textproduct: San Diego

SYNOPSIS

Periods of rain, wind, and mountain snow are expected Monday through Thursday. There is a slight chance of thunderstorms late Monday afternoon through early Tuesday morning. Lower elevation snow is possible Wednesday evening into Thursday, potentially as low as 3500 ft Wednesday night. Chances of precipitation decrease Thursday into the weekend.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

Key Points: *A winter storm will bring widespread heavy snow to the mountains above 6000 ft creating hazardous travel conditions Monday through Thursday. Snow levels as low as 3500 to 4000 ft Wednesday night may result in light accumulations through the Cajon Pass and along I-8.

*Gusty south to west winds are expected Monday from the coast to the deserts. Stronger and potentially damaging west winds along the mountain crests and adjacent desert slopes Tuesday afternoon through Wednesday. *Moderate to locally heavy rain is expected below the snow level. Ponding of water on roads and minor street flooding in urban areas possible. Increased flows on rivers, especially the San Diego, Santa Margarita, and Whitewater Rivers.

Areas of low clouds are developing over the coastal basin and should increase in coverage through the morning. Numerous high clouds continue to overspread the region ahead of an upper level low currently off the coast of Northern California. Today will be a few degrees cooler than yesterday with highs near to slightly below normal.

This low will dig south today before progressing eastward across So Cal on Monday. GEFS and EPS are still showing a weak to potentially moderate (40% chance) Atmospheric River moving into the area during the day Monday. Though narrow and quick moving, moderate to locally heavy precipitation is expected ahead and along the cold front with this AR early Monday afternoon through Monday evening, followed by additional post-frontal showers and a slight (15-20%) chance of thunderstorms for the mountains westward Monday evening into early Tuesday morning. Thunderstorms may produce brief heavier rainfall and small hail. In terms of snow, the NBM has snow levels rising to around 6500-7300 ft ahead of the front, though ensemble mean 700 mb temperatures of around -2C indicate snow levels closer to 7500 ft. This difference could limit snow accumulations on the front end of of the storm, though snow levels do fall rapidly behind Monday evening. See the Hydrology section for details on rain and snow amounts.

Gusty south winds will develop ahead of the cold front late Monday morning into the afternoon for the entire region, shifting to westerly and weakening for the coastal areas and valleys behind the front Monday evening, but remaining gusty in the mountains and adjacent desert foothills.

Moist cyclonic flow remains over the region on Tuesday for additional light showers. Strengthening gradients will generate strong west winds across the mountain crests and desert slopes, where gusts could exceed 70 mph. While most of these stronger winds will be over remote locations, there is the potential for damaging winds near far southern Lucerne Valley near the foothills of the mountains. These strong winds continue into Wednesday as the next short waves moves through. This will generate another round of more organized precipitation Tuesday night into Wednesday morning, followed by more light showers Wednesday afternoon. Snow levels during the more widespread and relatively heavier precipitation will range from 5000-5500 ft, falling to 4000-5000 ft Wednesday afternoon and evening. Another short wave moves through the mean trough sometime Thursday or Thursday night, though there is still uncertainty in the timing and track of this wave. Precipitation chances range from 20-40% for Thursday, lowering to below 15% on Friday and Saturday. Snow levels could fall as low as 3000-4000 ft by Thursday morning. If there are any appreciable showers during this time, it could lead to minor accumulations through the Cajon Pass (I-15) and in spots along I-8 near Pine Valley and Crestwood Summit, which could impact travel. Transitory upper level ridging will bring warmer and dry weather on Sunday.

AVIATION

151100Z... SCT-BKN low clouds based 2000-3000 ft MSL have filled the coastal basin with about 60% coverage of MVFR cigs. Coverage will continue to be especially patchy across southern SD County. The remaining clouds will eventually lift above 3000ft MSL around 15-17z before scattering out completely 18-21z. VFR conditions prevail in the afternoon with BKN high clouds in place AOA 20,000ft MSL. Clouds based 3-4 kft redevelop over the coastal basin foothills around 00z this evening, then gradually increase in coverage to the coast tonight between 04-08z, though remaining very patchy over SD County. Vis reductions where clouds and terrain meet.

