An abundance of low level moisture settled across southwest Colorado as an area of low pressure trekked overhead. This led to the development of dense fog in the Animas River Basin.
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The episode as NWS GJT wrote it up.
An abundance of low level moisture settled across southwest Colorado as an area of low pressure trekked overhead. This led to the development of dense fog in the Animas River Basin.
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 Sep 27, 10:48 PM MST, 3 hours 38 minutes earlier.
Area Forecast Discussion National Weather Service Grand Junction CO 1148 PM MDT Mon Sep 27 2021 .SHORT TERM...(Tonight through Tuesday night) Issued at 121 PM MDT Mon Sep 27 2021 Satellite imagery clearly shows circulation around the closed low that looks to be centered over central New Mexico at this hour. Short range CAM guidance was indicating some convection firing around noon over the San Juans but this has yet to really materialize thanks to cloud cover from earlier this morning. However, the best CAPE remains over the San Juans, along with some fairly strong divergence aloft. With some clearing taking place do expect some convection to take place...just a bit later than guidance suggested. Moving northward, CAM guidance was also showing some convection over the West Elk, Elk Mountains, Grand Mesa and a passing threat along the Bookcliffs. Thanks to the location of the low, convection will move from east to west (or southeast to northwest). Visible imagery and webcams do show some development at this hour so the going forecast for a few showers/storms looks good. PWATs have increased by about .2 inches over the last 24 hours so though the biggest threat remains gusty outflow winds, a quick downpour also can't be ruled out. Latest 15min HRRR guidance is showing the strongest convection to move through the San Juans and southern valleys from about 5 to 6PM before dying down after 9PM. Again, look for some gusty winds with some brief downpours. A few showers may persist overnight but these will be very isolated in nature. Things get more interesting Tuesday as the low moves northeast to the Front Range while a trough approaches from the northwest. Once again, the San Juans will be favored for precip around noon but as the low continues to progress northeastward, plenty of lift and increased moisture will lend themselves to increased precipitation. Like today, the ingredients look to be in place for some convection but cloud cover may inhibit the timing and strength of any convection. For now, precip looks to favor all the higher terrain through 6PM and then shifts to the northern half of the CWA heading from mid-evening onwards...following the closed low. In fact, the heaviest QPF is progged to fall from 9PM Tuesday night through Wednesday morning for the northern valleys. The approaching trough/surface cold front will also aid in increased lift so some steady stratiform precip with some embedded convection looks probable. Snow? Wouldn't be surprised to see a dusting to an inch near 11K feet. Expect temperatures to be cooler Tuesday thanks to increased clouds and precip with a noticeable dip Wednesday. .LONG TERM...(Wednesday through Monday) Issued at 121 PM MDT Mon Sep 27 2021 Model solutions continue to remain in decent agreement that the best forcing for ascent associated with the main upper level trough will focus precipitation generally from I-70 northward. This precipitation will last into the evening and is expected to come to an end overnight. Temperatures Wednesday will be 10-15 degrees below average across this portion of the area, while remaining more seasonal to the south. With the blocky pattern in place...and being the shoulder season...model consensus has been inconsistent at best. Models continue to indicate that energy from the stalled trough over the Rockies will split off and drop south, but agreement ends there. Some models, namely the EC, want to keep this parcel of energy further north over Arizona and bring wetter conditions to southwest Colorado and southeast Utah for Thursday night into Friday. The other camp, with the GFS and NAM on board, have the energy parcel much further south, over the Baja peninsula, and a much drier forecast. Stay tuned for subsequent forecasts, as model solutions will likely continue to change from run to run and, hopefully, will fall into better agreement for the late week period. Temperatures for this period are also a bit of a question mark due to the lack of model agreement and the subsequent large spread in the MOS guidance. So, for the time being, expect temperatures to be right around seasonal. At this point, the weekend forecast is looking drier as a shortwave ridge is expected to push up over eastern Utah and western Colorado. Should this forecast verify, the weekend will bring a welcome pause from unsettled weather with temperatures near seasonal values. && .AVIATION...(For the 06Z TAFS through 06Z Tuesday night) Issued at 1142 PM MDT Mon Sep 27 2021 Showers with isolated thunderstorms will linger through the early morning hours with gusty winds to 35 kts possible. Storms will redevelop over the high terrain Tuesday afternoon with storm activity and coverage increasing across northeast to east-central Utah and northwest to west-central Colorado Colorado this evening as a cold front approaches from the west. Expect VFR conditions with VCTS at most sites after 18-20Z and better chance of TAF sites to be impacted by prevailing precipitation after 23-01Z. Sites can experience MVFR conditions in showers with gusty outflow winds to 40kts also possible. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && SHORT TERM...TGR LONG TERM...BGB AVIATION...MDA
Hazardous Weather Outlook National Weather Service Grand Junction CO 412 AM MDT Mon Sep 27 2021 COZ001>014-017>023-UTZ022>025-027>029-281015- 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- 412 AM MDT Mon Sep 27 2021 This hazardous weather outlook is for eastern Utah and western Colorado. .DAY ONE...Today and Tonight A disturbance will continue to move eastward across New Mexico, bringing scattered showers and storms to the southern mountains and adjacent valleys. Storms will be capable of brief moderate rain and wind gusts to 40 mph. Isolated storms possible further north over the central mountains as well. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Showers and storms spread northward and become more widespread Tuesday and Wednesday as another system pushes in from the west. Temperatures will be much cooler with below normal readings. Drier air pushes in from the west late in the week with afternoon storms possible over the high terrain along the divide. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed.
The national radar mosaic for the moment the first event began.
1 dense fog. Local standard time, as filed.
Dense fog was observed across the Animas River Basin including at the Durango-La Plata County Airport as visibility was reduced to less than one quarter of a mile for an hour or so.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Colorado in 2021Every state in 2021This day in weather historyWarnings in Colorado, September 2021
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: 977475.