Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 200328
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
827 PM PDT Mon Oct 19 2020

.SYNOPSIS...
A persistent ridge aloft will gradually give way to a broad
troughing over the West as cold Canadian air sags south. It will
still be warm over Southern California on Tuesday, but noticeable
cooling will slowly spread inland Wednesday and Thursday. There will
even be a chance of some light rain or drizzle as the marine layer
deepens inland late in the week. Santa Ana winds may follow early
next week as cold high pressure builds into the Great Basin.

&&

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

Marine clouds have been gathering over the CA Bight this evening,
but had made little inland progress through 8 PM PDT. The sfc
pressure gradients remain fairly weak, and winds were mostly light,
except through and below the desert passes where some gusts of 25-30
MPH have been reported. The 00Z Miramar sounding had a 6C inversion
based near 1700 FT MSL. Winds were light westerly below the
inversion. No weather updates are planned this evening.

From previous discussion...

The highlights from yesterday remain unchanged. Upper level
ridging will continue over the region through Wednesday, leading
to continues above normal temperatures. The marine layer has
deepened and so low clouds will extend each night into the inland
valleys with fog expected to only be confined to the mesas.

Changes occur during the later part of the week and will be
noticeable! An upper level trough will be digging in from the
Pacific Northwest this weekend with phasing of an upper level cut
off low over the Pacific. This trough will move across SoCal late
this weekend or Monday and you will feel the change in the weather
with the temperature fall. A more dramatic deepening of the
marine layer (to 5000 FT+) will occur in advance of the trough
with drizzle possible late Friday night into Saturday and
measurable light rain possible sometime Sunday-Monday (depending
on the speed of the trough) from the coast to the mountains. Mean
rainfall amounts off the GFS and EC ensembles are in the
0.05-0.15" range. We do believe any rainfall that occurs will be
light/insignificant.

As the upper trough exits to the east, surface high pressure will
build southward into the Great Basin early next week. This
synoptic scale weather pattern is conducive for the development of
a Santa Ana wind event here across Southern California and so is
on our (human) radar. Some model runs indicate at least moderate
strength is possible sometime during the October 26-28 time
period. Low confidence on timing/strength, but this is something
we are monitoring closely and will continue to do so in the days
ahead as fuels are extremely dry and any rain that does fall
preceding this would do little to mitigate potential critical
fire weather conditions early next week. Stay tuned.

&&

.AVIATION...
200310Z...Coast/Valleys...Low clouds with bases 1200-1500 ft MSL and
tops to 1800 ft MSL will continue to fill in along the coast this
evening, reaching the coastal airports around 06-08Z then spreading
about 20-25 mi inland overnight. KONT should be near the edge of the
low clouds with a small chance of CIGS 11-15Z. Vis 2-4 SM in BR
likely where clouds near higher terrain in the valleys, including at
KONT. Clouds and vis restrictions clearing 17-19Z. Low clouds will
begin to spread back inland after 21/01Z with slightly higher bases
and tops.

Mountains/Deserts...A few clouds AOA 20,000 ft MSL and unrestricted
vis through Tuesday night.

&&

.MARINE...No hazardous marine weather conditions today through
Saturday.

&&

.FIRE WEATHER...
Elevated fire weather conditions will continue afternoons and
evenings through Wednesday due to the very dry conditions, low RH,
and above average temperatures.

Onshore flow will strengthen on Thursday, pushing cooler, marine air
well inland. Stronger onshore flow will continue through the
upcoming weekend, possibly generating periods of scattered, light
rain, and drizzle along and west of the mountains. Amounts will be
light, but the high RH and cooler conditions will ease any fire
weather concerns. Drier and warmer weather will arrive again early
next week, with a period of gusty Santa Ana winds possible.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

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

$$

PUBLIC...10/Gregoria
FIRE WEATHER...10
AVIATION/MARINE...Rodriguez


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