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

At mid-evening, low clouds were quickly spreading into the San Diego County valleys, with patchy low clouds along the Orange County coast. Elsewhere, the areas of smoke continued from the 3 big fires in SoCal to otherwise mar the clear skies. The stratus should cover most areas west of the mountains by Fri morning, though current guidance is suggesting that drizzle will not occur tonight or early Fri as the stratus thickness may not be enough despite the coastal eddy.

From previous discussion...

Cyclonic flow aloft, lower heights and a coastal eddy will maintain a deep marine layer, with low clouds spreading inland to the foothills during the nights and mornings.

Sunday through Thursday...
A reinforcing shortwave from the Gulf of Alaska will bring even colder air into the mean trough over the western US. It will deepen the trough, forming a closed upper low over central CA by Monday morning. This low will fill and eject to the northeast out of the base of the mean trough, to be replace by yet another reinforcing shortwave which will form another closed upper low somewhere over CA next Thursday.

As a result, SoCal will get a few degrees of additional cooling on Monday near the coast and 10 to 15 degrees of cooling for inland areas. High temperatures in the mountains could be as much as 20 to 25 degrees below average on Monday.

The marine layer will also deepen further, with some light precipitation possible at times from the coast to the mountains starting late Sunday and possibly continuing through Monday night.
Since the bulk of available moisture will be in the boundary layer and very limited, significant accumulations are not expected.

Strengthening onshore flow will produce gusty westerly winds on Monday...strongest in the mtns and adjacent desert areas. The exact timing is still uncertain as there is still a significant spread among ensemble solutions. The winds will weaken for Tuesday through Thursday but the dominant trough will remain in place, giving us temperatures below seasonal averages through next Thursday. How much below seasonal averages remains uncertain.

AVIATION
130430Z...Areas of near-surface smoke and smoke layers aloft from ongoing fires near Santiago Peak (Airport Fire), Wrightwood (Bridge Fire) and Running Springs (Line Fire) will continue through Fri evening. Local VIS will be below 3SM in smoke, with local BKN CIGs 5000-10000 feet MSL. Most impacts are expected in the mountains of San Bernardino and Riverside counties as well as the Santa Ana mountains.

Low clouds will continue to expand from the San Diego County coast through most of the valleys by 06Z Fri, through most of Orange County by 09Z Fri and most of the Inland Empire by 12Z Fri. Bases will be 1500-2300 ft MSL with tops to 3200 ft MSL. Areas of higher terrain above the valleys will be obscured 08Z-16Z Fri. Clearing will occur 15Z-18Z Fri in the valleys and 17Z-20Z Fri near the coast. Stratus will spread back inland Fri evening.

Areas of winds through mountain passes into deserts with gusts 25-35 kt will mostly dissipate 07-09Z Fri.

MARINE
Stronger winds will occur in the outer coastal waters Monday and Monday night with gusts 20-30 knots possible. Otherwise, no hazardous marine conditions are expected through Tuesday.

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

SGX WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...None.