textproduct: San Diego

SYNOPSIS

An Atmospheric River will bring widespread heavy rain and mountain snow above 8000 ft late Tuesday into Wednesday, with more scattered showers continuing into Thursday (Christmas Day). Gusty southerly winds can be expected across much of the area, including near the coast and over the coastal waters on Wednesday. There are additional chances for widespread rain, mountain snow, and elevated winds in the mountains and deserts Friday through Sunday with decreasing chances into Sunday.

DISCUSSION...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.

* 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 wind gusts 30 to 45 mph on Wednesday for the coast and valleys and Tuesday afternoon through Thursday evening for the San Bernardino mountains and High Desert. Make sure to secure outdoor furniture. Tree damage is possible.

* Additional chances of rain, wind, and mountain snow Thursday night through Sunday. Snow levels are expected to lower to 6500-7000 ft by Friday morning and 5000-5500 ft by the weekend creating slick conditions on mountain highways.

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 County to San Diego County deserts. After the rain band passes, scattered showers are expected to continue with locally heavy rain into Thursday morning (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. South winds are expected to increase on Tuesday afternoon, with the strongest winds expected during the day Wednesday. Peak wind gusts of 60-70 mph, locally up to 80 mph possible on the coastal slopes of the San Bernardino County mountains with gusts 45-65 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.

There still remains some uncertainty in the upper level pattern for late Thursday 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 as early as late Thursday continuing into Friday, with scattered light showers or dry conditions by the weekend. 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. In addition to rain and mountain snow chances winds are expected to increase over the mountains and into the deserts, with timing of the strongest winds dependent on timing and position of the incoming trough. 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.

AVIATION

222100Z....Coast/Valleys...Fog has cleared the southern portions of the San Diego coast with the Orange County coast clearing after 20Z. Low clouds will linger along the immediate coast in areas through the afternoon, before moving back onshore after 23/00Z. Bases initially 700-900 ft MSL will steadily rise overnight, likely to above 1500 ft MSL by 12Z Tuesday, and clouds will spread to cover the coastal basin. Areas of DZ early Tues morning will locally lower cigs/vis through the end of the TAF period.

Otherwise...SCT-BKN high clouds with unrestricted VIS continues through Tuesday morning.

MARINE

No hazardous marine conditions expected through Tuesday morning. An approaching storm system will lead to strengthening winds and building seas beginning Tuesday afternoon. Winds initially around 15- 20 kts gusting to 25 kts Tuesday afternoon will increase late Tuesday night, potentially to gale force with wind gusts upwards of 30-35 kts. This will bring steep seas of 7-10 feet. Additionally, periods of moderate to heavy rain Wednesday will lead to areas of poor visibility. Strong winds and steep seas are expected to continue through early Thursday morning. Winds will weaken slightly by Thursday morning, but likely remain hazardous to small craft through most of the day. Winds weaken and turn northwesterly Friday.

BEACHES

Frontal winds begin to make their way into the area Tuesday night into the early morning hours wednesday creating gusty winds and elevated surf heights of 6 to 8 ft. This will create hazardous swimming conditions at area beaches through Saturday morning.

HYDROLOGY

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 through 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 early 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) allowing for 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 Tuesday afternoon through early Thursday the current deterministic forecast is: - Cajon Pass westward: 7-10", locally higher - East of Cajon Pass: 5-8"

For remaining areas current deterministic forecast for Tuesday afternoon through early Thursday is: - Orange County: 3-4", highest north - Inland Empire: 2.5-4", highest northwest - San Diego County Coasts/Valleys: 2-3.5", highest north - Santa Ana Mountains: 3-5" - Riverside/San Diego County Mountains: 3-5" - High Desert: 1.5-3", highest west - Low Deserts: 0.75-1.5", 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 5000-5500 ft by the weekend.

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 8.9 ft early Thursday morning with the Santa Margarita expected to peak at 11.5 ft late Wednesday night. Interested parties should continue to monitor the river forecast as the forecasted peak and timing are highly dependent on the timing of the heaviest rainfall.

SGX WATCHES/WARNINGS/ADVISORIES

CA...High Surf Advisory from 11 PM Tuesday 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 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.

High Wind Watch from Tuesday afternoon through Thursday afternoon for Apple and Lucerne Valleys-San Bernardino County Mountains.

PZ...Gale Watch from Tuesday afternoon through Wednesday 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.