Low level moisture remained trapped across eastern Utah and, when combined with clear skies and light winds, another round of dense fog developed in some of the northeast and east-central valleys.
Every event with a place to put it; 2 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS GJT wrote it up.
Low level moisture remained trapped across eastern Utah and, when combined with clear skies and light winds, another round of dense fog developed in some of the northeast and east-central valleys.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Grand Junction, CO wrote its last area forecast discussion before the first event at Dec 19, 4:35 PM MST, 4 hours 25 minutes earlier.
Area Forecast Discussion National Weather Service Grand Junction CO 435 PM MST Sat Dec 19 2020 .SHORT TERM...(Tonight through Sunday night) Issued at 242 PM MST Sat Dec 19 2020 Lingering northwest flow behind the departing trough in continuing to bring high clouds overhead. Stratus and fog lingered in the lower valleys till late morning with the Uinta Basin holding on a bit longer, but the faster upper flow is allowing some of the stratus to erode. This has caused the Uinta Basin, including Vernal, to remain much colder than forecast under this persistent thick shield of low stratus. Anticipate some patchy fog to redevelop in some of the snow covered valleys overnight with colder temperatures as some areas across the central and southern zones clear out partially overnight. Cloud cover looks to persist up north with the northwest flow through Sunday. Northwest flow will also favor upslope snow across the Elkheads and Park Range in northwest Colorado, where 2 to 6 inches are possible, especially at the higher peaks. Rates will remain light to moderate at times, but mostly light, and with totals falling below advisory thresholds with limited road impacts. Elsewhere, conditions will remain dry through Sunday night with below normal temperatures, although a few degrees higher than today with a slow warming trend into early next week due to low level inversions and limited mixing. .LONG TERM...(Monday through Saturday) Issued at 242 PM MST Sat Dec 19 2020 Light, orographically driven showers will linger over the Park and Gore ranges through Monday morning under continuing northwest flow. A transient ridge will move through by Monday afternoon, with the ridge axis crossing the CWA Monday night and bringing drier conditions and an end to the orographic showers. Under this ridge, Monday and Tuesday will be relatively quiet with temperatures about 5 degrees or so above average. Our next system looks to move in Tuesday evening. While the medium range models are in good agreement that there will be a system moving through in the Tuesday-Wednesday timeframe, that's about where the agreement ends. The ECMWF, ensemble ECMWF, and the Canadian models both have the trough moving slower and digging further south compared to the GFS/GEFS. Although the GFS has been flip-flopping a bit between runs, as the 06Z run seemed more in line with the slower, wetter solution, but the 12Z run is again drier and more progressive. Regardless of the solution, the models are all in agreement that the CWA will see some precipitation, centered around the Tuesday evening through Wednesday late morning/early afternoon timeframe, with the focus for snowfall favoring the northern and central Colorado mountains. As is often the case with these digging, open waves, we could see accumulating snowfall drop as far south as the San Juans, with the northwest slopes more favored at this time. Will continue to monitor how this situation develops. Temperatures take a nosedive Tuesday night into Wednesday with the passage of the previously mentioned system. However, a ridge is expected to build in behind the trough, allowing temperatures to rebound towards the end of the week. Strong valley inversions are likely under this ridge though, so a situation where the hills warm faster than the valleys is expected. && .AVIATION...(For the 00Z TAFS through 00Z Sunday evening) Issued at 432 PM MST Sat Dec 19 2020 Fog and low stratus remains in some low valleys this afternoon. Anticipate that VEL and CNY will continue to see IFR/LIFR conditions overnight as northwest flow keeps the low stratus banked up in these valleys. Things are a little more uncertain in places like DRO and GUC so kept them VFR. All other terminals will see VFR conditions for the rest of the period. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && SHORT TERM...MDA LONG TERM...BGB/MDA AVIATION...TGJT
Hazardous Weather Outlook National Weather Service Grand Junction CO 407 AM MST Sat Dec 19 2020 COZ001>014-017>023-UTZ022>025-027>029-201200- Lower Yampa River Basin-Central Yampa River Basin- Roan and Tavaputs Plateaus-Elkhead and Park Mountains- Upper Yampa River Basin-Grand Valley-Debeque to Silt Corridor- Central Colorado River Basin-Grand and Battlement Mesas- Gore and Elk Mountains/Central Mountain Valleys- Central Gunnison and Uncompahgre River Basin- West Elk and Sawatch Mountains-Flat Tops- Upper Gunnison River Valley-Uncompahgre Plateau and Dallas Divide- Northwest San Juan Mountains-Southwest San Juan Mountains- Paradox Valley/Lower Dolores River- Four Corners/Upper Dolores River-Animas River Basin- San Juan River Basin-Southeast Utah-Eastern Uinta Mountains- Eastern Uinta Basin-Tavaputs Plateau-Arches/Grand Flat- La Sal and Abajo Mountains-Canyonlands/Natural Bridges- 407 AM MST Sat Dec 19 2020 This hazardous weather outlook is for eastern Utah and western Colorado. .DAY ONE...Today and Tonight Light snow showers will continue across the Park and Gore Ranges in Northern Colorado with an inch or two of new snow will fall by late tonight. Otherwise, dry weather is expected today and tonight. .DAYS TWO THROUGH SEVEN...Sunday through Friday The next chance for snow comes Tuesday afternoon into Wednesday with the northern mountains again favored. Early indications are that snowfall generated by this system will be light to moderate. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected today or tonight, however spotters are encouraged to report snowfall amounts to the National Weather Service in Grand Junction.
The national radar mosaic for the moment the first event began.
2 dense fog. Local standard time, as filed.
Dense fog was observed across the Grant Flat and Arches region including at the Canyonlands Field Airport as visibility was reduced to less than one quarter of a mile for several hours.
Dense fog was observed across the Eastern Uinta Basin including at the Vernal Regional Airport as visibility was reduced to less than one quarter of a mile for several hours.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Utah in 2020Every state in 2020This day in weather historyWarnings in Utah, December 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 925783, 925784.