FXUS66 KSGX 230417

Area Forecast Discussion
National Weather Service San Diego CA
917 PM PDT Sun Apr 22 2018

Weak onshore flow will result in more widespread low clouds and fog
nights and mornings, otherwise fair and mild weather will continue
this week under weak high pressure aloft. Low pressure will move
toward the north coast of California late in the week, strengthening
the onshore flow, and deepening the marine layer across the
Southland. Areas of night and morning coastal low clouds and fog
will expand into the valleys, along with slight cooling.



by 8 PM PDT...low clouds were expanding over the inner coastal
waters and over some coastal areas.. Inland, skies remained mostly
clear. The 00Z Miramar sounding indicated a 24 hour cooling of about
4 degrees C near 1500 Ft MSL...verifying a slight build in the
marine layer. Surface pressure gradients had a 3-4 MB onshore trend
from yesterday as well. Peak onshore wind gusts were in the 25 to 35
MPH range in the mts/deserts.

Based on the pressure gradient trends, expect a shallow marine layer
will persist through the night allowing some fog to form. the best
chance of dense fog appears to be over the mesas and other higher
coastal terrain, however a very weak inversion limits confidence in
development and persistence of the fog in any one area. The marine
layer may even be shallower on Mon night/Tue morning, continuing a
threat of coastal fog. Added visibility qualifier "1/4 mile or less"
to parts of the coastal and far western valley zones, otherwise no
forecast changes.

From previous discussion...

.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.

Tranquil weather will prevail across 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.


30415Z...Coast...Areas of low stratus/fog will continue to develop
overnight within 5-10 miles of the coast. Bases will be 600-900 feet
MSL with the chance they could lower to 400-500 feet MSL in some
areas after 09Z, with tops mostly below 1000 ft MSL. Areas of vis 3-
5 miles will occur with local vis 1/4 mile or less possible,
especially over higher coastal terrain. Timing of cigs at airports
is somewhat low confidence, and areas with BKN/OVC cigs could go SCT
at times overnight. Most areas should clear by 16Z Mon, with
unrestricted vis after 16Z Mon. More low stratus/fog should develop
Monday evening.

Valleys/Mountains/Deserts...FEW-SCT clouds at/above 25000 ft MSL
with unrestricted vis will occur through Monday evening.


Patchy fog may develop overnight tonight, but the chance of
visibilities below 1 NM is low. Fog could develop Monday night as
well. 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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