16-19
seconds will arrive late Thursday and continue Friday, with the peak
Friday morning. Due to the direction around 270 degrees, the swell
will bring very high surf of 10-16 feet to San Diego County, before
diminishing rather quickly Friday night. Due to the southwest
orientation of the beaches in Orange County, surf will be 8-13 feet
in this area. A High Surf Warning is in effect Thursday afternoon
through Friday in San Diego County and a High Surf Advisory is in
effect for the same times in Orange County.
Also, coastal flooding will be possible with the high tides Thursday
evening and especially Friday morning, where a Coastal Flood
Advisory has been issued.
&&
.HYDROLOGY...
More widespread rain will begin to move in late tonight and continue
through Thursday afternoon. The heaviest and most widespread rain
will begin to move into Orange, San Bernardino and Riverside
counties around 4-5 AM, and into San Diego County around 6-7AM.
Widespread rain will continue through about 2-3PM Thursday, then
gradually taper off Thursday evening and night.
Forecasted rainfall totals tonight through Thursday:
Orange County Coast: 1 to 1.50 inches
San Diego County Coast: 0.50 to 1 inch
Valleys: 0.75 to 1.50 inches
Riverside/San Diego Mtns: 1 to 2 inches
San Bernardino Mtns: 1.50 to 3.00 inches
High Desert: 0.25 to 0.60 inch
Lower Deserts: 0.15 to 0.30 inch
Peak hourly rainfall rates look to range from 0.40 to 0.60" per
hour, which could result in some low-lying road flooding, runoff,
and some minor flows on recent burn scars.
With saturated soils, runoff could be significant for areas north of
San Diego County where a Flood Watch has been issued for Thursday.
The CNRFC currently has the Santa Margarita River at Ysidora and the
San Diego River at Fashion Valley both reaching monitor stage late
Thursday night. Flooding of low-water crossings are likely near
Fashion Valley.
Rain will end from northwest to southeast Thursday evening and
Thursday night.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 2 AM to noon PST Thursday for Orange County
Coastal Areas-Orange County Inland Areas-Riverside County
Mountains-San Bernardino and Riverside County Valleys-The
Inland Empire-San Diego County Coastal Areas-San Diego
County Mountains-San Diego County Valleys-Santa Ana
Mountains and Foothills.
Coastal Flood Advisory from noon Thursday to 6 PM PST Friday for
Orange County Coastal Areas-San Diego County Coastal Areas.
High Surf Warning from noon Thursday to 6 PM PST Friday for San
Diego County Coastal Areas.
Flood Watch from late tonight through Thursday afternoon for
Orange County Coastal Areas-Orange County Inland Areas-
Riverside County Mountains-San Bernardino County Mountains-
San Bernardino and Riverside County Valleys-The Inland
Empire-Santa Ana Mountains and Foothills.
Wind Advisory until to 6 PM PST Thursday for
Apple and Lucerne Valleys-San Bernardino County Mountains.
Winter Storm Warning from 4 AM to 6 PM PST Thursday for San
Bernardino County Mountains.
Winter Weather Advisory from 4 AM to 6 PM PST Thursday for
Riverside County Mountains.
High Surf Advisory from noon Thursday to 6 PM PST Friday for
Orange County Coastal Areas.
PZ...Gale Warning until 4 PM PST Thursday 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/HYDROLOGY...Connolly/JMB
AVIATION/MARINE/BEACHES...Gregoria