Area Forecast Discussion
Issued by NWS Salt Lake City, UT
Issued by NWS Salt Lake City, UT
463 FXUS65 KSLC 201029 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 429 AM MDT Fri Sep 20 2024 .SYNOPSIS...A typical early fall pattern will continue across the Beehive State through the next week. An upper level disturbance will bring the threat of showers and thunderstorms to portions of southern and eastern Utah Saturday. A weak cold front will cross northern and central Utah Monday. && .SHORT TERM (Through 12Z/6AM Sunday)...Satellite imagery reveals a closed low positioned over southern California early this morning, which will end up dictating much of the sensible weather through the weekend across Utah and southwest Wyoming. As the closed low continues to make its way inland, showers and thunderstorms will develop across far southwestern UT this afternoon, increasing in coverage heading into the evening hours. Gusty microburst winds will be the main threat, though any training in storms over Zion NP will produce a minor flash flood threat as well. Given strong dynamics with this closed low and sufficient moisture, these showers will likely continue overnight, slowly shifting eastward with the progression of the low. As the low lifts into the Four Corners region Saturday morning, precipitation chances will expand further north, staying mostly east of I-15 through the afternoon. Scattered showers and thunderstorms on Saturday will result in a heightened flash flood risk for prone areas in slot canyons or near recent burn scars, particularly for areas further east, with mean QPF ranges between 0.1-0.25" across southern UT. As the low exits the area late Saturday, precipitation chances will largely dissipate. The low passing just to our south will also result in gusty easterly canyon winds along the Wasatch Front and Cache Valley Friday night/Saturday morning, with the highest gusts expected in the most prone areas in Davis County. HREF max gusts top out around 40 mph, though canyon mouths could see brief periods up to 45-50 mph. .LONG TERM (After 12Z/6AM Sunday)...To close out the weekend, deterministic global solutions and ensemble solutions begin the period in good agreement regarding the large-scale pattern. We kick things off with a closed low over/near the Colorado Front Range on Sunday, with lingering early Sunday cyclonic flow extending westward into extreme eastern Utah, with a very low (<15% chance) for a shower for these extreme eastern areas Sunday. Otherwise, Sunday is expected to be precipitation-free with temperatures just a shade below seasonal normals. The journey into the long term portion of the forecast is a short one before model spread is introduced, with deviating solutions arriving as early as Monday. First, we`ll dive into where solutions are aligned: there is agreement that a ridge will build offshore of the western CONUS with a trough diving down the periphery of the ridge across the Rocky Mountains Monday and Tuesday. Two forecast scenarios are in play regarding the evolution of this trough. There is a subset of the ensemble solution space (as well as the deterministic ECMWF) that closes a low off directly over Utah on Monday, with the low lingering around through Wednesday. This would result in cool, showery conditions for the start of the workweek. This outcome is represented by a minority of the solutions, and interestingly the NBM is more reflective of this solution, with mentionable precipitation across the southern half of Utah on Monday and closer to the Arizona border Tuesday. The alternate scenario is one with a progressive trough which would maintain seasonable temperatures and dry conditions for the early portion of the workweek. Ensemble spread then decreases (with an associated increase in forecast confidence) for the mid to late period, with consensus showing positive height anomalies gradually building across the Great Basin, resulting in a gradual warming and drying trend through the remainder of the week. While there are a few ensemble members that want to bring a trough through the northern tier and knock down the ridge, the overwhelming consensus maintains positive height anomalies/ridging across the region, resulting in stellar weather to kick off the official start of the Autumn season. && .AVIATION...KSLC...VFR conditions are expected through the period, with southeasterly drainage winds becoming northwest around 17Z. Cumulus buildup expected over terrain during the afternoon, with a very low (<15% chance) for an isolated afternoon terrain-based shower. Winds most likely to remain northwest Friday night, but confidence in wind direction is low due to the emergence of an easterly gradient over the region. .REST OF UTAH AND SOUTHWEST WYOMING...Area of low pressure is forecast to track just south of Utah through the period, bringing scattered showers and thunderstorms to southwest Utah beginning during the late afternoon hours. Elsewhere, afternoon terrain-based cumulus will develop into isolated showers, especially over central Utah. Any shower/storm will be capable of producing gusty/erratic winds. && .FIRE WEATHER...A low pressure system will slide across the Desert Southwest Friday and Saturday, lifting northwards across CO Saturday night. With this system, expect showers and thunderstorms to develop over southwestern UT Friday afternoon, then spreading eastward overnight and into Saturday morning across much of southern and eastern UT. Gusty outflow winds are likely with these storms. The highest chances for wetting rains are over the southern mountains on Saturday, particularly between Bryce Canyon and Capitol Reef. Expect improving overnight recoveries through Sunday night. A mostly dry cold front will push through northern and central UT Monday afternoon and evening, though there is a small chance for some showers across southern UT. High pressure is then likely to build, producing warmer and drier conditions heading into midweek. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...None. WY...None. && $$ Cunningham/ADeSmet For more information from NOAA`s National Weather Service visit... http://weather.gov/saltlakecity