895 FXUS66 KSGX 041105 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 305 AM PST Sun Feb 4 2024 .SYNOPSIS... A slow moving trough with an atmospheric river will bring a prolonged period of rain and mountain snow and a moderate to high risk of flooding today through Tuesday, heaviest on Monday and Tuesday. Gusty south winds develop ahead of the trough today, mainly near the coast and in the San Bernardino County mountains and deserts. Lighter winds follow for the remainder of the week. Additional weaker troughs will continue periods of showers Wednesday through Friday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... .SHORT TERM (Today-Tuesday)... Mid-level and high level moisture moving along the north fringes of weak ridge is bringing light showers this morning. So far rainfall totals have been around one-tenth inch or less. This elevated moisture will lift north and out of the area later this morning as the long wave trough off the coast rotates from a more east-west axis to a north-south one. Gusty south west winds develop ahead of the trough this afternoon into Monday morning, mainly near the coast and over the San Bernardino Mountains and deserts. Wind gusts along the immediate coast will be around 35-45 mph, while in the mountains high desert they will be around 40-50 mph. Isolated gusts as high as 65-70 mph are possible along the desert slopes and adjacent foothills of the San Bernardino County Mountains. Winds Monday afternoon and Tuesday will be relatively light. Deeper moisture associated with an atmospheric River begins to spread into Central and parts of Southern CA later today, first in Orange County/western San Bernardino County by the late morning and San Diego County and Riverside County this evening. This will be the beginning of a long period of precipitation as the trough remains relatively stationary through early Tuesday, keeping a weak to moderate atmospheric river aimed directly at So Cal. Some weak instability exists this afternoon into early Monday when slight chances of thunderstorms remain in the forecast. The instability and deep moisture will only exacerbate the potential flood issues as rainfall rates increase to 0.50"/hr or more, especially over Orange County and the coastal slopes of the San Bernardino Mountains. The thunderstorm threat decreases for the remainder of the day Monday, but increases again on Tuesday afternoon as the trough draws near and colder air aloft overspreads the region. Rainfall rates at this time will likely exceed 0.50"/hr again in some spots. Activity eventually begins to wane late Tuesday as the trough finally begins to progress east and takes the AR with it. While rainfall rates outside of any thunderstorms will be rather modest, mostly around 0.15-0.30"/hr, the relentless rain and impressive totals will likely cause flooding issues across much of the area. Current rainfall totals for today through Tuesday evening are as follows: Orange County: 4-5", up to 7" in the Santa Ana Mountains San Diego County Coasts/Valleys/Mountains: 2-4" Inland Empire: 2.5-5", highest near the foothills of the San Bernardino Mountains Riverside County Mountains: 2-4" San Bernardino County Mountains: 4-8", locally up to 11" on the south facing slopes Apple/Lucerne Valleys: 1.5-2" Coachella Valley: 0.7-1.5", locally up to 3" near the San Gorgonio Pass San Diego County Deserts: 0.5-0.75" The Weather Prediction Center (WPC) has a slight-moderate risk of Excessive Rainfall over Orange County today, a moderate risk for most areas along and west of the mountains on Monday and a high risk across the San Bernardino Mountains, and a slight risk on Tuesday for areas along and west of the mountains. The deserts have a marginal to slight risk each day. A Flood Watch remains in effect for the entire forecast area later today through Tuesday afternoon. As far as snow goes, snow levels will rise from around 6,000 ft this morning to 7,000-8,000 ft this afternoon through Monday evening, then falling to around 5,500-6,000 ft by Tuesday afternoon. Multiple feet of snow are forecast for areas above 8,000 ft. Between 7,000-8,000 ft totals will range from around 9-20 inches, and between 6,000-7,000 ft around 4-9 inches, most of which will occur on Tuesday. It should surprise no one that it will be cold through