MARINE

No hazardous marine conditions are expected through tonight. An incoming storm will generate strong winds with high and choppy seas beginning Monday, weakening a bit on Tuesday before increasing again on Wednesday. Seas expected to be around 8-12 feet with wind gusts 25-35 knots, with briefly higher to 40 knots. A Gale Warning is in effect from 7 AM Monday through 7 AM Tuesday and contains more information. There is also a slight chance of thunderstorms over the waters Monday afternoon/evening.

BEACHES

Increasing westerly swell (9-11 feet with an average period of about 10-12 seconds) will produce large breaking waves up to 8-12 feet on most beaches across Orange and San Diego Counties from Monday afternoon through Wednesday. Widespread, strong rip currents are also likely with these waves and dangerous swimming conditions expected. A High Surf Advisory is in effect and contains more information.

HYDROLOGY

A winter storm will bring widespread moderate to heavy precipitation and mountain snow on Monday, followed by scattered showers and a slight chance of thunderstorms Monday evening into early Tuesday morning. Light showers continue Tuesday, then another short wave will produce more organized precipitation with lower snow levels on Wednesday. Additional light showers are possible Thursday into early Friday. Precipitation chances are currently below 15% for Friday and Saturday, but there is the potential for light showers at times.

Peak hourly rainfall rates during the frontal passage on Monday will range from 0.40-0.70" per hour. Current forecast rainfall totals for Monday and Tuesday will be 0.75- 1.40" for the coast and valleys, 1-3.5" for the mountains, 0.20-0.80" for the High Desert, and 0.10-0.50" for the low deserts, locally up to 1" near the San Gorgonio Pass in the far northern Coachella Valley. Much of this rain will fall on Monday, with light to locally moderate showers possible on Tuesday. A Slight Risk of excessive rainfall is forecast for Orange County into the lower slopes of the San Bernardino Mountains on Monday.

If higher end rainfall amounts materialize across the northern Coachella Valley, increased flows on the Whitewater River could cause flooding concerns at some of the low water crossings. Latest river forecast ensembles have around a 22% chance of the San Diego River reaching Monitor Stage on Monday and Wednesday.

Snow levels at the onset of precipitation are currently forecast around 6500 ft for the San Bernardino Mountains and increasing to around 7000 ft in San Diego County. This will reduce the flash flood threat on upper portions of the burn scars. The snow level falls to around 5000-5000 ft Monday night as precipitation turns lighter and more showery. For the first round of snow Monday and Tuesday, 5-12 inches expected at 6000-7000 ft, 12-18 inches expected at 7000-8000 ft, and up to 26 inches above 8000 ft.

For the 48 hour period of 4 AM Wednesday to 4 AM Friday, the NBM has a 30-50% chance of 0.50" or more for the coasts and valleys, a 40-70% chance in the mountains, and a 5-10% chance in the deserts. For this same time period, there is around a 25% chance of at least 6" of snow at the 5500 ft level, increasing to around a 40% chance at 7000 ft. Chances of accumulating snow (0.1" or more) are around 50% through the Cajon Pass and along higher portions of I-8.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

CA...Wind Advisory from 7 AM to 8 PM PST Monday 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.

High Surf Advisory from 10 AM Monday to 4 AM PST Thursday for Orange County Coastal Areas-San Diego County Coastal Areas.

Winter Storm Watch from Monday morning through Thursday morning for San Bernardino County Mountains.

Winter Storm Watch from Monday morning through Thursday morning for Riverside County Mountains.

High Wind Watch from Monday morning through late Monday night for San Diego County Mountains-San Gorgonio Pass near Banning.

Wind Advisory from 6 AM to 10 PM PST Monday for Apple and Lucerne Valleys.

Wind Advisory from 10 AM Monday to 1 AM PST Tuesday for San Diego County Deserts.

PZ...Gale Warning from 7 AM Monday to 7 AM PST Tuesday 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.