Area Forecast Discussion
Issued by NWS San Diego, CA

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228
FXUS66 KSGX 202026
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
122 PM PDT Sat May 20 2017

.SYNOPSIS...
High pressure aloft will continue very warm weather inland through
Tuesday, while a sea breeze keeps coastal areas cooler. Some patchy
fog is possible by morning, but more likely Monday morning as
onshore flow gradually increases. Cooler all areas by Wednesday as
low pressure moves inland to the north, and the marine layer deepens
under increasing onshore flow for the latter half of the week. This
will push daytime temperatures back below average, and result in
gusty westerly winds across the mountains and deserts. Warming
returns next weekend as low pressure weakens aloft.

&&

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

Satellite imagery this afternoon showed completely clear skies
across the Southland, including the CA Bight. Marine stratus
remained west of the islands at 1 PM PDT. Temps were running mostly
a few degrees above values observed yesterday. It was still quite
dry at Noon PDT with surface dewpoints west of the mts holding
in the 30s and 40s F. Surface pressure gradients were holding
at about 4 MBS offshore NV to KSAN, and weak onshore to the lower
deserts. Peak wind gusts were from the east at 25 to 35 MPH, and
mainly along the coastal foothills of the San Diego County mts.

With the nearest marine stratus well south along the coast of Baja
and west of the islands, any fog development is expected to remain
patchy along the coast through Sun morning. There will be a better
chance of stratus/fog nearshore and inland once the marine layer
offshore has a chance to get reestablished.

High pressure aloft over the EastPac today, will expand eastward
over the West Coast through Monday. This will gradually decrease the
offshore gradient at the sfc as the heat builds inland, and the
thermal low strengthens over the Lower Colorado River Valley. Low
pressure will be reinforced by the trough moving inland to the
north. This will result in gusty westerly winds over the mts/deserts
next week, and a building marine layer. Through Mon, significant
cooling will be confined to the coastal/western valley areas. By
Wednesday, it will progress farther into the valleys and foothill
areas, while the deserts remain hot. The breezier conditions
overnight will keep overnight minimums elevated across the lower
deserts as well.

Beginning Thursday, the cooling will be more noticeable across the
deserts, while daytime temperatures farther west remain well below
average. The trough over the northwest will begin to lift out next
weekend, allowing some recovery of heights/thickness across SoCal.
But any ridging looks fleeting as the main ridge axis again surges
into higher latitudes off the Coast, and some undercutting of the
westerlies along 30N ensues. Expect modest warming next weekend as
the onshore weakens and the marine layer subsides.

Any forecast issues will be timing the return and depth of the
marine layer and associated stratus/fog. For now, it appears
the best chance for any airport impacts would come late nights/
mornings Sun through Tue.

&&

.AVIATION...
202020Z...Primarily P6SM vis and mostly SKC for all areas through
21/0600 UTC. 21/0600-1600 UTC, there`s a chance that patchy dense
fog/low stratus will develop within 5 SM of the coast. Bases of low
stratus would be 200-400 ft msl with tops around 500 ft msl.
However, confidence in low stratus/dense fog impacting KSAN, KCRQ
and KSNA is low at the moment, with greatest chances for dense fog
occurring during the 21/1200-1500 UTC time-frame. Otherwise, P6SM
vis and mostly SKC continuing through Sunday morning.

&&

.MARINE...
Patchy dense fog may develop over the coastal waters early Sunday
morning into Sunday night, and perhaps into Monday as well. Also, 20
kt northwest wind gusts are possible each late afternoon and early
evening through Thursday over the outer waters.

&&

.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...JAD
AVIATION/MARINE...Harrison



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