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.
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.
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.
Periods of moderate to locally heavy rain are possible along the main cold front Monday afternoon and evening. With the cold front passage there is a 15-20% chance of thunderstorms Monday evening into early Tuesday morning. Peak hourly rainfall rates Monday afternoon and evening will range from 0.40-0.70" per hour. Overnight Monday into Tuesday morning, rainfall rates of up to 0.30" per hour are expected. Dry conditions possible Tuesday afternoon with only light isolated showers, most likely in the mountains. Current forecast rainfall totals for Monday and Tuesday are 0.90-1.50" for the coast and valleys, 2-3.5" for the mountains, 0.25-0.40" for the High Desert, and 0.10-0.30" for the low deserts. A Slight Risk of excessive rainfall is forecast for Orange County into the lower slopes of the San Bernardino Mountains on Monday.
The second and third round of rain Tuesday night into Wednesday and Thursday into Friday morning are expected to be lighter for lower elevations. Hourly rainfall rates for Wednesday are expected to be 0.20-0.30" per hour. Forecast rainfall totals for Wednesday and Thursday are 0.40-0.80" for the coast and valleys, 0.20-0.40" for the High Desert, and 0.10-0.25" for the low deserts.
If rainfall amounts trend up for the northern Coachella Valley, increased flows on the Whitewater River could cause flooding concerns at some of the low water crossings. The San Diego River at Fashion Valley is expected to reach monitor stage Monday and again on Wednesday. Interested parties should continue to closely monitor the river forecast and observed river levels this week. Current forecast indicates minor flood impacts to Avenida del Rio may occur. Please see the latest River Statement or CNRFC website for more information.
Snow levels at the onset of precipitation are currently forecast around 6500 to 7000 ft for the San Bernardino Mountains and increasing to around 7500 ft in San Diego County. This will reduce the flash flood threat on upper portions of the burn scars and higher elevations of area mountains. The snow level falls to around 4500-5000 ft early Tuesday morning as precipitation turns lighter and more showery. For the first round of snow Monday, 6-10" expected between 6000-7000 ft, 10-18" expected 7000-8000 ft, and up to 30" above 8000 ft. Cold air will remain in place through Thursday, with snow levels 4500-5500 ft Tuesday through Wednesday, potentially to dropping to 3500-4000 ft Wednesday evening into Thursday. For Wednesday through Thursday snow forecast has an additional 4-10" 5000-6000 ft, 10-16" 6000-7000 ft, and 16-20" above 7000 ft. Locally higher amounts (up to 24") possible above 8000 ft. The San Diego County mountains may see some accumulating snow, with totals of 1- 3", locally higher at the highest peaks. Chances of accumulating snow (0.1" or more) are around 50% through the Cajon Pass and along higher portions of I-8 Wednesday night into Thursday morning.
151815z, Satellite shows two cloud decks, one FEW-BKN around 2000- 5000ft MSL and another BKN AOA 20,000ft MSL. VFR prevailing today before more widespread low clouds around 2000-4000ft MSL bring high- end MVFR to low-end VFR conditions overnight and into Monday morning. VIS will be below 2SM for some valleys and where clouds intersect terrain. A significant system arrives after 12z Mon with widespread south to southwest gusty winds generally after 16z. Gusts up to 30 mph for the coasts and inland areas, with up to 50-60 mph for the mountains, desert foothills, and mountain passes. 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 prior to 14z Mon, with -RA thereafter. -RA possible for San Diego and Riverside counties after 17z. Rain will become more widespread through Monday afternoon with significantly reduced VIS and LIFR possible at times with heavier rain.
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, 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.
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.
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.
