Tion and marine discussions updated for 00Z TAF package.
Dry conditions with above average temperatures inland will continue into early this week. Periods of patchy fog expected near the coast and valleys through Sunday. An Atmospheric River will bring widespread heavy rain and mountain snow above 8000 ft late Tuesday into Thursday (Christmas Day). Additionally gusty southerly winds can be expected across much of the area, including the coast. There are additional chances for showers Friday and next Saturday but chances are lower.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Key Points:
* Widespread moderate to heavy rainfall Christmas Eve and Christmas Day with the potential for roadway flooding. Rapid rises in small streams and increased flow in main stem rivers expected.
* Snow levels are forecast to remain above 8000 ft through Christmas day, increasing potential for rock and mudslides along mountain highways, and debris flows in and below burn scars. * Gusty southerly winds are expected across the area with 80-90% 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 Friday and Saturday with lower snow levels, creating slick conditions on mountain highways.
At 12 PM a coastal eddy was spinning over the coastal waters, which has helped maintain fairly widespread low cloud coverage at the coast and in portions of the western valleys. With minimal changes in the weather pattern expected into Sunday the marine layer should remain deep enough for areas of dense fog to develop on higher coastal terrain tonight into Sunday morning. Highs will remain 5 to 10 degrees above average for inland locations through Monday, with highs along the coast near average.
A significant pattern change is expected by the middle of the week. An upper level trough and associated Atmospheric River will amplify off the West Coast. A short wave embedded in the trough lifts into Southern California Tuesday night into Wednesday, ushering in the first surge of moisture and widespread moderate to heavy precipitation. Current forecasts from CW3E WRF models are showing IVT forecasts as high as 600-700 kg/m/s aimed at Southern California Tuesday night through much of the day Wednesday. Even with the differences between global models and uncertainty in exact details, there is increasing certainty that periods of moderate to locally heavy rain will occur between the overnight hours of December 23 through December 25. Snow levels are expected to stay above 7500 ft through December 25, with snow levels above 8500 ft at the onset of the precipitation. This increases the chances for flooding, debris flows, and mud slides in the mountains, especially near burn scars. Additionally impacts from river flooding, especially near the San Diego river, are possible. See the Hydrology section below for additional information. Gusty south winds are also expected across the coastal areas, valleys, and the San Bernardino Mountains into the adjacent desert foothills likely starting late Tuesday night/early Wednesday continuing through Wednesday afternoon.
From Thursday onwards, there is increasing uncertainty in the evolution of the upper level pattern, in both the forward progression of the low and how far south it will dig. About 45 percent of ensemble solutions dig the trough of low pressure south with the axis off to our west which would continue to pull moisture into the area and continue the wetter pattern for Southern California for Thursday and Friday. By Saturday, 60 percent of solutions still have the trough in position to continue bringing precipitation to Southern California. Current forecast maintains chances of precipitation through the end of the forecast period (next Saturday). The closed low that may bring additional precipitation could be colder, which would increase chances of snow in the mountains although uncertainty in what the snow levels would be remains quite high. Additionally, westerly winds could increase depending on the final track of the low. Regardless, multiple days of precipitation, which will likely be heavy at times, is bringing a risk of flooding and difficult travel conditions around the holiday.
For late Tuesday through Friday morning, the chance for 5 inches or more of rainfall for the western portions of the to San Bernardino County mountains (Mt Baldy to I-15) are 85 percent with an 75 percent chance for 6 inches or more, and the chance for 8 inches or more of rainfall has increased to 50 percent. Strong southerly flow aloft (30-50 kt) will allow for orographic enhancement of rainfall on the coastal slopes of the San Bernardino mountains. In this type of weather pattern, this area tends to receive the most rainfall. For the eastern part of the San Bernardino County mountains (areas east of the Cajon Pass) chances of exceeding 5 inches are 70 percent, exceeding 6 inches are 60 percent and exceeding 8 inches are 15 percent.
Probabilities for seeing rainfall totals over 2 inches Tuesday night through Thursday night are listed below:
- OC/Santa Ana Mountains: 90-95% - Inland Empire: 60-90%, highest west - Riverside/SD Co Mts: 60-80% - SD Coast/Valleys: 60-80%, highest northwest SD Co - Deserts: 15-25% (lower deserts), 40-55% (high desert)
Probabilities for seeing rainfall totals over 3 inches Tuesday through Thursday night are listed below:
- OC/Santa Ana Mountains: 70-85% - Inland Empire: 40-85%, highest west - Riverside/SD Co Mts: 40-70% - SD Coast/Valleys: 30-60%, locally higher in northwest SD Co - Deserts: 10-20% (lower deserts), 20-30% chance (high desert)
Current deterministic forecast for Tuesday through Friday morning: - Orange County: 3.5 - 4.5", highest north - Inland Empire: 3-4", highest west - San Diego County Coasts/Valleys: 2-3", highest north - San Bernardino County Mountains: 4-10", highest west - Santa Ana Mountains: 4-6" - Riverside/San Diego County Mountains: 2-4.5" - High Desert: 1.5-3", highest west - Low Deserts: 0.9-1.5", locally 2.5" near the San Gorgonio Pass
Most of the precipitation across southwestern California through Christmas morning is expected to fall as rain with snow levels remaining above 8000 feet. By late evening on the 25th, snow levels may drop to 7500 ft, but precipitation is expected to be less widespread. Snow levels could drop even lower for Friday and Saturday, but exact levels and locations remains uncertain.
The San Diego River at Fashion Valley is currently expected to reach Action/Monitor stage by early Thursday morning. Based on current forecast, the river is expected to peak at 7.8 ft.
210000z, Coast/Western Valleys, Low clouds push back ashore after 02Z this evening, moving to 15-20 miles inland overnight. Bases initially near 800-1000 ft MSL may settle closer to 500-800 ft MSL overnight with a weaker coastal eddy, though confidence in these lower cloud bases is low. Clouds very likely to make it to vcnty KONT, with a 40-45% chance for CIGs at the airport itself by 10Z. VIS restrictions down to 0-3 SM for inland valleys and higher inland terrain, with 4-6 SM along the coasts and coastal mesas. Clouds scattering back to the coasts 17-19z Sunday morning. Clouds at 800- 1100 ft MSL move back onshore after 02Z Mon.
Otherwise, FEW-SCT high clouds with unrestricted VIS through this evening.
No hazardous marine conditions are expected through Tuesday morning. South winds will strengthen late Tuesday in advance of a Pacific storm. Winds and seas are expected to increase as a result, with seas near 8-10 ft and wind gusts upward of 30-35 kts.
Ca, None. PZ, None.
