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. Low elevation snow is possible Wednesday evening into Thursday potentially as low as 3500 ft Wednesday night/Thursday morning. Periods of rain/snow and elevated winds continue into Thursday. Chances of precipitation decrease Friday into the weekend.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

Update: The latest model data reflects that the first initial showers ahead of the frontal boundary associated with the first system moving across the region tomorrow should begin over the inner waters of the SoCal bight by around 10 AM, or possibly earlier, and begin to move up over the northwestern areas of Orange County shortly before Noon tomorrow. This line of showers (and possibly even a few embedded thunderstorms), will move over the IE between 12-2 PM, and then eventually will track over San Diego County by around 2 to 5 PM during the afternoon hours tomorrow. Between these hours, this will likely be when the heaviest rainfall occurs, which may possibly produce rainfall rates up to 0.8 inches per hour for some of the southern facing slopes of the San Bernardino Range. This may possibly need flash flooding concerns for the burn scar areas, and these will be closely monitored. After this first initial push of heavy precipitation moves through, showers, and possibly a few thunderstorms, will become more intermittent by later Monday night and into Tuesday, before the next wave moves through with even colder temperatures and reinforce the chance of additional precipitation across the region.

Snow levels will begin around 6500-7000 ft across the San Bernardino Mountains, tapering upwards towards 7500-8000 ft ahead of the cold front. After passage, the snow level will drop down to around 5500- 6500 ft going into Monday night, and then even further down to 4500- 5500 ft by Tuesday night. Snowfall amounts for some of the highest peaks located within the San Bernardino Mountains could approach up to 3 ft locally. At lower elevations, there could also be flooding concerns near the Lytle Creek area. Once snow levels drop enough to received snow in the Big Bear Lake recreational area, there could be amounts upwards of 12 to 18 inches of snow possible at the base of the ski resorts, and possibly more than 2 ft at the summits of both Summit and Bear Mountain. For the mountains of Riverside and San Diego counties, there could be similar amounts of possibly more than 2 ft of snow at San Jacinto through Tuesday, and amounts possibly of 3 to 6 inches for Mt Laguna. It will also become quite windy for some locations, with gusts approaching 70 mph through San Gorgonio Pass, and other gusts of greater than 60 mph for some of the high terrain and locations prone to strong winds, and also within some of the lower desert communities, where there can also be blowing dust as a result. Please see the latest Winter Storm Warnings and Wind Advisories for more information.

Previous discussion submitted at 430 PM:

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.

A winter storm will bring rounds of rain, mountain snow, and gusty winds to the area through Thursday. An upper level low pressure currently off the coast of Northern California will dig south and east Monday, pulling moisture from a weak atmospheric river into Southern California. This will bring widespread moderate to locally heavy rain and mountain snow Monday afternoon into Tuesday morning. Isolated showers could continue into mid-Tuesday morning, most likely in the mountains, with dry conditions possible elsewhere. A robust shortwave will move through the main troughing pattern Tuesday night into Wednesday, sparking another round of widespread rain and mountain snow. There still remains uncertainty in overall precipitation totals for Tuesday night into Wednesday but most ensemble guidance is indicating less precipitation for lower elevations than the first round. A third shortwave is then expected to move east across the area Thursday into early Friday, which could again spark additional rounds of widespread precipitation. There remains some uncertainty in how far south that final shortwave will dig, but there are indications additional rounds of at least light precipitation are possible. Details surrounding expected rain and snowfall can be found in the Hydrology section below.

In addition to rain and mountain snow with each shortwave passage, gusty onshore winds can be expected. Onshore flow has already begun to increase this afternoon, with some elevated southerly winds gusting near 30 mph in the San Bernardino mountains. Peak gusts this afternoon into Monday morning will be around 35 mph. Southerly winds will increase Monday morning ahead of the main cold front. The strongest winds are expected to occur with and the few hours after the passage of the cold front. The front will move from northwest to southeast across the area starting in Orange County/Inland Empire early Monday afternoon, reaching San Diego county by late Monday afternoon. For the coast, widespread gusts of 30 to 40 mph expected, with some local gusts up to 45 mph possible with the frontal passage. For the deserts, gusts 40 to 55 mph expected. The mountains and desert mountain slopes are expected to see gusts 55 to up to 65 mph. Gusty winds could result in hazardous travel conditions, particularly for high profile vehicles; falling tree limbs; or uproot shallow rooted trees. Winds will weaken briefly early Tuesday morning before increasing again for the mountains and deserts ahead of the second shortwave. 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 San Bernardino mountains. These strong winds continue into Wednesday. A third round of increased winds are possible for Thursday, with lower confidence in timing and speeds.

Precipitation chances lower to around 10% for Friday as most ensembles have the final shortwave off to the east by then. High temperatures will be well below normal through the week, as much as 10-15 degrees below on Wednesday. Slow warming is expected Friday through the upcoming weekend.

AVIATION

160600Z...BKN around 20,000ft MSL continue to stream in and are largely limiting satellite's view of lower clouds. Based on surface obs, the lower clouds are based generally around 2000-3000ft MSL, but are largely FEW-SCT along the coasts at this hour, with better coverage found across the inland valleys. As a storm system approaches, these cloud bases should lift overnight, lifting above 3000ft MSL by 12z Monday. VIS may fall below 2SM for some inland valleys and where clouds intersect terrain.

A significant system arrives after 12z Monday with widespread south to southwest gusty winds generally after 16z. Gusts up to 30kts for the coasts and inland areas, with up to 45-55 kts for the mountains, desert foothills, and mountain passes. Winds will generally shift from south-southwest to westerly following a front that pushes through after 21z Monday. Strong up/downdrafts near mountains, BLDU with reduced VIS in the deserts, and sporadic LLWS possible around sunrise Monday for inland areas.

VCSH for parts of Orange and San Bernardino counties from 15-20z Monday before the main precipitation arrives, largely -RA filling in thereafter, spreading into Orange County by 20z, and the Inland Empire/San Diego County 21-23z. The main band of heavier rainfall will push through starting around 22z Monday in Orange County, and sweeping eastward through the mountains by about 02z Tuesday. Likely VFR CIGs near 2000-3000ft will accompany the -RA with VIS down to 3- 5SM in RA/BR. The heavier rainfall could drop VIS below 1SM with accompanying CIGs around 700-1000ft MSL. Behind the main band, off an on lighter rain showers remain possible into the overnight hours Monday night into Tuesday. Intermittent CIGs near 2500ft MSL remain possible with these showers and VIS occasionally down to 3-5SM.

MARINE

An incoming storm will generate strong winds with high and choppy seas beginning Monday, weakening a bit on Tuesday before increasing again on Wednesday, prevailing into Thursday night. 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 Friday morning. 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.

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 Warning from 10 AM Monday to 9 AM PST Thursday for Riverside County Mountains-San Bernardino County Mountains.

Wind Advisory from 10 AM Monday to 1 AM PST Tuesday for San Diego County Deserts-San Diego County Mountains-San Gorgonio Pass near Banning.

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

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.