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Area forecast discussion
National Weather Service San Diego California
1020 PM PDT Sat Sep 14 2019

high pressure will slowly lose it's grip on the region this weekend,
resulting in a cooling trend Sunday into Monday. Some patchy, dense
fog is possible again tonight into Sunday morning along the coast,
spreading farther inland by Monday morning as the marine layer
deepens ahead of a trough moving inland across the west. The trough
will bring strong and gusty west winds over the mountains and
deserts, and noticeable cooling.


Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino

Evening update:

No significant changes were necessary this evening. The 00z sounding
from Miramar shows a very weak marine inversion near the surface.
There will be another threat of patchy dense fog late tonight and
Sunday morning near the coast. An incoming trough will begin a slow
cooldown Sunday, continuing through midweek.

Previous discussion, issued at 120 PM pdt:

Satellite imagery at 1 PM PDT still showed some patchy low clouds/
fog persisting along the San Diego and o.C. County coasts, otherwise
it was mostly clear over the County Warning Area. The only near-term forecast
concern is for further development of this area of fog/low clouds
along the coast. Confidence is low due to lack of coverage at this
time, but the potential is there, especially later tonight into
Sunday morning. Surface pressure gradients were mixed and quite weak
at midday keeping winds light.

High pressure over the SW today will shift east as a trough swings
east over the Great Basin on Monday. This shift will mean cooler
weather as the marine layer builds inland and gusty westerly winds
develop over the mts/deserts. A second, deeper trough will follow
over the west later in the week, reinforcing the cooling and gusty
onshore flow. It will also keep the weather dry.

The main weather concern today continues to be elevated fire weather
conditions due to the heat and low relative humidity (see fire weather discussion
below). This will ease a bit on Sunday as the atmosphere begins to
cool and the sea breeze carries more moisture inland. More
noticeable cooling, and building of the marine layer occur on
Monday. The onshore flow will increase late Monday/Monday night and
may reach advisory levels over the mts/deserts.

There is high confidence in the medium term as most guidance
indicates troughing over the west. This should keep the weather dry,
with a healthy onshore flow and temperatures near-to-slightly below
average. As of the latest 12z model solutions...there is better
agreement on a troughing pattern persisting through next weekend.
Some warming is likley due to modest height/thickness recovery, but
this would still leave daytime high temps (at present calculations)
slightly below average.



150400z...coast...mostly clear skies with unrestricted vis will
prevail through at least 08z sun. Patchy stratus/fog will likely
develop near the immediate coast, though confidence is low for any
cigs at san/crq/sna. If cigs occur, they will most likely be between
10z and 15z sun. Cloud bases will be 300-500 ft mean sea level with tops to 600
ft mean sea level. Areas of vis 1-3 miles with local vis below 1 mile could
occur 10z-16z sun. VFR conditions will prevail after 16z sun with
high clouds above 20000 ft mean sea level increasing. Stratus/fog will likely
be more extensive Sun night.

Valleys/mountains/deserts...mostly clear skies will prevail through
sun evening with some scattered-broken clouds possible in the
mountains/deserts sun and sun evening with bases mostly 10000 ft mean sea level
or higher. Vis will likely remain unrestricted through sun evening.


patchy late night and early morning low clouds and fog will likely
reform late tonight through Sunday morning, with periods of
visibility below 1 mile, locally down to 1/4 mile.


Fire weather...
hot again today with many relative humidity values near or below 15% more than 25
miles inland at midday. Westerly winds with the afternoon sea breeze
will gust 15 to locally 25 miles per hour at times, keeping an elevated fire
weather threat into the evening. A cooling trend begins in Sunday,
and is amplified into Monday as onshore flow strengthens and relative humidity

Strong west winds will develop over the mts/deserts by late Monday
through Monday night at 25-30 miles per hour with peak gusts 40-50 miles per hour on the
deserts slopes and near the passes. These winds will be accompanied
by higher relative humidity initially, but it will fall to around 15% or lower by
Tuesday morning in some areas, followed by decreasing wind. So
several hours of near critical fire weather conditions could develop
Tuesday morning on the ridges and desert slopes before the winds
drop off.


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


Sgx watches/warnings/advisories...

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