Isolated light showers will continue this morning along and west of the mountains. A weaker Pacific storm will move through the area this afternoon through Tuesday evening, bringing widespread rain and mountain snowfall for areas above 5,000 feet. Isolated showers continue into Wednesday with moisture wrapping around the north side of the low pressure system. A third system may bring additional by late Thursday or Friday, though details with this system still remain uncertain.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Isolated light showers continue to linger this morning as deep layer moisture and onshore flow persist, otherwise high clouds are moving across the region ahead of the next upper level low. This low will track south along the coast today, slowly moving east across So Cal Tuesday and Wednesday. The main band of precipitation associated with the cold front moves through this afternoon and evening, bringing another round of widespread precipitation and a slight (10-15%) chance of thunderstorms with it. Brief, heavy rain with rates upwards of 0.50"/hr are possible within a narrow band right along the cold front. Isolated to scattered showers will follow the front late tonight all the way into early Wednesday. Ensembles are showing at least weak instability (mean CAPE of around 100 J/kg) for the mountains westward Tuesday afternoon, for another slight (10%) chance of thunderstorms. Depending on how the low tracks and how slowly it moves off to the east, we could remain in wrap around moisture on the north and west sides of the low through the day Wednesday. If it tracks slowly enough for moisture to remain into the afternoon, we could see another round of showers as daytime heating/destabilization occur, predominantly over the mountains and deserts.
Storm total precipitation with this low are forecast to be around 0.50-0.75" for the coasts to the mountains, with amounts of 1-1.6" on the south/southwest facing coastal slopes, 0.10-0.30" in the deserts, and less than 0.10" in the deserts. Desert amounts could end up being higher should the wrap around showers materializes. Snow levels will be much lower with this system and have trended lower since yesterday, now bottoming out at around 4500-5000 ft across the San Bernardino Mountains and 5000-5500 ft in Riverside/San Diego Counties Tuesday morning. This will be in the colder air behind the front after the majority of the precipitation occurs though, so do not expect much accumulation at these elevations (less than 0.5" of snow). Snow levels will be closer to 6500-7000 ft in the main precipitation band. Snow totals for 5500-6500 ft will be near 1", 2-4" between 6500-7000 ft, and 4-8" above 7000 ft. Again, this amounts could end up increasing if the wrap around showers occur, especially for the northern slopes of the San Bernardino Mountains.
A brief break in precipitation occurs Wednesday night through Thursday afternoon. Ensembles are starting to come into better agreement with the track of the next system, though consequential differences remain. They have trended away from the inside slider track, which now only accounts for about 10% of the ensemble space, meaning the potential for this storm to miss us with that track has decreased. Around 20% of the members, the majority from the GEFS, follow a track close to the deterministic GFS, swinging the upper low further west and south before moving inland across Northern Baja. The trajectory of the short waves around this low will determine where the heavier precipitation occurs. A further west and south track would favor San Diego County with lower precipitation amounts to the north. A slight westward but less southerly track would favor the coastal areas, and a more direct route (much like the early week system) would more evenly distribute precipitation across the region. The range of storm total precipitation remains massive - anywhere from zero to over an inch for the coasts to the mountains.
Once this upper level low departs (sometime Sunday or Monday), weak upper level ridging will build in from the west bringing drier weather and high temperatures returning to normal. Otherwise highs will remain well below normal for the week, at times as much as 15-20 degrees below normal.
171030z, SCT-BKN low clouds with cigs varying between 1800-4000 feet MSL along with iso -SHRA into this afternoon. A frontal band arrives from the northwest starting in Orange County around 00Z and into San Diego by 06Z, bringing RA, cigs 1000-2500 feet MSL, vis 1- 5SM, and a southerly to westerly wind shift. Higher terrain obscured in FG/RA during this time and through tonight. Cigs/vis lifting and/or scattering out near the coast after 07Z Tuesday.
A cold front will arrive late this afternoon with a distinct wind shift from southerly to westerly. The strongest winds will be the westerly winds behind the front this evening. Check the Small Craft Advisory for details. There is a slight chance of thunderstorms this afternoon and evening, and again Tuesday afternoon. Any thunderstorm could produce lightning, gusty erratic winds, choppy seas, and/or a waterspout. No hazardous marine conditions are expected Tuesday night through Thursday, but the next storm system has the potential to bring hazardous winds and seas Thursday night and Friday.
Elevated surf of 3 to 5 feet, with sets to 6 feet on west facing beaches is expected tonight and Tuesday. This will generate a high risk of rip currents.
There is a slight chance of thunderstorms and lightning this afternoon and evening, and again Tuesday afternoon.
Ca, None. PZ, Small Craft Advisory from 4 PM this afternoon to 2 AM PST Tuesday 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.