Tuesday, with highs lower to 10-15 degrees below normal by Tuesday. && .LONG TERM (Wednesday-Sunday)... But wait, there's more! We remain under a long-wave trough through the end of the week with several short waves passing through California. Mostly light showers will continue at times Wednesday and Thursday with snow levels between 4,500-5,000 ft. Another much, much weaker and shorter lived AR nudges into the region on Friday, bringing more widespread moderate rainfall. Additional accumulations of 0.25-0.50" for Orange County and the Inland Empire, 0.75-1" for the San Diego coasts/valleys, 0.75-1.75" in the mountains, and less than 0.10" in the desrts is expected for the second half of the week, though confidence is a little lower given ensemble spread. Another 3-8" of snow is forecast for the mountain communities, this time with San Diego County getting in on the snow action. The light at the end of the tunnel appears on Saturday when showers finally come to an end. Wednesday will be the coldest day of the week followed by slow warming through Sunday, though highs remain well below normal. Sunday we might actually transition to a dry period for So Cal, at least for a few days. && .AVIATION... 041015Z...Coast/Valleys/Mountains...FEW-BKN clouds with bases 4000- 15000 ft MSL today. Cloud bases lowering after 03Z Mon to 2000-3000 ft MSL in Orange County and the Inland Empire. Lower cloud bases reaching San Diego County 08-10Z Mon. ISOLD -SHRA will occur through 22Z Sun, with SHRA increasing after 00Z Mon in Orange County, the Inland Empire, and coastal slopes of the San Bernardino mtns. 15-20% chance of TSRA near KSNA, KONT, and KSBD 00Z-12Z Mon. SHRA will slowly spread south into late Mon AM. Local vis down to 2-5 SM in SHRA 00Z Mon through 18Z Mon. Lower cigs and vis expected after 18Z Mon in areas of +RA. Higher terrain will be obscured into Mon AM in clouds/SHRA. Deserts...SCT-BKN clouds with bases at/above 8000 feet MSL will continue into Monday with unrestricted surface VIS. SHRA possible as early as 10Z Mon. && .MARINE... Gusty south to southeast winds are expected today into Monday evening with peak gusts 25-30 kt. A combination of wind waves and a 9-12 second southwest swell will generate seas 10 to 12 ft. Seas could locally reach 14 ft on Monday. Strong winds and choppy/steep seas will create hazardous boating conditions Sunday through Monday. South winds will weaken Monday evening before becoming briefly easterly. Winds will eventually transition to westerly Tuesday evening into Wednesday. Periods of moderate to heavy rain are expected Sunday night through Tuesday as well as a chance of thunderstorms Sunday night into early Monday and again on Tuesday. && .BEACHES... An incoming southwest swell from 240 degrees of 8-12 feet with an 9- 12 second period is expected generate high surf today into Monday night. In Orange County surf of 4 to 8 ft expected, in San Diego County surf of 6 to 10 ft expected. Surf will be highest on Monday. Strong rip currents and hazardous swimming conditions are expected. There is a chance of thunderstorms at beaches Sunday night and Monday. Additional chances of thunderstorms at the beaches on Tuesday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...High Surf Advisory from noon today to 10 PM PST Monday for Orange County Coastal Areas-San Diego County Coastal Areas. Flood Watch from this afternoon through Wednesday morning for San Diego County Coastal Areas-San Diego County Deserts-San Diego County Mountains-San Diego County Valleys. Wind Advisory from 8 PM this evening to 10 AM PST Monday for San Diego County Coastal Areas. Flood Watch from noon PST today through Tuesday afternoon for Apple and Lucerne Valleys-Coachella Valley-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 Gorgonio Pass Near Banning-Santa Ana Mountains and Foothills. Winter Storm Warning until 6 PM PST Tuesday for Riverside County Mountains-San Bernardino County Mountains. Wind Advisory from noon today to 10 PM PST this evening for Apple and Lucerne Valleys. Wind Advisory from noon today to 10 AM PST Monday for Orange County Coastal Areas-Orange County Inland Areas. PZ...Small Craft Advisory from 6 AM this morning to 10 PM PST Monday 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 AVIATION/MARINE/BEACHES...CO