Widespread heavy rain is expected late this evening into Wednesday across Southern California. After the initial rain band moves from northwest to southeast across the area on Wednesday, scattered locally heavy rain is expected to continue across much of the area into Christmas day. Snow levels will remain high through the duration of the event increasing the flood threat for the mountains, particularly in San Bernardino County. In addition to widespread heavy rain on Wednesday, gusty southerly winds are expected for the coast, valleys, San Bernardino mountains, and High Desert. Another round of more widespread moderate to heavy rain is expected sometime late evening on Thursday into Friday morning. Drier conditions are looking more likely for sometime Saturday and Sunday. We're monitoring the potential for additional precipitation around New Year but there is still considerable uncertainty in the forecast.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Key Points:
* Widespread moderate to heavy rainfall Christmas Eve with the potential for roadway flooding and localized flash flooding from the coast to deserts. Rapid rises in small streams and increased flow in main stem rivers expected.
* The highest risk for flash flooding on Christmas Eve is for the San Bernardino County mountains with a high risk for excessive rainfall that could result in damaging and potentially life- threatening flash flooding, mud slides, and debris flow. The threat will be highest during the day on Wednesday.
* Gusty south winds will develop tonight and continue through Wednesday evening with gusts 30 to 40 mph, locally up to 45 mph near the coast and to 55 to 75 mph along and below the desert slopes of the San Bernardino County mountains. Make sure to secure outdoor furniture. Tree damage is possible.
* Scattered showers will continue on Christmas Day with another round of more widespread showers for Thursday night into Friday. Snow levels will lower to around 7000 feet for Thursday into Thursday evening, then lower to around 6000 feet by late Friday creating slick conditions on mountain highways above 6000 feet.
An Atmospheric River is still on track to impact Southern California on Wednesday. At 1 PM, areas of light rain are streaming across much of the area, including portions of the deserts. So far only minimal amounts had accumulated with most places less than 0.10" inch over the last 6 hours. As we get into tonight locally heavy rain may develop, most likely on the coastal slopes of the San Bernardino County mountains. The heaviest and most widespread rain is expected through the day Wednesday, with the heavy rain moving northwest to southeast from Orange County to San Diego County deserts. After the rain band passes, scattered showers are expected to continue with locally heavy rain into Thursday (Christmas Day). Latest guidance shows a significant decrease in IVT (atmospheric moisture) during the day Thursday, bringing the potential for periods no precipitation for some areas. Snow levels are expected to remain quite high through the duration of the heaviest precipitation, but are expected to lower later in the week. Please see the Hydrology section below for more information on expected rainfall rates, totals, and snow level information.
In addition to periods of heavy rain, gusty south winds are expected. Strongest winds expected during the day Wednesday. Peak wind gusts of 60-70 mph, locally up to 75-80 mph possible on the desert slopes of the San Bernardino County mountains with gusts 45- 60 mph into the High Desert. Winds will stay elevated in San Bernardino county mountains and High Desert into Thursday. For the coasts and valleys gust of 30-45 mph are expected, with the strongest winds expected with the main band of rain.
Models are coming into better agreement that the next round of widespread moderate to heavy rain will be Thursday evening into Friday. Guidance is indicating a more organized band of rain will move across the area sometime late Thursday into Friday, with some lingering scattered showers through the day Friday eventually tapering off some time Saturday. In addition to rain and mountain snow (7000 ft and above), winds are expected to increase over the mountains and into the deserts, with timing of the strongest winds shaping up to be sometime Thursday night into Friday. Overall, rainfall rates and amounts are starting to shape up to look less than what is expected for Wednesday, but additional rainfall on already saturated soils may result in additional areas of flooding.
Scattered mostly light showers are expected continue into this evening. Rain is expected to become heavy at times late tonight along the south slopes of the San Bernardino County mountains with heavy rainfall spreading southward across the area for late tonight through Wednesday morning with the heavy rainfall continuing through Wednesday evening. Scattered showers may continue on Thursday (most likely in the morning) for some locations with another round of more widespread showers for Thursday night into Friday. The snow level will rise to above 9000 feet for early Wednesday morning, fall to around 7000 feet on Thursday through Friday morning, then fall to 4500 to 5000 feet for early Saturday morning as precipitation tapers off.
Through Wednesday night, for Orange and southwestern San Bernardino Counties, rainfall could range from 2.5 to 3.5 inches near the coast to 6 to 8 inches on the coastal slopes of the San Bernardino County mountains with isolated totals to around 12 inches on the coastal slopes to the west of the Cajon Pass. Rainfall will decrease from north to south with rainfall across far southern San Diego County around 1.5 inch near the coast to 2.5 inches in the mountains. For the high desert, 1.5 inch to 3.5 inches near the foothills of the San Gabriel Mountains is expected with 0.75 to 1.5 inch for the lower deserts. Hourly rainfall rates are expected to be 0.50-1 inch per hour, locally higher in any convective bands or thunderstorms that develop.
