textproduct: Cheyenne

KEY MESSAGES

- Severe weather will be possible Sunday afternoon with a Marginal Risk of strong to severe thunderstorms along the Interstate 80 corridor.

- Another round of accumulating late season snow is increasingly likely on Monday, mainly above 5000 feet in elevation.

- 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 335 AM MDT Sun May 17 2026

Potent storm system appears on track over the next 48 hours, so mainly made some tweaks to the current forecast and upgraded most of the Winter Storm Watches. Also, issued Winter Weather Advisories for the nearby valleys and high plains near the mountains.

Today's Strong/Severe Thunderstorm risk...

For this morning and today, current KCYS radar loop shows scattered showers developing across the mountains and lifting northeast across the area. A midlevel shortwave/vort max out ahead of the main storm system is forecast to lift northeast across Wyoming early this morning. This feature may bring increasing rainfall rates for a time around sunrise. Can't rule out some thunder as well. All high res models show quiet conditions for a time later this morning and early this afternoon before a more potent shortwave moves eastward across the area ahead of the developing storm system to our west as it digs south into northeast Nevada and Utah. This shortwave will bring our next threat of strong to possibly severe thunderstorms, mainly along Interstate 80 and south of the North Platte River valley this afternoon and this evening. SPC continues with the Marginal Risk for severe storms as the event doesn't look nearly as good as it did a few days ago. This is mainly because of a potential stratus deck forming this morning and remaining over the eastern plains for much of the day. With a reinforcing shot of cooler air today/secondary cold front, it may be difficult to get much instability going before the forcing from the shortwave moves over the area, with high res model guidance showing several bands of fast moving showers and embedded thunderstorms lifting northeast across the area. With lower freezing levels today and plentiful forcing, can't rule out some marginal hail and strong gusty winds...mainly near the Colorado border.

Much needed widespread precipitation and southeast Wyoming heavy snowfall...

The main event starts this evening and overnight. This is going to be a complex late season winter storm with the possibility of several additional winter weather headlines added/expanded by Monday. There is also some potential of a bust (mainly over Carbon and Albany counties) due to the nature of the cold front, surface low, and how they interact with the higher terrain.

All models continue to show the western double-barrel low pressure center, currently over Oregon, digging southeast into Utah and western Colorado today and tonight. This upper level low is forecast to become dominant by tonight as a strong cold front slides south and then stalls near the Colorado/Wyoming border with surface cyclogenesis taking place over northwest Colorado. With these systems, the strongest lift is usually to the north and northwest of the surface low, which places this area over central Wyoming and south central Wyoming...including Carbon, Albany, and possibly Platte and Laramie counties. With strong frontogenesis for more than 6 to 12 hours and very good convergence, decided to upgrade the Winter Storm Watch to a Warning for most of the mountain zones and included the lower elevations of northern and central Carbon County and the Arlington/Elk Mountain area. Adjacent zones north of Interstate 80 will see snow, but a later transition from rain to snow, so issued Winter Weather Advisories for the Shirley Basin, central Laramie Range, southern Laramie Range Foothills, and Converse county for 2 to 5 inches of snow but little road impacts. Areas south of I-80 and the rest of the I-25 corridor from Wheatland to Cheyenne remain in the area of lowest confidence with snow amounts and change over times, so kept some accumulation but didn't issue any headlines for these areas quite yet. The Laramie Valley will be fighting easterly downslope winds off the Laramie Range during most of this event, so kept them out of any headline for now. Further west, expect there to be a very sharp gradient with snowfall rates south of Interstate 80 across south central Wyoming. Further east, expect a later transition time to snowfall...which may bump into the early daytime hours where snow may struggle to accumulate. For this time of the year, typically need 1+ inch/hr snowfall rates to overcome the warm pavement during the daytime. As of right now, only seeing this criteria being met over Carbon and portions of Albany county, hence the Winter Storm Warning. These conditions are possibly further east towards I-25, but not nearly as confident at this time. Also, the 00z ECMWF have backed off on QPF amounts a little bit along the I-25 corridor and further east, which is also influencing this decision. Areas well east of I-25 including western Nebraska will still see good precip amounts mainly in the form of rain. Rain should mix with and change over to snow at times on Monday, but any snowfall should mainly fall during the daytime hours. Little if any snow is expected to accumulate due to the high mid-May sun angle.

