251 FXUS65 KVEF 122252 AFDVEF Area Forecast Discussion National Weather Service Las Vegas NV 252 PM PST Tue Feb 12 2019 .SYNOPSIS...Moisture associated with our next system will reach the southern Sierra Nevada tonight. The moisture will continue to stream eastward across the region Wednesday and Thursday with good chances of mountain snow and valley rain. Additional storms systems over the weekend will keep the region cooler than normal along with a threat of rain and snow showers. && .SHORT TERM...through Friday. The focus of the short term remains a developing atmospheric river event, which could prove to be significant with inland penetration across the region. Microwave satellite imagery already shows impressive poleward transport of tropical moisture, already as far as 30N over the eastern Pacific. Trends are expected to continue as upper-level trough will continue to sag south out of the Gulf of Alaska and phase with low pressure currently just east of Hawaii. The fire hose of moisture will take aim first at Northern California, then work southward and intensify as it does so. Precipitation will begin in the Sierra by Wednesday morning and intensify through the day. Wet-bulb effects might briefly lower snow levels to around 5500' to start the event, but should quickly rise through the day Wednesday, potentially to 9000' or above with the heaviest precipitation. This could limit major accumulations to above 8000' with transition zone of moderate amounts down to 7000'. Just where that snow level ends up will have impacts on any rain-on- snow runoff and flooding concerns, but minor flooding is possible with over QPF amounts over an inch forecast for the Owens valley, and in excess of 3-5" liquid/liquid equivalent in the Sierra, with snow amounts of 1-3 feet above 8000', and in excess of 3-5 feet for the crest. The AR will continue to slide southward on Wednesday, with quite a few models indicating moisture funneling inland around the south of the Sierra and Tehachapis, producing some precipitation chances in the southern Great Basin and local mountains on Wednesday. Fairly low confidence for precipitation and amounts in those areas for that time frame. The AR continues sinking south under cyclonic flow and strengthening Wednesday night into Thursday, and attention turns to the southern California mountains. PWATs are forecast to be near or at records for several hours. It looks like 18Z -00Z Thursday is the best window for maximum precipitation for the mountains in/around San Bernardino county, with plenty of spillover into places like Morongo Valley and Yucca Valley. Flooding becomes a concern for these areas and eastward with the progression of the AR Thursday. Decided to hoist a flash flood watch for southern portions of San Bernardino and Mohave counties for Thursday for the heavy rain and runoff risk. General amounts across the Mojave desert look to be around 0.25", but amounts closer to or in excess of 1" are possible if not likely in areas of the Flash Flood Watch. Would not be surprised to see areas like Las Vegas receive very little rain, while the mountains receive plentiful rain. Snow levels will likely rise above the 8000' mark on Thursday, which will limit accumulation in areas like Kyle Canyon in the Spring Mountains. Snow amounts there have been cut/shifted upward once again. Lee Canyon may pick up significant snow, so current advisory still in good shape. Areas of concern if the moisture plume wobbles a bit further north would be the Virgin and Muddy River basins. Anyone with concerns in these basins are not out of the woods, and should continue to monitor forecasts for updates. The area really starts to shut off Thursday night and Friday as the AR gets shoved even further southward, with only lingering shower activity in the mountains from colder air taking back over. However, it may take some areas several days to respond to the mountain rainfall, like the Big Sandy River in southern Mohave county. .LONG TERM...Friday night through Tuesday. A trough axis is forecast to swing through our area Friday night on the southern periphery of the big, cold low over the Pacific Northwest. This will provide enough forcing to wring out some more precip over the Sierra and the higher terrain of the southern Great Basin, possibly extending as far south as northern Mohave County. It will also kick up westerly winds, and cannot rule out needing a wind advisory for the places prone to enhanced west winds such as Barstow. Behind the trough axis, precip chances should mostly come to an end Saturday. The next storm is forecast to roll southward through California Sunday, bringing another round of precip chances to much of our area. Depending on how quickly it moves out, precip chances should end either Monday or Tuesday. Temperatures will be below normal through the period under the influence of long wave troughing. && .AVIATION...For McCarran...Clouds will slowly lower and thicken through Wednesday, with ceilings possibly falling to 8000 feet or below along with a slight chance of showers Wednesday afternoon. A stronger storm is expected to bring worse conditions Thursday afternoon into Friday morning, with low ceilings and visibilities possible along with gusty south to southwest winds. For the rest of southern Nevada, northwest Arizona and southeast California...Precip chances will begin in the Sierra this evening and spread across much of the area by Wednesday afternoon. Expect lowering ceilings with mountain obscurations. After a brief lull Wednesday night, a stronger storm will bring deteriorating conditions Thursday into Friday morning, with widespread valley rain and high mountain snow, leading to significant terrain obscuration. && $$ SHORT TERM...........STEELE LONG TERM/AVIATION...MORGAN For more forecast information...see us on our webpage: http://weather.gov/lasvegas or follow us on Facebook and Twitter