The San Bernardino County mountains are part of a high risk area for excessive rainfall for Wednesday morning through Wednesday night, areas with a heightened risk for damaging and potentially life- threatening flash flooding. Hourly rainfall rates of 0.50-1 inch are likely for several hours on the coastal slopes of the eastern San Gabriel Mountains from before dawn on Wednesday through Wednesday afternoon, locally to around 1 inch per hour from thunderstorms or bands of heavier showers.
The San Diego River at Fashion Valley and the Santa Margarita River at Ysidora are currently expected to reach Action/Monitor stage. Based on current forecast, the San Diego river is expected to peak at 7.5 ft Wednesday night with the Santa Margarita expected to peak at 11.1 ft late Wednesday night/early Thursday morning. If heavier rain develops over either of those two river basins, the rivers may exceed their currently forecast peaks.
Additional rainfall during the day on Thursday from showers will range from around 0.50 inch north to less than 0.25 inch further south, locally to around 1 inch on the coastal slopes of the San Bernardino County mountains. Locally higher amounts possible where any convective showers or thunderstorms develop Thursday morning.
For Thursday night and Friday, a weaker atmospheric river will bring another round of more widespread showers with a slight chance of thunderstorms. For Orange and southwestern San Bernardino Counties, rainfall could range from around 0.75 inch near the coast to 1 to 2 inches on the coastal slopes of the San Bernardino County mountains with isolated totals to around 3 inches on the coastal slopes to the west of the Cajon Pass. Rainfall will decrease from north to south with rainfall across far southern San Diego County around 0.25 to 0.50 inch. For the high desert, 0.25 to 0.50 inch is expected with less than 0.25 inch for the lower deserts. First take of hourly rainfall rates for late Thursday into Friday look to range anywhere from 0.3-0.75 inches per hour. It's possible that there will be additional chances of thunderstorms sometime late Thursday into Friday, with higher rainfall rates expected where thunderstorms develop.
232130z. A high broken to overcast layer of clouds will continue to cover the area around 20-25k ft MSL through the TAF period. Low clouds with bases 1500-5000 ft MSL expected along the coast with inland areas getting as low as 300-700ft MSL after 12-14Z Wednesday. Light rain chances through the day today with increasing chances through this evening and overnight. Vis reductions 1-4 SM are expected as rain chances increase. The main rain band will move in from the northwest to the southeast towards the end of the TAF period and will bringing moderate to heavy precipitation with vis reductions 1-3 SM along with strong south to SE winds.
The approaching storm system has increased winds out of the south to southeast and will build seas this afternoon. Winds initially around 15-20 kts gusting to 25 kts will increase early Wednesday to gale force, with wind gusts upwards of 35 kts. This will bring steep combined seas of 6-9 feet. Additionally, periods of moderate to heavy rain Wednesday will lead to areas of poor visibility. Winds diminish slightly late Wednesday through early Thursday, but winds and seas will remain hazardous to small craft. Another round of stronger southwest winds expected late Thursday into early Friday, with gusts around 25 kts. Winds weaken and turn northwesterly Friday.
Strengthening south winds tonight into Wednesday will lead to elevated surf 5 to 8 feet with sets to 9 feet, highest for south- facing beaches. This will lead to hazardous swimming conditions, as well as possibly minor tidal overflow for south-facing beaches from a combination of winds and elevated surf through Wednesday, especially during high tides. Winds diminish along with surf late Wednesday, but more breezy winds along with large fresh swell will lead to surf building again Friday-Saturday. A Surf Advisory is in effect from tonight to Saturday afternoon.
Ca, High Surf Advisory from 11 PM this evening to 11 AM PST Saturday for Orange County Coastal Areas-San Diego County Coastal Areas.
Wind Advisory from 3 AM to 10 PM PST Wednesday for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Coastal Areas-San Diego County Valleys-Santa Ana Mountains and Foothills.
Flood Watch from late tonight through late Wednesday night for San Diego County Coastal Areas-San Diego County Deserts-San Diego County Mountains-San Diego County Valleys.
Flood Watch from 10 PM PST this evening through Wednesday 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 Gorgonio Pass near Banning-Santa Ana Mountains and Foothills.
High Wind Warning until 3 PM PST Thursday for Apple and Lucerne Valleys-San Bernardino County Mountains.
PZ, Gale Warning until 4 PM PST Wednesday 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.
