222 FXUS65 KSLC 272319 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 419 PM MST Sun Dec 27 2020 .SYNOPSIS...A low pressure system off the California coast this afternoon will move onshore tonight, then track east into southern Utah Monday afternoon. Widespread precipitation is expected across the southern two-thirds of Utah tonight through at least early Tuesday afternoon. High pressure aloft will expand east across the Great Basin midweek. && .SHORT TERM (Through 12Z Wednesday)...The upper low sitting just off the California coast late this afternoon will move onshore late tonight, then move quickly into southern Utah Monday afternoon. This low will gradually evolve into elongated open trough as it moves through eastern Utah Monday evening, then exit the state to the east by Tuesday morning. The near 700mb baroclinic zone that started the day over southern Nevada then east across far northern Arizona has lifted north into southern Utah in advance oft the approaching upper low this afternoon. Increasing southwest flow into this boundary has resulted in low-level warm advection coincident with the upglide region into the southern mountains. This thermally-driven and orographically enhanced lift has produced light snow across the southern mountains this afternoon. Waning jet support late this afternoon will likely keep this precip on the light side as it works north into central Utah this evening. Anticipating a fairly rapid increase in both the areal coverage and intensity of precip beginning late Monday morning as the upper advances towards southern Utah. Precipitation will remain along and south of the deformation axis across northern Utah, with the heaviest precip across central/southern Utah in the area of best synoptic-scale lift in the eastern flank of the low. Lift with this storm is not expected to exceptionally strong, but with a fairly deep moisture profile will plan on significant snowfalls on Monday through Monday night. Precipitation will begin to turn more showery and decrease in areal coverage and intensity late Monday night as the now open trough works into the far eastern portion of the state. Organized precip will likely wind down Tuesday morning, with just lingering instability showers through the afternoon. The high amplitude upper ridge along the west coast Tuesday will produce an anti-cyclonic northerly flow downstream from the ridge across Utah/western Wyoming Tuesday night. A somewhat drier air mass will accompany the northerly flow, leading to gradually clearing skies late Tuesday night and into early Wednesday. .LONG TERM (After 12Z Wednesday)...The flow pattern remains active in the long term, though disturbances that move across the Great Basin are mostly shearing and weakened due to transit through the mean long wave ridge off the Pacific coast. The next disturbance should arrive on Thursday. This system has a different orientation than Monday's storm in that it is coming in from the northwest, and the most favorable flow and dynamics will be across northern Utah (versus the central and south for Monday's event). Still lack of any moderate to strong moisture tap and weakening dynamics will limit the system's potential to produce more than a quick shot of snow. GEFS and EPS means as well as WPC clusters are in agreement on the timing of Thursday's system. GEFS 7H temperature plumes show around minus 10 to minus 12 degrees on average during the peak of storm with good packing of the individual members, bringing confidence of an all snow event. Snow amounts for valley locations of the north are not all that impressive (an inch or two at best). NBM showed slight chance PoPs mainly mountains north of SLC on Saturday. Not much to focus on in the GEFS or EPS means at that time, but the operational GFS has a system moving through the northern Rockies Saturday with the tail end shearing quickly across Utah and the Great Basin. Looking at GEFS plumes shows a few members generating light precipitation at SLC and LGU but most members do not, so confidence is not high on this event. Even if it does pan out, most of the state will remain dry. By Sunday the feature of interest on the GEFS and EPS is the trough over the Pacific Northwest and British Columbia. The details of shortwaves associated with this features are not clear this far out, but the mean trough does set up the potential for more systems to impact northern Utah. Roughly half of the GEFs and EPS members show light precipitation at SLC on Sunday, and about two-thirds of the members do the same for Logan. && .AVIATION...Light northwesterly winds currently in place at the KSLC terminal are expected to switch back to the southeast between 03- 05z. Cigs at the terminal are expected to lower rapidly to near 6000ft AGL by sunrise Monday morning, with vicinity showers developing south of the terminal after sunrise. Periodic MVFR or high end IFR conditions in light snow are expected to develop Monday afternoon and evening. No accumulations are expected on surfaces until possibly after sunset Monday evening/night. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...Winter Weather Advisory from 10 AM Monday to noon MST Tuesday for UTZ004-012>016. Winter Weather Advisory from 1 AM Monday to 3 PM MST Tuesday for UTZ517-518. Winter Weather Advisory from 10 AM Monday to 3 PM MST Tuesday for UTZ010. WY...None. && $$ SHORT TERM...CONGER LONG TERM...BILLINGSLEY AVIATION...MERRILL 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