textproduct: San Diego

SYNOPSIS

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.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

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.

HYDROLOGY

In terms of precipitation amounts, for the 48 hr period from 4 AM Monday - 4 AM Wednesday, the NBM chances for at least 0.50" are around 90-100% for the mountains westward, 40-75% over the high desert, and 15-45% over the low deserts. Chances of at least 1" are around 45-85% for the coasts and valleys, 70-100% for the mountains, 10-25% for the high desert, and 5-15% for the low desert. Hourly rain rates will be near 0.40-0.70"/hr as the system on Monday moves through. Rates will be much lower on Tuesday and Wednesday.

Current official forecasted rainfall totals range from around 1.25-2" for the coasts and valleys, 2-5" inches in the mountains, 0.50-1.50" for the high desert, highest near the foothills of the mountains, and around 0.30-0.75" for the low desert, locally around 1.75" near the San Gorgonio Pass. If these higher amounts verify over 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 15% chance of the San Diego River reaching Monitor Stage on Monday and Tuesday.

Significant snowfall is possible with this storm system next week. At the 6500 ft level in the San Bernardino and Riverside County mountains, there is around a 75% chance of at least 12" of snow from 4 AM Monday through 4 AM Wednesday and around a 25-35% chance of at least 12" down at the 5000 ft level. Given the higher snow levels in San Diego County, chances of at least 12" at Palomar and Mount Laguna are around 15-20%. Storm totals for the week could approach 2 feet above 7000 ft in the San Bernardino and Riverside County Mountains.

Additional light rain and mountain snowfall may occur later in the week with low confidence on exact totals.

AVIATION

141100Z...FEW-SCT low clouds based 1500-2000 ft MSL are passing offshore near the coast. Occasional cigs from these clouds are possible at coastal sites (10-20% chance) through 16z. Patchy FG has developed in the southern Inland Empire, lowering VIS locally down to 1/4SM. FG scatters out 16-18z Sat. Increasing low clouds based 2500-3500 ft MSL develop around coastal and foothill areas after 01- 03Z Sunday, and then fill in coverage into the inland areas, filling the entire coastal basin by around 06z. VIS reductions where clouds intersect with terrain.

VFR conditions prevail elsewhere.

MARINE

No hazardous marine conditions are expected through Sunday night. A storm system will generate strong winds and high and choppy seas likely hazardous to small craft Monday, continuing through Wednesday. Seas expected to be around 9-13 feet with wind gusts 25- 35 knots. There is also a slight chance of thunderstorms over the waters Monday afternoon/evening.

BEACHES

Combined elevated swell (about 9-11 feet with an average period of about 10-12 seconds) from the west and southwest (240-280 degrees) will likely provide 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 are expected.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

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