In addition to this, models have increased east to northeast winds with the reverse pressure gradient across Carbon and western Albany counties. This makes sense since models have trended more aggressive with cyclogenesis to the south across Colorado. Increased winds along Interstate 80 with forecast gusts up to 50 MPH. This will present another problem...possible Blizzard conditions. Although low confidence, sustained winds over 40 mph with gusts over 50 mph should be enough to loft the wet snow flakes...and with falling snow near 1 inch/hour, can't rule out periods of very low vis around one quarter mile.

Very cold into Tuesday...

Thankfully, this storm system won't stick around for too long as it will already be in its weakening phase by Monday afternoon, as the remnant open wave trough quickly ejects east over the area. Snow or rain/snow mix will gradually come to an end mid to late Monday afternoon. With that said, impacts will not complete go away with a remaining snowpack likely and possibly some clearing skies. Expect record low temperatures or near- record low temperatures Monday night and early Tuesday morning with temps possibly lowering into the teens to low 20s. Expect far eastern Wyoming and western Nebraska to be slightly warmer with the lack of a snow pack, but still could see record lows in the mid to upper 20s. This classifies as a very late hard freeze for the region and we'll likely need Freeze watches or warnings shortly. For now, will see how the event plays out before committing. Plus it's still 4 to nearly 5 periods out, so we have some time to make additional adjustments.

LONG TERM /TUESDAY THROUGH SATURDAY/

Issued at 216 PM MDT Sat May 16 2026

The long term period will begin under an anomalously cold air mass in the wake of a powerful winter storm, with record low temperatures across Eastern Wyoming and the Nebraska Panhandle on Tuesday morning (record low for Cheyenne on Tuesday is 27 degrees set back in 1966). Unsurprisingly, high temperatures on Tuesday will remain 15-20 degrees below climatology mainly in the 40s west of the Laramie Range, and 50s east, as we remain under the influence of a broad, positively-tilted trough centered over the Rocky Mountain West. Areas with the most snow cover will be the coldest spots, as snowfall has a high albedo and is highly effective at radiating back into space. Further, melting and sublimation of snow are cooling processes, so this will further limit warming even given the high May sun angle. Tuesday night will again feature record low to near record low temperatures as we remain under weak isentropic descent and cold air advection with partly clear skies which should allow for another night of effective radiative cooling.

The gradual warm up continues as we head into Wednesday and are situated in a zonal flow regime as well as in the left-entrance region of a departing 90-100 knot 250 mb jet, which isn't exactly a favorable relative location for widespread ascent and precipitation. However, given that numerous vorticity maxima will be propagating along the base of the trough combined with differential heating of the sloped terrain, chances for a few high-based showers and thunderstorms are in the forecast. These are likely to produce more wind than beneficial rain (it is Wyoming, of course) as forecast soundings are showing inverted-V profiles and MUCAPE around 100 J/Kg.

Thursday we see another shortwave trough propagate within the longwave pattern, so chances for high-based showers and thunderstorms will remain in the forecast as high temperatures creep back up towards climatology in the low to mid 60s. Flow turns more northwesterly into Friday, however with the absence of any large- scale forcing for ascent, conditions will remain mostly dry with high temperatures around seasonal values.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1134 AM MDT Sun May 17 2026

An active 24 hours ahead with multiple rounds of showers and thunderstorms ahead of a potent winter storm that will impact Southeastern Wyoming and Western Nebraska overnight. MVFR will be in place in the vicinity of any showers and thunderstorms this afternoon into this evening with CIGS at or below 2k ft, in addition to erratic and gusty winds as high as 50-60 knots. Into the overnight hours we begin to transition to snow at the Wyoming terminals, can expect more widespread IFR to LIFR conditions with visibilities as low as 1/4 SM and CIGS below 1 kft especially in areas of heavy, blowing snow. Expect wind gusts to pick up to 30-35 knots into the early morning hours for all terminals, with prevailing winds out of the NE ranging from 25-30 knots at the Wyoming terminals and 20-25 knots for the Nebraska terminals.

CYS WATCHES/WARNINGS/ADVISORIES

WY...Winter Weather Advisory from midnight tonight to 6 PM MDT Monday for WYZ101-105-106-117. Winter Storm Warning from 6 PM this evening to 6 PM MDT Monday for WYZ103-104-110-114-116. Winter Storm Warning from midnight tonight to 6 PM MDT Monday for WYZ109. Winter Weather Advisory from 10 PM this evening to 6 PM MDT Monday for WYZ112. NE...None.