Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 212052

Area Forecast Discussion
National Weather Service San Diego CA
152 PM PDT Sat Apr 21 2018

High pressure aloft and weak onshore flow will bring warmer
weather through Monday with a gradual return of night and morning
coastal low clouds and fog. For Tuesday through Friday...a low
pressure system off the West Coast will begin to move towards the
Pacific Northwest late in the week. This will slowly strengthen
onshore flow across southern California and bring slow deepening
of the marine layer. Areas of night and morning coastal low clouds
and fog will spread into portions of the valleys with a cooling
trend spreading into the valleys.



Water vapor imagery shows an upper level area of low pressure off
Baja and moving east. This is sending some high level cloudiness
into Southern California. Otherwise, it is a noticeably warmer day
today away from the coast with most locales into the 70s and 80s
and in the 90s across the lower deserts.

The upper low will move east over Baja through Sunday. The big
question is will there be fog affecting the coast overnight into
Sunday morning. We think the probability of this is low given some
mid level height falls and the fact that there are no low clouds
anywhere near the SoCal coast at this writing. Hi-res models have
been way too bullish with fog formation so they really aren`t much
of a help. The pressure gradient from SAN-TPH will become onshore
so this should bring at least some low cloudiness to the coast
late tonight. However, dense fog potential looks very low at this
time. Evening shift can monitor trends to see if this potential

Sunday-Monday will feature similar weather conditions under high
pressure aloft dominating the weather. This will lead to dry
weather and warm temps with plenty of sunshine. Nighttime and
morning low clouds will move onshore the coast and into the inland

For the middle part of next week, global models show a trough
cutting off and meandering off the California coast. Dry weather
will continue with some slight cooling west of the mountains, but
the deserts will see little changes in temps. Winds will increase
from the mountain tops to the deserts as southwest flow increases.

The big uncertainty lies on where the cut off low will go through
late next week and global models are at odds in a big way. The
latest GFS takes it over SoCal (with rain chances and cooler
temps) while the EC takes it inland over far northern CA and
Oregon with a ridge building locally (dry and warmer). Given such
high uncertainties, no changes were made to the extended


212025Z...SCT-BKN clouds at or abv 20000 ft MSL with unrestricted
vis through this afternoon. Patchy low clouds and fog possible along
the coast and up to 5 miles inland this evening into Sunday morning,
with the potential for CIGS below 700 ft MSL and areas of vis 3 SM
or less. Confidence is low that there will be impacts at any TAF
sites. Any impacts at TAF locations will be after 06Z Sun and any
low clouds and fog will likely clear between 14Z and 16Z.

Otherwise a few high clouds and unrestricted vis through
Sunday morning.


No hazardous marine weather is expected through Wednesday.




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