Winter Weather in California, May 1 to 2, 2018

Area Forecast Discussion
National Weather Service San Diego CA
923 PM PDT Tue May 1 2018

.SYNOPSIS...
A low pressure system over the southwest states will bring cooler
weather and periods of showers into Wednesday. Some snow
accumulation is possible in the San Bernardino and San Jacinto
Mountains through Wednesday morning. For Thursday through Saturday,
high pressure will bring dry weather with a warming trend. Then
cooler Sunday through Tuesday as onshore flow spreads cooling
inland.

&&

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

...RRM updated for latest storm total rainfall...
...Record report issued for numerous record low maximums today...

Clouds were widespread from the mountains to the coast this evening.
At 9 PM PDT...Radar showed scattered showers, mostly light, over SW
San Diego and San Bernardino County. The 00Z Miramar sounding had
the marine inversion level near 7300 FT MSL. Instability was
marginal and PW stood at 0.57 inch. Greatest precip totals (0.5 to
0.90 inch) over the past 24 hours, were logged on the coastal
slopes. Some higher amounts may have occurred there, but ALERT gage
reports from San Bernardino County were not available. At lower
elevations, most areas had less than one-tenth of an inch. Surface
pressure gradients were 7-8 MBS KSAN to the deserts, and were
supporting gusty winds over the mts/deserts. Passes were strongest
with peak gusts 45 to 55 MPH.

The 00Z NAM has the disturbance moving through the Bight nailed,
based on water vapor imagery. It will pass south in the next couple
of hours and has only generated a few scattered showers. It has
another disturbance slipping south just inland overnight as
northerly flow develops around the back side of the upper low
centered near Las Vegas. The latest 00Z WRFEMS run has dramatically
reduced the forecast precip over the region tonight. But the 00Z
run of the NAM and the latest HRRR keep showers going over the
San Bernardino Mts and even coastal San Diego County tonight
with this feature, so even though amounts are light, the threat of
rain and mountain snow continues into Wednesday morning although
snowfall amounts reaching Advisory level look doubtful at this
point. The snow level should be 5000 to 5500 FT.

No changes to the current forecast for now despite the lower
precip amounts now in the forecast. LAXQPSSGX has been updated.

From previous discussion...

One last shortwave will move across our area Wednesday morning.
Precipitation is expected to increase in coverage, and will be a
little heavier due to an increase in mid-level moisture. Snow levels
will hover around 6500 ft today and dip to 5500 ft tonight. Snow
accumulations will mainly be 1-3 inches above 5500 ft, however up
to 6 inches are possible where stronger showers develop. A Winter
Weather Advisory has been issued for areas above 5500 ft in the
San Bernardino and Riverside County Mountains. High-res models
show showers decreasing in coverage by Wednesday afternoon, and
ending by late Wednesday evening.

High pressure then builds over the region through Saturday. High
temperatures will increase to near normal Thursday, then jump to
5-10 degrees above normal Friday and peak at 5-15 degrees above
normal on Saturday. Weak inland offshore flow will limit the
inland push of the seabreeze Friday, then onshore flow will
increase through early next week as a weak upper-level trough
moves across the Pacific Northwest Sunday into Monday. This will
help lower highs back to seasonal normals by Monday. The ridge
will create a shallow marine layer this weekend, with minimal
stratus coverage along the coast. Low clouds will push more into
the valleys early next week as the marine layer deepens.

&&

.AVIATION...
020330Z...Coast/Valleys/Coastal Mountain Slopes...Low cloud layers
with bases 1500-3500 feet MSL and tops to 6500 feet MSL will
continue through Wednesday evening. SCT -SHRA will continue through
15Z, then taper off during the afternoon and evening. VIS may lower
to 2-4SM in heavier showers, with cloud bases down to 800 feet MSL
at times. Mountain obscurations likely in clouds/fog/precip through
Wednesday evening.

Desert Mountain Slopes/Deserts...ISO showers possible over the
deserts through Wednesday afternoon. Otherwise SCT-BKN clouds 5000-
10000 ft MSL and unrestricted vis.

&&

.MARINE...
Showers will continue across the coastal waters through early
Wednesday afternoon. Otherwise, no additional marine weather hazards
are expected through Sunday.

&&

.SKYWARN...
Skywarn activation is not requested at this time, but may be
requested later this afternoon. Weather spotters are encouraged
to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory from 8 PM this evening to 11 AM PDT
     Wednesday for Riverside County Mountains-San Bernardino
     County Mountains.

PZ...NONE.
&&

PUBLIC...10/JJT
AVIATION/MARINE...SS