000 FXUS66 KSGX 290451 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 951 PM PDT Thu Mar 28 2024 .SYNOPSIS... A cold Pacific storm moving down the West Coast will bring widespread rain and mountain snow to Southern California this weekend. Steady precipitation Saturday morning will transition to showers and isolated thunderstorms Saturday night and Sunday. Lingering showers Monday will be followed by dry and warmer weather Tuesday and Wednesday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... EVENING UPDATE... Low clouds have quickly developed across the coastal basin and even into some of the mountain foothills this evening. The 00Z KNKX sounding shows the marine inversion based near 1800 ft MSL, but based on areal extent of low clouds its evident that it has already deepened significantly since then. Otherwise there were scattered high clouds moving across the region. Breezy onshore flow is occurring with gusts generally 25-35 mph in the mountains and deserts, locally near 50 mph through the San Gorgonio Pass. These winds will continue into Friday, becoming more widespread across the deserts during the afternoon and evening. Timing of Wind Advisory for the deserts was adjusted to include the winds on Friday. PREVIOUS DISCUSSION... An atmospheric river ahead of a trough deepening down the West Coast Friday will bring a period of steady moderate to heavy rain to Southern California Saturday. Showers and isolated thunderstorms are forecast Saturday night and Sunday in the cold, unstable airmass behind the cold front. A Flood Watch has been issued from late Friday night through Sunday evening. The Weather Prediction Center has a risk of Excessive Rainfall and increased flood potential from the coast to the mountains this weekend. A strong shortwave diving south out of the Gulf of Alaska tonight will deepen a longwave trough down the West Coast Friday. This trough taps into a conveyor belt of moisture over the Pacific that will make landfall over the Southwest Friday night and Saturday. Moderate to heavy rain over Santa Barbara Friday evening will move south across the LA basin Friday night, and then spread south into Orange, San Bernardino, Riverside and San Diego Counties Saturday morning. This is when steady moderate to heavy rain is likely from the combined effects of the 500 mb positive vorticity advection, lifting along the surface cold front and the orographic enhancement over the mountains. Rainfall rates may exceed 0.50"/hr in the AR Saturday morning. After the AR passes Saturday afternoon, the cold core low behind the trough will settle over the Southwest. A second trailing vort max will help fuel scattered showers and a thunderstorms in the unstable airmass Saturday night and Sunday. A Winter Storm Watch has been issued for the mountains above 5,000 feet this weekend. Snow levels Friday night and early Saturday will rise to 6,000 feet in the warm air advection ahead of the cold front, then fall to between 4,500 and 5,000 feet Saturday night, and hover between 5,000 and 5,500 feet Sunday. Snow accumulations will be greatest in the San Bernardino Mountains above 7,500 where over a foot of snow is expected. Showers will linger into Monday in the broad cyclonic flow trailing the upper low which will dig all the way to central Baja Monday evening. Ridging slowly builds over the Southwest Tuesday through Thursday for warmer days. && .HYDROLOGY... Rain and mountain snow will begin as early as Friday night in Orange County and the San Bernardino Mountains, then move south across the Riverside and San Diego Counties Saturday morning. On Saturday night and Sunday, showers will be fairly widespread with locally heavy rain rates in any thunderstorms in the cold unstable airmass behind the trough. Forecast Rain Totals Through Monday: Coastal and Valley........1.00"-2.00" Mountains.................2.00"-3.50" locally 5.00" in the SBD mtns Highs Deserts.............0.50"-1.00" Lower Deserts.............0.25"-0.50" The snow level near 6,000 feet late Friday night and Saturday morning will fall to 4500 feet Saturday night, then fluctuate between 5000 and 5,500 feet Sunday. Projected Snow Totals Through Monday: 4500-5500 ft...0-3" 5500-6500 ft...3-6" 6500-7500 ft...6-12" Above 7500 ft..12+" The current forecast for the San Diego River at Fashion Valley has the river rising on Saturday, but remaining below the low-water crossings. With additional rainfall on Sunday, the river could rise sufficiently to flood the low-water crossings late Sunday. && .AVIATION... 290430Z...Coast/Valleys...Areas of mostly BKN CIGs will continue through 18Z Fri, with some airports likely SCT TEMPO BKN. Bases will rise from 1500-2000 ft MSL through 07Z Fri to 2000-3500 ft MSL 07Z- 18Z Fri with tops to 5000 ft MSL. Local vis 2-5 miles in BR will occur in the valleys 07Z-18Z Fri. Otherwise, occasional high clouds above 20000 ft MSL will occur through Fri evening, with clouds developing in the 3500-7000 ft MSL layer Fri evening. Mountains/Deserts...Local terrain obscurations in clouds will occur below 5000 ft MSL on the coastal slopes through 18Z Fri. Areas of west winds 20-30 kt with local gusts 35-45 knots will occur mainly from the mountain crests east/north through the desert slopes and into adjacent deserts through Fri evening, along with MOD UPDDFs and local LLWS. Otherwise, occasional high clouds above 20000 ft MSL will continue through Fri evening with unrestricted VIS. && .MARINE... No hazardous marine conditions are expected through Friday. A Small Craft Advisory has been issued for southerly winds 20-25 knots with gusts to 30 knots and choppy seas Friday night and Saturday morning. A distinct wind shift from south to west will occur with a front moving through the coastal waters Saturday morning, with weaker winds and less chop thereafter. There is a slight chance of thunderstorms Saturday night through Sunday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Flood Watch from late Friday night through Sunday afternoon for Orange County Coastal Areas-Orange County Inland Areas- Riverside County Mountains-San Bernardino County Mountains- San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Coastal Areas-San Diego County Mountains-San Diego County Valleys-Santa Ana Mountains and Foothills. Winter Storm Watch from Friday evening through Sunday evening for Riverside County Mountains-San Bernardino County Mountains. Winter Storm Watch from Saturday afternoon through late Sunday night for San Diego County Mountains. Wind Advisory from 2 PM Friday to 7 PM PDT Saturday for Apple and Lucerne Valleys-Coachella Valley-San Diego County Deserts-San Gorgonio Pass Near Banning. PZ...Small Craft Advisory from 11 PM Friday to 5 PM PDT Saturday for Coastal Waters from San Mateo Point to the Mexican Border and out to 30 nm-Waters from San Mateo Point to the Mexican Border Extending 30 to 60 nm out including San Clemente Island. && $$ PUBLIC...SS/Moede HYDROLOGY...Moede AVIATION/MARINE...Maxwell