047 FXUS65 KSLC 080408 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 1008 PM MDT Thu Oct 7 2021 .UPDATE...Made minor edits to the forecast this evening. Lead shortwave (per WV, obs and SPC mesoanalysis) is currently lifting northward across northern Utah this evening, directly responsible for the development of the ongoing convective complex rolling northward, mostly along and north of I-80 as of 10PM MDT. Locally heavy rain and gusty outflow winds will remain threats as this activity rolls northward. Farther south, convection generally struggled to initiate as optimal shear/instability balance wasn't reached. Have thus lowered PoPs here through 6AM. A cluster of thunderstorms is currently moving northeastward along/north of a Price to Sunnyside line. 00Z CAMs indicate the current batch of showers/storms over northern UT will continue to propagate north and east with continued development expected again north of I-80 owing to increasing large-scale ascent. This activity should largely diminish around sunrise. Meanwhile, an amplified trough will swing into northern California Friday morning. Multiple shortwaves will rotate north and east across the forecast area through the day in the presence of increasingly favorable jet dynamics. Thus, precipitation remains on track to blossom from SW to NE through the morning. The previous discussion follows. See updated SLC aviation discussion below as well. && .SYNOPSIS...An active weather pattern will prevail across the region through the weekend and into early next week, as a series of increasingly cold storm systems pass through the area. && .SHORT TERM (Through 12Z/6AM Sunday)... Late this afternoon a stalled baroclinic zone remains in place near the Utah/Idaho border. A wave moving through Nevada will continue to track eastward along this boundary and across northern Utah this evening and overnight. Thunderstorms continue to develop ahead of the wave over northeastern Nevada and southern Idaho, with a few initial thunderstorms starting to develop over northern Utah in the increasing large scale ascent. Through this evening, expect showers and thunderstorms to grow upscale as moving out of northeastern Nevada and into northwestern Utah where they will be rooted in the surface based-instability south of the front and able to develop a cold pool balanced with the increasing speed shear. This will support an organized convective line moving tracking mainly along and north of I-80 this evening, arriving into the Northern Wasatch Front and Cache Valley areas by 8-10 PM this evening. Thunderstorm wind gusts 40 to 55 will be most common, with a few isolated severe gusts to around 60 mph possible. Highest confidence is wind gusts will be across northwestern Utah, with decreasing confidence as storms arrive into the northern Wastach Front, as storms may decouple from the boundary layer after sunset limiting wind mixing to the surface. SPC continues to outline this portion of the state in a Marginal Risk. The initial round of showers and thunderstorms will weaken and exit into Wyoming overnight, however more scattered showers are likely to remain in place over northern Utah Friday morning in the vicinity of the baroclinic zone. A southern stream wave will eject ahead and phase with a northern stream wave digging into the Pacific Northwest, bringing a prolonged period of synoptic forcing for precipitation across the state. Precipitation will fill ahead of the ejecting wave across southwest into central Utah Friday morning through early afternoon. Precipitation rates will largely be light to moderate, but with southwesterly flow this will result in orographic enhancement around the Pine Valley Mountains and over and south of Brian Head toward Zion National Park. Prolonged moderate will likely develop flows on many dry washes and slot canyons in the area. Meanwhile across northern Utah a surface low is expected to develop over northeastern Nevada and track into northern Utah Friday afternoon and evening ahead of the upper-level low. This will track along the baroclinic zone in place in northern substantially increasing low-level convergence ahead of the low, into the boundary, with upper-level divergence and cooling aloft steeping mid-level lapse rates. Thus given unseasonably high PW and CAPE values, and the subtropical sourced moisture advection with the southern stream wave moving into the boundary, there is an elevated risk of heavy rainfall Friday afternoon and evening. The focus for heaviest rainfall will set up along an axis from Tremonton/Brigham City over to Logan. Training and efficient heavy rain production are possible along this corridor, and thus have issue a Flood Watch for Friday afternoon through Friday evening, with the main concern being areal flooding of basements, poor drainage areas, urban areas and small streams. Otherwise, much of the rest of the state will see beneficial precipitation. This storm system will be on the warmer side, with snow levels initially over 10kft dropping to around 7-8kft by Saturday morning. However, this will bring the first signficiant snow to the high Uinta Mountains, and have issued a Winter Weather Advisory above 9kft where accumulations of 4 to 8 inches are expected, with locally higher totals to 12 inches at higher elevations. The high peaks of the Wasatch will also likely get in on some of this snow into Saturday morning, especially from the central Cottonwoods northward where linger low-level moisture, westerly flow and lake/orographic enhancement will allow precipitation to linger. Impactful snow accumulations are likely expected to remain above road level, except for the Bald Mountain Pass in the High Uintas, at the very top of Little Cottonwood and over Monte Cristo summit. By later Saturday into Sunday