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,
.Morning Update,
Clouds and Sun are moving over the region after a beautiful sunrise. The next incoming weather system to impact our area is currently near the Central Coast, giving showers from Santa Barbara to San Luis Obispo up to the Bay Area. Hi-res models show this line of showers that is currently near Ventura/Santa Barbara to move from west to east starting by the mid to late afternoon hours across the LA Basin, then moving into parts of San Diego County by this evening after 6 PM, where evening commutes may be impacted. Tuesday still looks to be the coldest day of the week with highs in the 50s/60s for lower elevations and 30s/40s for mountain areas. The forecast remains on track for this system to rotate over the region, leaving the continued chance of rain and high elevation snow for many through Tuesday into early Wednesday.
.Previous Discussion (238 AM Monday),
An incoming low pressure system 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 around1", 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.
171630z, FEW-SCT low clouds based 2-6 kft will linger through the coastal basin through the morning. A frontal precipitation band arrives from the northwest starting in Orange County around 01-02Z Tue, reaching San Diego by 05-06Z, and pushing over the mountains and into the low deserts by 09-10Z. Expect a few hours of cigs 1000- 2500 feet MSL and vis 1-5SM through the coastal basin with the passage of this band. Cigs/vis lifting and/or scattering out with generally VFR conditions, thereafter. Higher terrain obscured in FG/RA during the frontal passage and through the overnight hours. SCT -SHRA/SHRA begins to develop 12-18Z Tue, reintroducing the threat for intermittent MVFR cigs/vis through the coastal basin.
A cold front will arrive late this afternoon with a distinct wind shift from southerly to westerly. The westerly winds behind the front will gust 20-25 knots at times. 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.
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.
