Mild and dry this weekend with breezy south winds developing over the mountains and deserts Sunday afternoon. A winter storm will bring moderate to heavy rain, mountain snow, and gusty winds Monday and Tuesday along with a slight chance of thunderstorms Monday evening. Snow levels fall through Wednesday, with elevations above 5000-6000 feet likely to see heavy snow and significant impacts. Additional chances of showers on Thursday, lowering to around 10% Friday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
, New Aviation Discussion,
There are a few patches of low clouds over the coastal areas and valleys, most widespread over the souther Inland Empire, with clear skies elsewhere. Expect low clouds to clear by late morning and scattered high clouds to move in this afternoon. More widespread low clouds across the coastal areas and valleys tonight into Sunday morning as the marine layer deepens with an approaching upper level trough. Otherwise high temperatures a few degrees above normal today, lowering to near to slightly below normal on Sunday. Breezy south to southwest winds develop across the mountains and San Bernardino County deserts Sunday afternoon and evening with peak gusts around 35 mph.
An upper level low currently off the Pac NW coast will slowly drop south through Monday morning before a negatively tilted short wave progresses across Southern CA. This wave is still forecast to have a weak Atmospheric River associated with it. Gefs and EPS ensemble means currently have Integrated Vapor Transport (IVT) peaking during the day Monday (though GEFS is a little slower) somewhere between about 300-500 kg/m/s, though the EPS mean and deterministic EC are on the stronger end. The EPS has a 45-60% chance of IVT greater than 500 kg/m/s for a brief period on Monday, though GEFS probabilities are closer to 30%. Though relatively narrow and short duration, this has the potential to produce moderate to heavy precipitation ahead of and along the cold front on Monday when hourly rainfall rates could exceed 0.50"/hr. There is a 15-20% chance of thunderstorms for the mountains westward Monday afternoon, which could produce locally higher rainfall rates. The snow level has trended a little lower with this AR as well, currently around 6500 ft for the San Bernardino Mountains and increasing to around 7000 ft in San Diego County. This will allow for more snow accumulation on Monday for places like Big Bear, while somewhat reducing the flash flood threat on upper portions of the burn scars. Wpc has also introduced a Slight Risk of excessive rainfall for Orange County into the lower slopes of the San Bernardino Mountains on Monday. The snow level falls to around 5000-5000 ft Monday night as precipitation turns lighter and more showery. We remain under cyclonic flow for the remainder of Tuesday, allowing for scattered showers to continue. See the Hydrology section for details on rain and snow amounts.
In addition to precipitation, gusty south winds will develop ahead of the cold front on Monday with widespread gusts of 25-35 mph for the coastal areas and valleys, 45-55 mph in the mountains and deserts, and isolated gusts of 65 mph on the wind prone desert mountain slopes/canyons. West winds behind the front will generally impact the beaches, mountains, and deserts.
There are still some timing differences in the ensembles regarding the passing of the next short wave on Wednesday, which will generate another round of widespread precipitation. Current NBM has highest probabilities of precipitation Tuesday evening through Wednesday morning, though around 34% of the ensemble space is a bit slower, bringing the wave through later in the day Wednesday. This next trough will be colder, with snow levels dropping to around 4500-5200 ft Wednesday morning and 4000-5000 ft Wednesday afternoon, highest south. Ensemble spread continues into Thursday with the progression of the next short wave and the position of the mean trough. A position too far inland (around 57% of the ensemble space) would limit the precipitation potential, while a slower/further west track with a deeper trough (the remaining 43% of members) would allow for more showers. This is especially critical as snow levels could fall as low as 3000-3500 ft across the San Bernardino Mountains and down to 4000 ft in the San Diego County Mountains in the colder/wetter scenario Thursday morning, leading to the risk of accumulating snow through the Cajon Pass/I-15 and near Pine Valley and Crestwood Summit on I-8.
Precipitation chances lower to around 10% for Friday as most ensembles have the short wave 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 occurs Friday and Saturday.
141830z, Clear skies and light winds should prevail for much of the day. Low clouds develop offshore tonight and will move inland beginning around 00z Sunday. Bases are expected to be higher, generally 1500-3500ft MSL, rising a bit through the night. Localized areas of VIS below 3SM for inland valleys and coastal mountains overnight. For KSAN, clouds may clear after 09z Sun, with intermittent MVFR CIGs through 16z, and VFR beyond then. SCT-OVC high clouds AOA 20,000ft MSL move in Sunday.
VFR conditions prevail elsewhere with increasing high clouds AOA 20,000ft MSL Sunday.
No hazardous marine conditions are expected through Sunday night. A 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 9-13 feet with wind gusts 25-35 knots, with briefly higher to 40 knots. A Gale Watch is in effect 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 of 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.
Ca, High Surf Advisory from 10 AM Monday to 4 AM PST Thursday for Orange County Coastal Areas-San Diego County Coastal Areas.
PZ, Gale Watch from Monday morning through late Monday night 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.
