Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 222057

Area Forecast Discussion
National Weather Service San Diego CA
157 PM PDT Sun Apr 22 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 patchy 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.



.SHORT TERM (Tonight through Tuesday)...

Hi-res models have been way too aggressive with marine layer
stratus the past couple of nights and again are depicting a solid
marine layer stratus infiltrating the coast and inland valleys.
Current visibile satellite imagery shows some stratus around San
Clemente Island with another area along the northern Baja coast,
slowly creeping north. However, a clear sky is seen across all of
Southern California this afternoon.

The big challenge is the extent of low clouds and especially any
fog tonight. Given a developing onshore weak pressure gradient
tonight and some stratus seen moving towards the area, believe
that low marine layer cloudiness will be more prevalent tonight.
Fog is a higher uncertainty and of low confidence, though patchy
fog is possible.

Tranquil weather will prevail acoss the area through early next
week as zonal flow then weak upper level ridging dominates.
This will result in pleasant weather conditions with plenty of
sunshine aside from nighttime and early morning low cloudiness
potential. Temps will be similar each day with above average
readings continuing.


.LONG TERM (Wednesday through Saturday from previous
A low pressure system off the West Coast will move slowly towards
the Pacific Northwest with the GFS farther south and a little
faster and the ECMWF slower and farther north with the ECMWF
ensemble mean in between and favored for now. Onshore flow will
strengthen with slow cooling for inland areas for Wednesday
through Saturday with periods of gusty southwest to west winds in
the mountains and deserts. The marine layer will slowly deepen
with slow cooling spreading into the valleys and with areas of
night and morning coastal low clouds and fog spreading farther
into the valleys.


221930Z...Coast...A few patches of low clouds and fog linger at the
beaches this afternoon with no impacts at TAF sites. Low clouds
could begin to increase in coverage and spread inland around 03Z
Mon, but timing is uncertain. Expect cloud bases 400-800 ft MSL and
areas of vis below 3 SM extending up to 5 miles inland by 12Z Mon.
Vis of 1 mile or less will be possible on higher coastal terrain.
Low clouds and fog expected to lift between 14Z and 16Z Monday.
Otherwise, FEW-SCT clouds at/above 25000 ft MSL through late

Valleys/Mountains/Deserts...FEW-SCT clouds at/above 25000 ft MSL
with unrestricted vis through late tonight.


Patchy fog may develop over the coastal waters each morning through
Wednesday. Otherwise no hazardous marine weather is expected through


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




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