NWS San Diego, CA wrote its last area forecast discussion before the first event at Dec 27, 2:00 PM PST, 6 hours earlier.
Area forecast discussion, Dec 27, 2:00 PM PST, 6 hours before
Area Forecast Discussion
National Weather Service San Diego CA
200 PM PST Sun Dec 27 2020
.SYNOPSIS...
Mostly cloudy and cooler conditions will prevail this afternoon
ahead of an approaching Pacific storm system. Rain with mountain
snow will begin late tonight and continue at times through Monday
night. The rain showers will be briefly heavy, and snowfall will be
locally heavy in the mountains late tonight through Monday night
with difficult travel conditions. Gusty winds will occur from the
mountains into the deserts and also at times along the coast Monday
into Monday night. The storm will move east Tuesday, followed by
gusty, dry, northerly winds Tuesday night and Wednesday, with
possible strong northeast winds Friday. Temperatures will mostly be
below normal until around the New Year when they will recover to
near normal.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Mostly thick high clouds continued moving over the region, and the
clouds played a big role in temperatures mostly only being around 60
west of the mountains, though cooling aloft played a role as well.
The winter storm system will bring precipitation starting late
tonight, mostly after midnight, with the first wave a little ahead
of what was previously expected, mostly before sunrise Monday. Thus,
widespread precipitation should begin by sunrise Monday, then taper
a bit before another wave and great instability occurs Monday
afternoon, and then the trough axis Monday evening. Best
available precipitable water amounts continue to be around 0.75"
both very early Monday morning and Monday evening, though no
atmospheric river is expected as IVT values barely top 250 kg/m/s
and briefly. Some of the showers may be heavy as they move rapidly
from west to east, with a slight chance of thunderstorms. HRRR and
local WRF both show maximum hourly precipitation rates remaining
mostly at/below 0.40" per hour, and that combined with the low snow
levels, will keep the debris flow risk relatively low, but
definitely not zero, in burn areas. Above 4500 feet in the
mountains, snow will be falling and heavy accumulations are likely
into Monday night above about 5000 feet, with over a foot likely in
some locations at/above 7000 feet. Gusty southwest winds will
occur at times late tonight through Monday evening, with local gusts
in the mountains and desert slopes around 55 MPH, and even a few
gusts over 25 MPH could occur at the coast at times. A Winter Storm
Warning is in effect for this storm. For details on expected
rainfall, please see the Hydrology discussion below.
A transient ridge will build over the West in the wake of the storm,
turning our winds back offshore late Tuesday into Wednesday. These
winds will be dry and gusty, but relatively cool, keeping temps a
bit below average. Another, weaker trough will dig over the West
later in the week, passing by to the NE. Winds will turn briefly
back onshore around Thursday before turning back offshore New
Years Day when Santa Ana conditions will be possible. Temperatures
should increase a bit starting Friday under the ridging, with no
strong signals after that as we will be under a more progressive
weather pattern with fewer strong ridges and troughs. Dry weather
will prevail Tuesday through Saturday, with small precipitation
chances early the following week when troughs pass by to the north
in that progressive west-to-east pattern.
&&
.AVIATION...
272130Z...Coast/Valleys...BKN clouds at/above 10,000 ft MSL will
prevail through this afternoon. Low clouds will begin to spread back
inland after 00Z Monday, with bases around 1500-2500 feet MSL.
Isolated showers could begin as early as 06Z Monday, becoming more
widespread overnight and into Monday morning. Lower vis is possible
during heavier showers.
Deserts/Mountains...Increasing clouds at/above 10,000 ft MSL and
unrestricted vis through this evening. Increasing mid and some low-
level clouds tonight, with mountain snow expected and vis
restrictions affecting obscured coastal slopes.
&&
.MARINE...
An approaching storm system will bring scattered showers starting
early tonight, with periods of rain through Monday night.
West winds associated with this system will arrive early Monday
morning, with gusts on Monday afternoon that will near 30 knots
across the Outer Waters, and up to 25 knots for the Inner Waters.
Combined seas 7 to 12 feet are expected, highest across the Outer
Waters late Monday and through Tuesday. In addition, there will be a
chance of thunderstorms on Monday, along with the possibility of a
few waterspouts. A Small Craft Advisory is in effect for all of the
coastal waters beginning at 3 AM Monday and lasting through noon
Tuesday. Conditions will improve by Wednesday.
&&
.BEACHES...
Building west swell due to an approaching storm system will raise
surf across area beaches today, with widespread elevated surf
conditions likely, and a few sets to 7 feet possible in San Diego
County. Surf will increase on Monday, peaking between Monday
afternoon and Tuesday, with sets to 8 feet. In southern San Diego
County, sets may reach 9 to 10 feet Monday and Tuesday. There will
also be a chance of thunderstorms with lightning at the beaches on
Monday. A High Surf Advisory is in effect through 10 AM Wednesday.
&&
.HYDROLOGY...
The Pacific storm is slightly faster than earlier expected, so
precipitation will likely become widespread between around 1 AM and
4 AM Monday. Precipitation will generally be widespread and showery
through the day Monday and Monday evening. There is sufficient
instability and cold air for low topped thunderstorms. Rainfall
rates could be briefly heavy, up to around 0.40 inch per hour, and
some showers will contain small hail. The more steady rainfall and
heavier accumulations will be on the coastal mountain slopes where
an extended period of nearly saturated, upslope flow will add up
over time. The showers/tstorms should be moving along at 25-30 MPH.
This will limit rainfall accumulation in any one spot.
Given the dry conditions, the modest atmospheric moisture
availability, the showery nature of this event, and the speed at
which the showers will be moving, significant runoff will be
limited. Those areas most susceptible to minor flooding will be
urban, and with poor drainage, and where heavy showers repeat or
train over the same area. Mainstem river and small stream flooding
is not expected. Even the coastal slope areas, where storm totals
could reach two inches or more, should be able to absorb much of
this over time. The stark exception of course is fresh burn scars
which are much more susceptible to runoff and debris flows if
rainfall rates become heavy, which there is a small chance of and
will be monitored.
This system will remain progressive, limiting overall precipitation
totals. Latest storm total precipitation expected....0.60-1.20 inch
west of the mountains...1.00 to 2.50 inches mountains...and 0.15 to
0.35 deserts. Locally higher amounts are possible in spots,
especially the southwest facing mountain slopes.
With snow-levels down to 4000-4500 FT, snowfall should be a major
impact for travelers in the mountains. Even the higher mountain
passes could get icy for a time. For now 3-6 inches of snow looks to
be widespread above 4500 FT, with areas of 8-12 inches or more above
5500 FT, and 12+ inches at/above 7000 FT, highest in San Bernardino
County and the San Jacinto Mountains.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning from 5 AM Monday to 4 AM PST Tuesday for
Riverside County Mountains-San Diego County Mountains.
Winter Storm Warning from 1 AM Monday to midnight PST Monday
night for San Bernardino County Mountains.
High Surf Advisory until 10 AM PST Wednesday for Orange County
Coastal Areas-San Diego County Coastal Areas.
PZ...Small Craft Advisory from 3 AM Monday to noon PST 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/HYDROLOGY...Maxwell
AVIATION/MARINE/BEACHES...Connolly