drier and anticyclonic flow will bring and end to precipitation across the area. .LONG TERM (After 12Z/6AM Sunday)... The early winter-like potent storm system continues to be the dominate weather maker from Monday through Wednesday. So if you want a day to play in the outdoors, Sunday will be the best day under mostly sunny skies, although temperatures will be about 5-10 degrees below normal, which means upper 50s to lower 60s most valleys. Now the storm of interest! What we know is that this will be a cold system and snow will occur in all mountains and widespread rain will occur in the valleys. The questions to be answered will be who will receive the most precipitation, how much, and which valleys will have the best chance of receiving their first snowfall. The ensemble means have been in great agreement on developing this cold deep trough over the Great Basin from Monday through Wednesday, but there are still some differences in the exact depth and the trough axis location. The deterministic model runs have shifted slightly westward with the core of the coldest air moving southward across Nevada which the latest 12Z EC ensemble supports vs the farther east solution of the trough axis of the GEFS along the NV/UT border. The NBM appears to be leaning more heavily towards the EC solution. Have gone this direction and then some with increased PoPS across northern Utah Monday morning and from the southwest portion of the CWA to the Uintas by Monday afternoon. Have backed off on PoPs in the Uinta Basin as the NBM has had a wet bias on the leading edge of storms there in the past. With warm advection wrap around precipitation becoming the dominant feature for precip production, have upped the PoPs where this looks most prevalent which will be along the mountain spine and across the I-15 corridor. Depending on the intensity and how cold the boundary air will be will determine whether snow will reach down to the valley floors. Right now it looks like western valleys above about 5000 ft will received some snow, in particular from Kanosh to Cedar City. Another area that typically would be of concern for getting a surprise snow under wrap around precip is in Castle Country. However, since the low level cold air will remain mainly west of the mountains, and no antecedent cold air in place, upslope flow is not expected to bring snow levels down to the valley floor there during this storm. By the time the colder air arrives it will most likely be downsloping and drying conditions developing. Central and northern mountains above 7500 ft should receive 6-12 inches of snow with locally more possible while 4 to 10 inches are more likely in the southern mountains. The storm system moves out of most of the CWA by Wednesday, although some showers will remain under the colder air aloft over the northeast portion of Uta, primarily the Uintas. Temperatures will rebound from being 20-30 degrees below normal on Tuesday, to 10-20 degrees below normal Thursday. Mostly sunny skies return Thursday. && .AVIATION... KSLC...(UPDATE): The main area of thunderstorm activity has pushed north and east of the terminal. Outflow boundaries have been producing isolated gusts up to 24 knots. These winds will continue to diminish over the next few hours, but winds will stay out of the north through 06z, before switching to a southerly flow. Vicinity showers will continue to develop through midnight but should bring minimal impacts to the terminal REST OF UTAH AND SOUTHWEST WYOMING...Widespread VFR conditions can be expected across the southern portions of the airspace through 12Z Friday, while scattered showers and thunderstorms prevail across the northern half. Isolated mountain top obscurement is possible in heavy rain. After 12Z Friday a moist southwest flow will spread across the entire airspace with periods of heavy rain obscuring most mountain tops. MVFR conditions will be prevalent across the mountain spine from near Cedar City northeastward to SW Wyoming. && .FIRE WEATHER... Showers and thunderstorms will track across northwest Utah late this afternoon and evening bringing a corridor of likely wetting rains. Otherwise, a few showers and light rain (snow in the high Uintas) is expected farther southeast through this evening. A stronger low pressure system along the Pacific Coast will move inland and slowly cross the area beginning late tonight, and continuing through Friday and into the day Saturday. Widespread precipitation will accompany this system, with the heaviest precipitation falling across northern Utah, as well as locally around the Pine Valley Mountains and Brian Head area. The cold front associated with this system will cross the area Friday night, dropping snow levels to near 8000 feet across northern Utah by early Saturday morning. This will allow for a few inches of accumulation across the higher terrain of northern Utah, while cooler air spreads across all areas. The heaviest snow will remain above 9000 feet in the Uintas and upper Cottonwoods where 4 to 12 inches is possible. A colder storm system remains on tap for late Monday into Tuesday, which will bring the coldest temperatures of the season thus far. Widespread precipitation looks to accompany this system, with the potential for snow levels to fall to the valley floors by Tuesday morning. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...Flood Watch from Friday afternoon through Friday evening for UTZ103-107-110. Winter Weather Advisory from 6 AM Friday to 6 PM MDT Saturday for UTZ112. WY...None. && $$ ADeSmet/Woodward/Church/Struthwolf For more information from NOAA's National Weather Service visit... http://weather.gov/saltlakecity For frequently asked questions about the Area Forecast Discussion visit... http://weather.gov/saltlakecity/general/afd_faqs.php