Well-below-normal temperatures for the entirety of the forecast period. Additional precipitation and stronger winds expected through Saturday, with hazards for both the marine waters and the beaches prevailing through the weekend. Additional wind and rain/snow possible with another system Monday/Tuesday. A third system late next week may bring similar impacts but with lower confidence.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
This morning's sounding showed a decent amount of CAPE over coastal Southern California, evident with the line of thunderstorms that made its way across the marine waters early this morning, fizzling out once they moved ashore. Generally 0.25-0.50" fell this morning, but some thunderstorms that reached coastal communities were able to provide closer to 0.75" before sunrise. Radar currently has a few piddly showers over parts of the coastal and valley zones, with very little in terms of accumulation. In a lower-CAPE environment, chances of thunderstorms for the remainder of the day are very low, generally 5% or less, but on-and-off showers could continue at times through this evening.
The low responsible for our morning convection has transitioned to an open trough, currently positioned over Southern California according to mid-level water vapor satellite. With a tighter gradient, winds increase this afternoon and evening over the marine waters, mountains, and deserts, with intermittent gusts to 30 mph over the waters and 45 mph for the wind-prone spots in the mountains. Winds will be strongest tonight as the shortwave on the backside of the trough moves through, with winds slowly weakening through Saturday morning. Additional rain/shower activity will accompany this shortwave, although the very low thunderstorm chances point to lighter accumulation through Saturday morning compared to this morning. Expect up to an addition 0.25" through Saturday morning for most spots west of the mountains (and up to 0.50" of additional rainfall for the mountains and foothills). Marine and beaches will see impacts as well through Saturday, please see these sections below.
Any rainfall should quickly come to an end Saturday afternoon, with drier air filtering in behind the trough. Weak upper level ridging will build in late Saturday and early Sunday, with less cloud coverage. Sunday will be a nice break in between systems, as the next trough is poised to move through Monday/Tuesday. Models continue to struggle to resolve the next system, especially timing and strength, with the GFS a bit faster compared to the ECMWF. The timing of this system will dictate marine and beaches impacts with another round of strong winds and high seas. For the inland side, a front associated with the trough will bring gusty winds and additional precipitation, as well as isolated thunderstorm chances. While this next system will be cooler than the previous, trying to nail out snow levels with such mismatch in the global models is difficult, especially with the heavier precipitation likely to be pushing through on Tuesday. For now, snow levels are generally at or above 6000ft during the precipitation window on Tuesday, which could allow for some of the higher elevations and mountain roads to see low snow impacts. Depending on the track and strength, snow levels could range anywhere from 5300-7000ft+. Regardless of how this system shakes out, guidance has been trending drier with each model run, and the latest guidance indicates less than 0.25" for most locations Mon-Tue (0.50" for the mountains). Additionally, temperatures will fall Mon-Tue to around 8-17 degrees below normal.
Further on in the week, another system could bring additional wind and rain to SoCal Thurs/Fri, but there is low confidence in any discernible impacts as global models are displaced with the core of the low by well over 100 miles. It is possible that the low swings too far north for us to benefit from, but NBM persists with around a 50% chance for rainfall on Thursday and 20% for Friday.
Much of this weeks forecast depends greatly on how these lows move across and how far south they make it. As is typical, models are struggling with these shoulder season systems, even in the shorter 3-4 day window. Close eyes will be kept on how the Mon/Tue system develops, with the possibility of some snowfall for the higher terrain.
131800z, Coast/Valleys, ISO to SCT -SHRA expected through this evening with SCT-BKN low clouds based 3-6 kft. Low clouds become more BKN-OVC after 06z Sat as a band of -RA pushes in from the northwest. This precip band will pass through the coastal basin from NW to SE from 08-16z, producing occasional MVFR cigs/vis. Wind direction shifts from SW to NW with the passage of this band.
Mountains/Deserts, SCT -SHRA through this evening in mountains with cloud bases 3-6 kft occasionally obscuring higher terrain. More steady -RA and vis reductions expected with the passage of the precip band mentioned in the section above (08-16z Sat). W/SW winds increase through this afternoon with widespread gusts to 25-35 knots (locally 40-45 kts over typically windy ridges, desert foothills, and through passes). Winds will gradually taper off through Saturday morning.
Southerly winds will increase today, turning westerly and strengthening late this afternoon and continuing through Saturday morning. Swell and wind waves will increase considerably at the same time. A Small Craft Advisory is in effect through Saturday afternoon. Winds and seas diminish late Saturday and Sunday, before increasing again on Monday as the next weather system arrives.
A series of swells (15-17 seconds) from the west-northwest (280-290 degrees) will generate elevated surf starting this afternoon, which will increase to a peak of 6 to 10 feet for west and northwest facing beaches Saturday and Sunday. A High Surf Advisory is in effect for these two days with dangerous conditions for both swimming and surfing. Surf begins to wane Sunday afternoon but will remain elevated for west-facing beaches into Monday morning. Another northwesterly swell will arrive on Monday, extending the threat for high surf and strong rip currents.
Ca, High Surf Advisory from 4 PM this afternoon to 10 AM PST Sunday for Orange County Coastal Areas-San Diego County Coastal Areas.
Coastal Flood Advisory from 7 AM to 4 PM PST Saturday for Orange County Coastal Areas-San Diego County Coastal Areas.
PZ, Small Craft Advisory until 4 PM PST Saturday for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm- Waters from San Mateo Point to the Mexican Border Extending 10 to 60 nm out including San Clemente Island.
