textproduct: Cheyenne

KEY MESSAGES

- Marginal risk of strong to severe storms this afternoon, mostly limited to the Nebraska Panhandle including both gusty winds and hail.

- Accumulating snow expected Sunday night through Monday with Winter Storm Warnings for many of our high elevation and mountain zones and Winter Weather Advisories for attendant nearby zones.

- Near record cold Monday and Monday night will lead to widespread freezing temperatures, which may damage sensitive vegetation and outdoor irrigation systems.

- Slow warming trend by the mid to late week with temperatures near normal by Thursday and Friday. Chance of showers and a slight chance of thunderstorms each day through Friday evening.

SHORT TERM /THROUGH MONDAY NIGHT/

Issued at 230 PM MDT Sun May 17 2026

Notable instability has struggled to return to the region this afternoon, likely thanks to the progress of the boundary and clouds allowing some limiting of heating during the morning hours. But mesoanalysis does show some pockets of weak instability, and a few storms are forming just over the border in Colorado in a similarly unimpressive environment. With some stout shear in the southern Nebraska Panhandle we could see a storm or possibly two strengthen and produce some breezy winds or small hail thanks to low freezing levels, but at this point SPC has taken most of our area out of the Marginal Risk with only the southeastern Nebraska Panhandle remaining in, so our chances for stronger to severe weather are dwindling today.

Moving into this evening, we'll shift gears away from severe weather and return to winter, with Winter Storm Warnings/Advisories out for a majority of the western portion of our CWA. At the upper levels, a low pressure system will swing into the Four Corners and then around and up into the Northern Plains, while at the surface we'll see some impressive cyclogenesis across Colorado. Similar to previous thinking, we should see the most favorable forcing located across south- central Wyoming including portions of Carbon and perhaps even Laramie Counties. Models remain in solid agreement on strong snowfall across this area, and this update doesn't feature many significant changes. Of note though, have seen some consistent forcing near the Sierra-Madre range that upgrading to a Warning seemed worthwhile, and while the whole range may land just shy of a foot of new snowfall, expecting enough locations around 12-16 inches that a warning felt justified. A Winter Weather Advisory was also issued for the Laramie Valley, and while easterly flow may help to preclude stronger accumulations, northern portions of this zone as well as along the interstate should see enough accumulation or at least reduced visibility from heavy, wet, and blowing snow. Speaking of reduced visibility, 850 mb pressure gradients show some very stout gradients from Craig to Casper, and strong winds will accompany this snowfall with gusts of 30-50 mph possible. For now, still holding off on upgrading even further to Blizzard Warnings as confidence is just not high enough that we will see widespread reductions in visibility. But the HREF does paint some swaths of around 40-60% probability across Carbon County for Blizzard conditions, and this will need to be monitored closely moving into tonight in case confidence grows further.

With stout cold air overspreading the region and locations west of the Laramie Range expected to see snow on the ground, temperatures will plummet on Monday. Widespread highs are only expected to max at the low 30's to 40's, and by the evening these will bottom out as we move overnight into Tuesday morning. Skies should also begin clearing overnight which will aid in radiational cooling, and many locations across the CWA may tie or set new all time low temperature records. Outside of the mountain zones, every zone in our area is now under a Freeze Watch for this timeframe, though the Nebraska Panhandle may struggle in some locations to reach 28 degrees, the needed low for Freeze products. Nonetheless, unseasonable cold will be here and lows across the entire region are expected to reach at or below freezing (including teens in Carbon County), which will impact plants and crops as well as outdoor equipment. Take the appropriate precautions now for this late season freeze.

LONG TERM /TUESDAY THROUGH SUNDAY/

Issued at 230 PM MDT Sun May 17 2026

Certainly less active than the short term, but the long term period will still have some impacts, with the main story being the record- breaking cold in the wake of a vigorous winter storm. Tuesday morning will start out in the 20s in a cold advection regime, hence the issuance of freeze watches/warnings to account for agricultural impacts and recently planted vegetation given the recent mild pattern. In fact, LREF 10th percentile temperatures show Rawlins getting well into the teens for a low Tuesday morning, which seems reasonable given ample snow cover and cold advection. Tuesday will also feature high temperatures 15-20 degrees below climatology, resulting in highs in the 40s for our Wyoming counties and 50s for our Nebraska counties. We will likely dip below freezing again on Tuesday night especially in regions that don't melt away all of their snow (i.e. areas west of the I-25 corridor). On Wednesday, temperatures will gradually moderate into the upper 50s to near 60 east of the Laramie Range and upper 40s to lower 50s west given weak isentropic ascent/warm advection as the longwave trough responsible for our anomalously cool weather dampens. There may be enough lingering moisture for a few showers and thunderstorms primarily over the higher terrain, however probabilities of seeing QPF > 0.05" are running at about 25%, so any moisture is unlikely to be beneficial. Given dry boundary layers as shown on model forecast soundings show a very dry boundary layer with surface dewpoint depressions exceeding 40 degrees, showers and thunderstorms will likely produce far more wind than rain.

Temperatures gradually warm on Thursday and Friday as a series of shortwave troughs embedded in quasi-zonal flow traverse across Eastern Wyoming and Western Nebraska, leaving in low (30%) chances for high-based showers and thunderstorms. Highs will gradually increase to climatological values (mid-upper 60s for Cheyenne to near 70 for the Nebraska Panhandle). Heading into the weekend, we will begin to dry out and encounter a faster warming trend to above- average temperatures as weak mid-level ridging establishes itself over the Rocky Mountain West, with no widespread or beneficial precipitation chances in sight.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 557 PM MDT Sun May 17 2026

Widespread rain and snow showers, with isolated thunderstorms, are moving through the Wyoming terminals. Visibilities of 2 to 5 statue miles have been observed in the heaviest activity over KLAR and KCYS. Conditions will continue to deteriorate at Wyoming terminals overnight with IFR and LIFR conditions with increasing rain and snow shower activity. Particularly of concern with this system is guidance is trending towards higher snow and wind potential. Heavy snow and wind gusts to 50 kts may occur in the mountain basins to impact KRWL reducing horizontal visibilities down to 1/4 SM or less and vertical visibilities to the surface. At KLAR and KCYS, while not as severe in conditions as KRWL, will most likely be in LIFR conditions with heavy snow and ceilings below 500 feet. Shower activity at the Nebraska Panhandles will continue to increase through the forecast period to induce MVFR conditions at times, particularly at KBFF. It is uncertain at this time if the rain will transition to snow at these terminals.

CYS WATCHES/WARNINGS/ADVISORIES

WY...Freeze Watch from late Monday night through Tuesday morning for WYZ101-102-106>108-113-115>119. Winter Weather Advisory from midnight tonight to 6 PM MDT Monday for WYZ101-105-106-111-115-117. Freeze Warning from 5 AM to 10 AM MDT Monday for WYZ102. Winter Storm Warning until 6 PM MDT Monday for WYZ103-104-110- 112-114-116. Freeze Watch from Monday evening through Tuesday morning for WYZ104-105-109>111. Winter Storm Warning from midnight tonight to 6 PM MDT Monday for WYZ109. NE...Freeze Watch from late Monday night through Tuesday morning for NEZ002-003-019>021-054-055-095-096.