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000
FXUS66 KSGX 112102
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
202 PM PDT Wed Sep 11 2019

SYNOPSIS...
An upper level trough will pass through the Great Basin through
tonight. Surface high pressure will settle in on Thursday and
produce a weak Santa Ana easterly wind. Upper high pressure will
follow on Friday and Saturday and result in significant warming.
Next week, another unseasonably strong upper trough will bring a
cooling trend, mountain and desert wind and coastal low clouds.

&&

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

The marine layer is extensive over southwest California and up to
2700 feet for the top this morning but early mixing signs have
resulted in full clearing thanks to the upper trough passage. The
onshore flow will persist and then weaken tonight and Thursday
morning. Marine clouds will be less extensive tonight and only
confined to the immediate coast by Thursday morning due to the
onset of offshore flow and subsidence aloft.

The current weather pattern is somewhat unusual before mid
September. The anomalies indicate the 1022 MB surface high
pressure that builds over Utah on Thursday is 2 to 5 year return
interval. It is rare to see an offshore flow Santa Ana wind this
early. The flow is forecast to be weak, however it is widespread
down to San Diego County. Given 850 mb temperatures warm to 23 to
25C on Thursday we will see a big jump in afternoon highs on
Thursday. The offshore flow weakens on Friday but persists and the
air mass warms to 26-28C in response to the upper level high
pressure which shifts over southern California. The warmest days
will be Friday and Saturday with Inland Empire getting into low
100 range and deserts as hot as 112. Due the offshore flow
Thursday and then weakening on Friday the warmest day on the coast
will be Friday with highs much above average. Humidity will be
very low except at the beaches. Well into the 80s for the coast
and mid to upper 90s for inland valleys. Temperatures and heat
risk approach but not exceed advisory levels and night lows
on the cool side due to dry air.

Longer range models depict another unseasonable strong (stronger
than todays) upper trough for next week and it takes a similar
track through the Great Basin for a rapid cool down and deep
marine layer for Monday and Tuesday. While it is not strong it
does elevate fire danger again due to the onshore flow into the
desert slopes and mountains and a dry system.

&&

.AVIATION...
112045Z...Coast/Valleys...Mostly clear skies expected through the
afternoon. Low clouds redeveloping after 06Z Thursday with bases
1000-1500 ft MSL impacting SD/OC county locales with MOD confidence
on timing. Spotty areas of vis below 6 sm in some spots. HI
confidence of no cigs at KONT/KSBD tonight. Scatter out 15-17Z
Thursday.

Mountains/Deserts...Mostly clear with unrestricted vis through
Thursday morning.

&&

.MARINE...
No hazardous marine conditions expected through Sunday.

&&

.FIRE WEATHER...Elevated fire weather on Thursday due to weak
Santa Ana east winds and low humidity. Hot and dry conditions
with diurnal winds on Friday and Saturday. Onshore flow increases
on Sunday with cooling trend.

&&

.CLIMATE...August was record dry with no monsoon in mountains and
deserts. Early September had a few days last week. Ocean
temperatures running 75F at Scripps Pier, record for September was
set earlier this week at 78F. Temps on Friday and Saturday are
about 10F above average for highs.

&&

.SKYWARN...
Skywarn activation will not be needed today.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

PUBLIC/FIRE WEATHER...Tardy
AVIATION/MARINE...APR