textproduct: San Diego

SYNOPSIS

Dry weather with above normal temperatures inland will continue through Monday. Periods of patchy fog in the coastal areas and valleys each morning through Tuesday. An Atmospheric River will bring widespread heavy rain and mountain snow above 8000 ft late Tuesday into Thursday (Christmas Day). Gusty southerly winds during this same period expected for much of the area, including the mountains, marine waters, and coast. There may be additional showers Friday through Sunday with chances lowering from around 70% on Friday to 40% on Saturday, and only around 15-20% Sunday.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

Key Points:

* Widespread moderate to heavy rainfall and a slight chance of thunderstorms Christmas Eve and Christmas Day with the potential for roadway flooding and localized flash flooding. 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 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 60-70% chance of wind gusts exceeding 35 mph along the coast Tuesday night into Wednesday. Make sure to secure outdoor objects. Tree damage is possible.

* Additional chances of showers Friday through Sunday with lower snow levels, creating slick conditions on mountain highways.

Low clouds spread into the San Diego County valleys early in the evening, but are much slower to push into Orange County this morning. Elsewhere there are a few high clouds moving east over the area. Low clouds will clear inland by late morning, but could stick to the coastal areas through the day. Not many day-to-day changes through Monday as a weak upper level high remains to our south or southeast, with high temperatures remaining around 10-15 degrees above normal inland, low clouds and patchy fog across the coastal areas and valleys each night and morning, and occasional high clouds.

A pattern change and significant precipitation is still on tap midweek as a large upper level trough amplifies off the West Coast, with the base of the trough extending as far south as around 15N latitude. An existing Atmospheric River with this trough will push into So Cal late Tuesday, and the highly amplified trough will also help pull in additional moisture from our south, though most of this is forecast to just brush the far southeast portion of our forecast area, with the bulk moving into Imperial County and western AZ. Both the GFS and EC are still showing IVT in excess of 700 kg/m/s. Ensemble guidance shows light precipitation beginning as early as Tuesday afternoon for the northern areas, with the widespread heavier precipitation occurring Wednesday morning through Wednesday evening. Strong southerly flow will accompany this initial wave of precipiation, resulting in gusty south winds across the coastal waters, coastal areas, and San Bernardino Mountains and adjacent desert foothills. 850-700 mb flow of 30-40 kt will enhance precipitation along the coastal slopes of the San Bernardino Mountains, where storm total precipitation could end up in double digits in spots. The snow level will be very high during the heaviest precipitation, generally between 8500 and 9000 ft. See the Hydrology section for details on precipitation amounts.

Deeper moisture moves out of the area Wednesday night into Christmas day, but weak short waves continuing to move through the mean trough will maintain chances of showers and a slight chance of thunderstorms. Overall precipitation will be lighter during this period, but brief heavy showers could occur, especially in thunderstorms. The snow level falls to around 7500-8000 ft by Thursday evening, so only the higher mountain peaks will be getting a white Christmas.

The evolution of the upper level pattern is uncertain as we head into Friday, Around 41% of the members have the upper trough further southeast, which would produce higher precipitation amounts and lower snow levels across So Cal, while the remaining 59% keep the trough further northeast for less precipitation and higher snow levels. NBM precipitation chances on Friday are around 70-80% for the mountains westward, and 40-50% in the deserts. By Saturday, the ensemble solutions with the more progressive trough are pretty much dry (around 50% of the members), while the slower solutions have at least light precipitation. The slowest cluster, encompassing around 34% of the members, has precipitation continuing into Sunday. 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

211000Z...Coast/Western Valleys...Low clouds based 900-1500 ft MSL with tops to 1800 ft MSL are present along coastal areas and will make their way inland to 15-20 miles overnight. Clouds very likely to make it to vcnty KONT, with a 60% chance for CIGs at the airport itself by 12Z. VIS restrictions 0-3 SM over higher inland terrain. Clouds scatter back to coasts 17-19Z, then moving back onshore after 22/02Z with similar bases/inland extent to tonight.

Otherwise...SCT-BKN high clouds with unrestricted VIS today.

MARINE

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 Wednesday, with seas near 8-10 ft and wind gusts upward of 30-35 kts.

HYDROLOGY

Widespread moderate to heavy rainfall is expected Tuesday night through Thursday, heaviest and most widespread during the day Wednesday. Strong south winds will favor the coastal slopes of the San Bernardino and eastern San Gabriel Mountains, generally from the Cajon Pass westward.

For the San Bernardino/San Gabriel Mountains 4 AM Tue through 4 AM Friday, probabilities of rainfall totals of 5 inches or more are as follows: - Cajon Pass westward: 75-85% - East of Cajon Pass: 35-65%

Probabilities of 8 inches or more: - Cajon Pass westward: 45-55% - East of Cajon Pass: 10-20%

Probabilities of 10 inches or more: - Cajon Pass westward: 10-20% - East of Cajon Pass: 5% or less

For remaining areas 4 AM Tue through 4 AM Friday, probabilities of rainfall totals 2 inches or more: - Orange County/Santa Ana Mountains: 85-95% - Inland Empire: 50-90%, highest west - Riverside/San Diego County Mountains: 60-80% - San Diego County Coast/Valleys: 40-75%, highest northwest - Deserts: 10-15% (lower deserts), 25-45% (high desert)

Probabilities of 3 inches or more:

- Orange County/Santa Ana Mountains: 65-80% - Inland Empire: 30-75%, highest west - Riverside/San Diego County Mountains: 40-60% - San Diego County Coast/Valleys: 10-50%, highest northwest - Deserts: 5-10% (lower deserts), 10-20% chance (high desert)

Current deterministic forecast for Tuesday through Friday morning: - Orange County: 3.3 - 4.5", highest north - Inland Empire: 2.5-4", highest northwest - San Diego County Coasts/Valleys: 2-3.5", highest north - San Bernardino County Mountains: 4-9", highest west - Santa Ana Mountains: 4-5.5" - Riverside/San Diego County Mountains: 2-4.5" - High Desert: 1.5-3", highest west - Low Deserts: 0.85-1.5", 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 7500-8000 feet. Snow levels could drop even lower for Friday and Saturday, but confidence in snow levels is low as the spread between the NBM 25th and 75th percentile snow level is around 1500-2000 ft.

The San Diego River at Fashion Valley is currently expected to reach Action/Monitor stage by 10 PM Wednesday. Based on current forecast, the river is expected to peak at 7.8 ft at 1 AM Thursday, but the official forecast only goes through 4 AM Thursday. There is a 27% chance the river could reach minor flood stage beyond the forecast period.

SKYWARN

Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions.

SGX WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...None.