textproduct: Western and Central Wyoming

KEY MESSAGES

- Chilly temperatures return again tonight with lows in the 30s and middle 40s. Clouds will increase from the south through the overnight hours. There will be a 20 to 40% chance for low clouds/fog near Casper Tuesday morning through about 9 AM.

- Apart from shower and thunderstorm chances (30% to 70%) on Tuesday afternoon and evening, the week is forecast to be dry with above average temperatures by Thursday.

UPDATE

Issued at 1221 PM MDT Mon Sep 21 2026

A pleasant day is in store with afternoon highs in the 60s and low 70s. Midday satellite imagery is showing a low cloud deck across Johnson County. These clouds are forecast to slowly dissipate as the atmosphere mixes out. Other than diurnal cumulus over the higher terrain and occasional high clouds, most locations will have plenty of sunshine today. Cloud cover is forecast to increase overnight ahead of Tuesday's shortwave; thus, there is low confidence in fog formation for most locations on Tuesday morning. However, fog development is most probable (40% chance) in and around Casper, including along I-25 on Tuesday morning. The Casper area is more susceptible to fog formation due to a light east to northeast wind drifting low clouds in from eastern Wyoming. The remainder of the forecast is on track with Tuesday afternoon and evening shower and thunderstorm chances (30% to 70%) as a shortwave traverses the region.

DISCUSSION

Issued at 130 AM MDT Mon Sep 21 2026

Diminishing cloud cover across the CWA in wake of the weekend's storm system that brought much needed and ample precipitation, especially east of the Divide. IR shows warming cloud tops as the aforementioned system makes its way eastward across the upper Great Plains to the Great Lakes stretching southward all the way to the Gulf of America. WV shows the main finger of the PFJ well to the north across Canada pushing cyclonically around an upper level low over the Gulf of Alaska. Another weak low near California will be the next system to bring in storm activity for Tuesday afternoon and evening with its slow progression east and northeastward.

With the clear skies west of the Divide, expect fog development over some of the area. Jackson Hole looks to be the best chance for a heavier impact with the surrounding lakes and recent rainfall. Points further east of the Divide, the cloud cover looks to hinder much of the area but possible (10-20%) around the Casper area but confidence is low at this point in time dependent on cloud cover by Monday morning sunrise. Beyond that, expect light winds and seasonable temperatures into the afternoon with tranquil weather conditions to be had heading into the next system for later Tuesday.

The aforementioned closed low near California will become more of an open and progressive wave with its northeastward movement through the upper Rockies Tuesday afternoon and evening. Scattered rain showers and isolated thunderstorms likely (40-70%) for much of the area west of the Divide, with lower confidence to the east (20-40%). The strongest PVA anomalies look to occur across Sweetwater County through the Upper Green River Basin for the strongest storms to occur. The likely impacts will be that of heavy rainfall and gusty outflow winds. Hi-res models are pushing the 36 hour limit on its output, but it tends to be close enough at this point in time. The Winds will likely block much impact to the east, but strong winds will be the main factor pushing down into the Wind River Basin towards sunset Tuesday evening.

Beyond Tuesday's storm and shower activity, expect induced ridging to build in through the Rockies that will bring above average temperatures for the remainder of the work week. Winds will be relatively tame for the week as well with no indications of fire weather concerns at this point in time. The next system looks to push in by Friday night into the weekend as the aforementioned Gulf of Alaska low extends down into the Pacific Northwest as the PFJ dips into the Continental US. Early indications of model runs keep the main impacts north into Montana but still affects to be had in the CWA with precipitation and more seasonable temperatures for the weekend and into the following week.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 354 PM MDT Mon Sep 21 2026

VFR conditions prevail through the period. Winds follow diurnal trends, decreasing this evening. Tonight, low clouds are possible around KCPR between about 10z and 17z as a northerly wind advects clouds to the area. Confidence is not high on this occurring, but high enough to included a SCT group in the TAF. Confidence for fog at KJAC remains very low, as the latest HRRR model sounding indicates a dry surface.

For Tuesday, showers and thunderstorms start to move in from the southwest towards the later half of the afternoon, with increasing chances at KBPI, KPNA, and KRKS. Chances spread further northeast past the end of the period.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts.

RIW WATCHES/WARNINGS/ADVISORIES

None.