A Pacific storm will continue to impact the region through the weekend. Heavier and more widespread precipitation will develop this morning, including a chance of thunderstorms. The weather pattern will remain active with another winter storm expected to impact the region for Monday into Tuesday with another system expected to impact the region later next week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
.Morning Update,
Widespread light to moderate showers continue to overspread the area this morning as the area of low pressure off the coast slowly moves northeastward toward Southern California. Hourly rain rates west of the mountains have generally been near 0.20"/hr, with some spots near over one third of an inch. The heaviest rainfall with the storm system is currently moving onshore, so we can expect the most intense rain to occur across our region late morning into the early afternoon. Models show more intermittent showers occurring by the evening into the overnight hours. Hi-res models show the heaviest showers will be possible around the San Diego metro areas, where some post-frontal showers may produce hourly rain rates near 0.50"/hr. Not too much in way of thunderstorm activity is noted this morning, but a slight chance remains in the forecast through the night.
The forecast remains on track for isolated shower activity to remain into Thursday, mainly along and west of the mountains. We will continue to monitor the next weaker storm system that will move into the area on Monday and Tuesday, as well as a possible third weather system by later next week.
.Previous Discussion,
Scattered mostly light showers continue to lift north across the region this morning with rainfall rates generally 0.10 inch per hour or less. However, that will change drastically later this morning as the upper level closed low currently around 300 mi west of San Diego drifts eastward and a vort max lifts north over So Cal. This forcing combined with forecast Integrated Vapor Transport (IVT) of over 500 kg/m/s and PW near 1.5" will produce widespread moderate to heavy precipitation today, mainly between 6 AM and 4 PM. Rainfall rates for urban areas are forecast to peak around 0.30-0.50"/hr, and as high as 0.80"/hr on the south facing mountain slopes. There is a 15-30% chance of thunderstorms today as well, which could produce locally higher rainfall rates/amounts. Rainfall rates of this intensity will likely produce roadway flooding, especially in poor drainage areas, and bring a threat of mud and debris flows in and below the recent burn scars. A Flood Watch remains in effect from 4 AM through 10 PM today.
Additional total rainfall through Sunday evening will be around 1.50-2.25" for the coasts and valleys, 2-4" for the mountains, and 0.50-0.80" for the deserts. Snow levels will remain above 7500 ft through the day, lowering to 6500-7000 ft tonight into Sunday morning, then rising to around 7500-8000 ft during the day Sunday, with snowfall totals of 1-4" above 7500 ft.
Rain turns to scattered showers this evening, some of which could be heavy as thunderstorm chances continue, then weaken and become more isolated Sunday morning. Isolated light showers will persist through the day Sunday into Monday morning. We may get a brief break late Monday morning before the next round of precipitation moves in sometime during afternoon, heaviest and most widespread late Monday into Tuesday morning. This second system will be another slow mover with precipitation lingering into Wednesday. A weak Atmospheric River is forecast to accompany this system, but there is still a lot of spread in total rainfall amounts. For the coasts and valleys, NBM 25th to 75th percentile precipitation range anywhere from 0.10-0.20" on the dry end to around 1-1.5" on the wet end. The chance of 48 hr rainfall totals exceeding 0.50" is around 45-65%, and chance of exceeding 1" is 25-50%, lowest in the Inland Empire. Snow levels with this storm will be lower, falling to 5500-6000 ft Monday night, then rising to 6000-6500 ft for Tuesday afternoon into Wednesday. Drier weather will then follow for Wednesday afternoon through Thursday morning.
For the next potential storm later in the week, huge uncertainty remains. Ensembles are split about 50/50 on the low tracking south just off the coast (wetter) versus a more inside track (drier), and a few ensemble members are completely dry. The timing is also questionable, with precipitation onset as early as Thursday afternoon and as late as Friday morning or afternoon.
Aside from precipitation, high temperatures will range from around 10 to 20 degrees below normal through much of the week with periods of breezy southwest to west winds as each storm system moves through.
.151630z, -SHRA with cloud bases 3500-7000 feet MSL with occasional +RA with bases down to 1000-2000 feet and vis 1-5SM will continue into this afternoon. 15% chance TSRA, highest for the coast and in inland valleys through 22Z, then 25% chance TSRA migrates inland after about 21Z. Precip decreases and associated CIG/VIS reductions begin to lift from south to north after 23Z, with SCT -SHRA and ISO -TSRA continuing through 05Z. ISO-SCT -SHRA thereafter with BKN clouds based around 2500-3500 feet MSL.
Frontal band to bring south to southeast winds gusting 20-30 kts west of the mountains, highest along coast, through 22Z. Winds turn southwesterly after about 20Z and diminish along the coast, while WSW gusts 25-40 kts pick up over mtns and desert slopes through tonight.
Another round of heavier SHRA along the coasts and valleys along with breezy southeast winds possible after 08Z Sun, bringing lowered bases and reduced VIS, with high res guidance indicating highest chances in SD County for impacts.
South winds 15-25 kts through 21Z will bring steep, mixed seas 5-8 feet throughout the coastal waters. See the Small Craft Advisory for details. Winds become west-southwesterly and weaken after 21Z.
Additionally, heavy rain will lead to reduced visibility and thunderstorms embedded within the rain resulting in gusty, erratic winds, lightning, and possible waterspouts. A Marine Weather Statement in effect through today contains more details.
Winds strengthen again Monday afternoon ahead of the next storm system. There is still some uncertainty in exact intensity of winds, but gusts in excess of 20 knots are possible along with another round of precipitation.
West swell 4-5 feet will combine with a south wind swell to create elevated surf 4-6 feet with local sets to 7 feet through the late afternoon. Additionally, there is a chance (20-30%) for thunderstorms through late this afternoon. See the Beach Hazards Statement for more.
Ca, Beach Hazards Statement until 3 PM PST this afternoon for Orange County Coastal Areas-San Diego County Coastal Areas.
Flood Watch through this evening for Apple and Lucerne Valleys- Coachella Valley-Orange County Coastal Areas-Orange County Inland Areas-Riverside County Mountains-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Coastal Areas- San Diego County Deserts-San Diego County Mountains-San Diego County Valleys-San Gorgonio Pass near Banning-Santa Ana Mountains and Foothills.
PZ, Small Craft Advisory until 4 PM PST this afternoon 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.
