Dry conditions with above average temperatures inland will continue into Monday. Periods of patchy fog expected on higher coastal terrain and in the valleys tonight into Monday morning. An Atmospheric River will bring widespread heavy rain and mountain snow above 8000 ft late for Tuesday into Wednesday, with more scattered showers continuing into Thursday (Christmas Day). Gusty southerly winds will develop across much of the area, including near the coast and over the coastal waters Tuesday into Wednesday. There are additional chances for showers Friday through Sunday, but with decreasing chances into Sunday.
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. 27% chance of the San Diego River reaching minor flood stage.
* Snow levels are forecast to remain above 8000 ft into Christmas morning, increasing potential for rock and mudslides along mountain highways, and debris flows in and below burn scars, especially in the San Bernardino County mountains.
* Gusty southerly winds are expected across the area with 70-90% chance of wind gusts exceeding 35 mph along the coast Tuesday night into Wednesday. Make sure to secure outdoor furniture. Tree damage is possible.
* Additional chances of showers Thursday through Sunday. Snow levels are expected to lower to 6500-7000 ft by Friday morning, creating slick conditions on mountain highways.
This evening, the marine layer is about 500 ft shallower than yesterday afternoon with weak onshore flow continuing. Low clouds are forming in the coastal areas of San Diego County and increasing in coverage. By Monday morning, most of the coastal areas and western portions of the inland valleys will be covered by low clouds. Locally dense fog will likely form on elevated coastal and inland terrain. Temperatures this afternoon reached 80 degrees in some inland locations.
From previous discussion, With a weakening high pressure ridge aloft, high temperatures on Monday are expected to be 5 to 10 degrees above average for inland locations, locally up to 15 degrees above average for the mountains and deserts. Temperatures are expected to fall to near or below average for the rest of the week.
An Atmospheric River is still on track to impact Southern California on Wednesday. Periods of light to locally moderate rain may begin as early as mid-day Tuesday with increasing chances of precipitation into the overnight hours. Overnight Tuesday 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 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. 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. Winds are expected to increase on Tuesday afternoon, with the strongest winds expected during the day Wednesday. Peak wind gusts of 60-70 mph possible on the coastal slopes of the San Bernardino County mountains with gusts 30-40 mph possible in the coasts and valleys.
There still remains some uncertainty in the upper level pattern for Friday through Sunday. Global models continue to struggle with the progression of the low pressure system that develops near the Gulf of Alaska. The solutions with more ECMWF ensemble members show a faster eastward movement of the low which would bring additional chances of more widespread precipitation to the area sometime Friday into Saturday, but drier conditions Sunday. Solutions with more input from GEFS members show a slower eastward progression of the low which would continue our precipitation chances into at least Sunday. We will have to see how this evolves over the next few days, but overall the rain for late into the week into the weekend looks far less impactful, though snow could make travel difficult in the mountains should the snow level lower enough.
While moderate to locally heavy rain may begin Tuesday evening (especially for the mountains) guidance is coming into better alignment that the heaviest and most widespread rainfall is expected during the day Wednesday. Rain will move from northwest to southeast through the day Wednesday, with scattered showers lingering into the overnight hours and Christmas morning. Those scattered showers may be locally heavy. Additionally there is a slight chance of thunderstorms for Wednesday evening into Thursday morning and heavy rain is expected where thunderstorms develop. Peak rainfall rates are expected to range from 0.5-1 inch per hour, with the potential to be locally higher where embedded convective showers or thunderstorms develop. The coastal slopes of the San Bernardino mountains are expected to receive a significant amount of rainfall due to strong southerly flow aloft (30-50 kt) enabling orographic enhancement of rainfall and the warm airmass preventing snow formation below 8000 ft during the heaviest precipitation.
For the San Bernardino/San Gabriel Mountains 4 AM Tue through 4 AM Friday, the current deterministic forecast is: - Cajon Pass westward: 8-14" - East of Cajon Pass: 8-10" Most of the rainfall is expected to fall on Wednesday.
For remaining areas 4 AM Tue through 4 AM Friday, Probabilities of rainfall totals 3 inches or more: - Orange County/Santa Ana Mountains: 60-85% - Inland Empire: 30-75%, highest west - Riverside/San Diego County Mountains: 25-65% - San Diego County Coast/Valleys: 15-70%, highest north - Deserts: 5-15% (lower deserts), 15-25% (high desert)
Probabilities of 4 inches or more:
- Orange County/Santa Ana Mountains: 40-65% - Inland Empire: 15-50%, highest west - Riverside/San Diego County Mountains: 20-45% - San Diego County Coast/Valleys: 10-45%, highest north - Deserts: less than 5% (lower deserts), 10% chance (high desert)
Current deterministic forecast for Tuesday through Friday morning: - Orange County: 3.3-4.75", highest north - Inland Empire: 3-4.75", highest northwest - San Diego County Coasts/Valleys: 2.5-4", highest north - Santa Ana Mountains: 4.5-6.5" - Riverside/San Diego County Mountains: 3-6" - High Desert: 1.5-3", highest west - Low Deserts: 0.85-1.5", locally 2.5" near the San Gorgonio Pass
The higher rainfall amounts in the deterministic ranges above will likely be more localized to where heavier, more convective bands of rainfall develop.
Most of the precipitation across southwestern California through Christmas morning is expected to fall as rain with snow levels remaining above 7500-8000 feet. Snow levels could drop to 6500-7000 ft by Friday morning, potentially down to 5500-6000 ft by Saturday. Confidence in snow levels is low as the spread/difference between the NBM 25th and 75th percentile snow level is around 1500-2000 ft.
The San Diego River at Fashion Valley and the Santa Margarita River at Ysidora are currently expected to reach Action/Monitor stage by Wednesday evening. Based on current forecast, the San Diego river is expected to peak at 8.2 ft early Thursday morning with the Santa Margarita expected to peak at 10.9 ft late Wednesday night. There is a 27% chance the San Diego river could reach minor flood stage by Friday depending on the location and intensity of additional rainfall.
220330z. Coast/Western Valleys, Very patchy low clouds with bases 800-1000ft MSL have begun to push into San Diego County, likely to reach KSAN within an hour. Further spread up into Orange County will occur by 06-08z. VIS is generally 3-6SM, lower where clouds intersect terrain. Bases may fall a bit closer to 700-800ft MSL through 10-12z before clearing back towards the coast by 16-18z. Patchy low clouds develop again Monday evening, with similar bases but will be a bit slower to move inland, likely after 02-04z Tue.
.Otherwise, SCT-BKN high clouds with unrestricted VIS continues through Monday.
No hazardous marine conditions through Tuesday morning. South winds will strengthen late Tuesday in advance of a Pacific storm. Strong winds and building seas are expected then through early Thursday morning. Steep seas near 8-10 ft expected with wind gusts upward of 30-35 kts. Winds and seas weaken Friday.
Ca, Flood Watch from late Tuesday night 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 Tuesday afternoon 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.
PZ, None.
