Small Craft Advisory 9, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
955 PM PDT Sat Apr 4 2020

.SYNOPSIS...
A slow-moving winter-like storm will bring cold weather and periods
of rain and mountain snow through Wednesday night. Light rain
showers could begin late tonight or Sunday morning, but the most
widespread rain and mountain snow will be on Monday and Tuesday.
Heavy snow will impact the San Bernardino mountains. The storm will
exit the region Thursday followed by dry and warmer weather Friday
and Saturday.

&&

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

A strong winter-like storm with high impact potential will move
across Southern California next week. This deep closed low will
bring periods of rain, significant mountain snow, gusty winds and
unseasonable cold. Temperatures will be 15 degrees below average
Monday through Wednesday. For example, the forecast highs in Palm
Springs M/T/W are 67/67/67. The average high is 82. The forecast
highs in Riverside are 58/61/61. The average high is 73.

Spotty light rain or drizzle from a deep eddy-enhanced marine layer
is possible later tonight into Sunday. But the significant rain will
hold off until Monday when an Atmospheric River (IVT values of 300-
500 kg/m/s) moves in from the Pacific. Moist southwest flow ahead of
the trough off the coast is perfectly aligned to bring moderate to
heavy rain to the southwest slopes of the Orange, San Bernardino and
Riverside County mountains. WPC has a slight risk of excessive
rainfall and a risk of flash flooding in these areas Monday and
Monday night. Rain amounts of 2-4" are possible on the southwest
facing slopes of the San Bernardino Mountains through Monday night.

Tuesday and Wednesday will be cold days with scattered showers and
perhaps a few thunderstorms just about anywhere as the cold core low
slowly meanders from Pt Conception Tuesday to near Yuma, AZ on
Wednesday. The airmass will be somewhat unstable with 500 mb
temperatures falling to -25C and -28C during that time. Several
shortwaves in the broad cyclonic flow will add to the instability.
Snow will be heavy at times above 5500 FT in the San Bernardino and
Riverside County mountains Monday through Wednesday night where a
Winter Storm Watch will be in effect.

By Thursday, there is quite a spread in the model solutions, but
both the ECMWF/GFS 00Z runs have the storm center to the east of the
forecast area, suggesting an end to most precip west of the mts, but
with wrap-around possibilities, mts and deserts could still be
getting significant amounts of rain/snow. So far, next weekend looks
dry and warmer.

&&

.AVIATION...
050330Z...Coast/Valleys...BKN-OVC low clouds with bases mostly
ranging between 2000-3000 ft MSL will continue to fill into the
valleys overnight. Visibility will remain 6+ miles at TAF sites. Low
clouds will lift to around 3000 ft MSL after 16Z. Partial clearing
is possible in the valleys after 18Z, but low confidence in clearing
at the coast.

Mountains/Deserts...Mostly clear with unrestricted vis through
Sunday morning. Areas of low clouds and fog will impact the
mountains overnight, dropping visibility down to 1 mile at times.
Partial clearing is likely in the mountains after 18Z. Locally gusty
NW winds possible Sunday afternoon through Sunday night at KPSP/KTRM
with gusts to 20-25 kts.

&&

.MARINE...
An incoming storm system will produce showers and gusty winds over
the coastal waters starting Sunday afternoon. There is a slight
chance of thunderstorms Monday night, and again Wednesday afternoon
and evening. The cold front will produce gusty southwest winds 20-25
kt over the coastal waters on late Sunday night through Monday
evening, which could be hazardous to small craft.

&&

.HYDROLOGY...

A blend of model QPF solutions was used to achieve our current
rain and snowfall outlook for this event. Hopefully the disparate
model track projections for this storm will coalesce, giving higher
confidence for parsing out the most significant precip and timing
impacts. For now, Precip totals Sunday night through Thursday night
are as follows:

High deserts...1-1.5 inches. Isolated thunderstorms could produce
localized excessive runoff, otherwise most of the rain will be of
the light to moderate variety.

Low Deserts: 0.5-1.00 inches. No significant runoff issues forecast
at this time, although inflow into the Whitewater River will need to
be monitored and may cause low-water crossing impacts in the
Coachella Canal by Tuesday.

Coast/Valley: 1.00-2.25 inches. Some urban and small stream flooding
issues are possible later Monday through Tuesday morning when the
more widespread and heavier rainfall is expected.

Mountains: 3.00-6.00 inches with isolated heavier amounts possible
on the coastal slopes of the San Bernardino Mts. Less expected south
through Riverside and the San Diego County Mts where less upslope
enhancement is likely. A very significant April snowfall event looks
likely in the San Bernardino Mts and the San Jacinto Mts where feet
of snow are likely to fall above 6000 FT. Small streams may overflow
their banks for a time later Monday into Tuesday on the coastal
slopes when the most widespread moderate rainfall is expected.

River flooding is not expected, but flow will be higher than normal,
even for the Mojave. The Whitewater will need to be monitored. For
the San Diego River, any amounts greater than currently forecast
would cause sharp rises and will be monitored.

.SKYWARN...
Skywarn activation may be requested Monday. H

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Watch from Sunday evening through late Wednesday
     night for Riverside County Mountains-San Bernardino County
     Mountains.

PZ...NONE.
&&

PUBLIC...Moede
HYDROLOGY...10
AVIATION/MARINE...Connolly