textproduct: Cheyenne

KEY MESSAGES

- Warmer and drier weather will continue through Sunday, leading to near critical fire weather conditions.

- The potential for cool temperatures and widespread precipitation continues to increase for Monday afternoon through Wednesday morning. Thunderstorms, beneficial rainfall, and accumulating snow are all possible, but the details are still uncertain.

- Drier, warmer, and windier weather returns to the area for the second half of the work week.

SHORT TERM /THROUGH SUNDAY NIGHT/

Issued at 320 AM MDT Sat May 2 2026

GOES water vapor imagery shows an extremely dry airmass aloft moving from north to south across our area this morning. This airmass is located on the eastern periphery of a narrow ridge building into the northern Rockies region this morning. As the ridge strengthens aloft, we can expect temperatures on the rise, gaining about 10F for today's highs compared to yesterday. While surface dewpoints aren't terrible currently, the drier air aloft will begin to mix down to the surface by mid morning. Precipitable water will drop to around 0.1 to 0.2" this afternoon, which will be around the 2.5 percentile for much of the area. Look for dewpoints to drop into the teens to single digits, pushing RH down to around 8 to 12%. We will have very deep, well-mixed boundary layers and potent inverted-v soundings this afternoon. The very dry airmass will elevate fire weather concerns, but the winds will remain marginal. A light northwest breeze is expected to develop this afternoon, with consistent gusts around 15 to 25 mph. A handful of gusts up to 30 mph are probable today over the High Plains, but currently this is not expected to be consistent enough to warrant a Red Flag Warning. We will need to watch conditions closely through the day in the case that winds end up stronger than anticipated.

A shortwave trough will pass well to our north this afternoon. A weak surface high pressure system will settle in over the northern Plains behind this system, pushing into our area this evening. Winds will turn northeast over the plains and we should see positive moisture advection in the lower levels. The moisture should push up against the Laramie range by Sunday morning, but then retreat slightly during the day Sunday. Gusty westerly winds and continued very dry air will prevail for the southeast Wyoming I-80 corridor. While fuels are in greenup for most of this area, the breezy and dry conditions may spread into Laramie County, where the extreme lack of recent precipitation has stunted the spring greenup. We may have locally critical fire weather conditions in Laramie County, depending on how far the stalled boundary retreats to the east during the day. Sunday will be another warm day across the area, but mid to upper level moisture is expected to recover nicely. A broad upper level closed low moving into the West Coast will contribute some Pacific moisture, while another weak shortwave aloft will move down from the northwest Sunday afternoon. The latter feature is expected to come with overrunning isentropic lift ahead of the trough axis, which will be well-timed with peak heating in the mid afternoon hours. Areas along and north of the North Platte River will have better low-level moisture as well. The synoptic lift will kick off widely scattered showers and thunderstorms that should be able to tap into modest convective instability in the 200 to 500 J/kg range. While most of the activity will be up in South Dakota and northeast Wyoming, PoPs in the 20 to 40% range spread into portions of our area along and north of a roughly Douglas to Alliance line. Any showers or thunderstorms will have the potential to produce gusty winds. Further south, we may see some isolated high- based showers and perhaps a few rumbles of thunder, but measurable rainfall is unlikely.

LONG TERM /MONDAY THROUGH FRIDAY/

Issued at 320 AM MDT Sat May 2 2026

The primary of the medium range forecast is the precipitation potential for Monday afternoon through Wednesday morning. A very complex synoptic setup is expected to develop during this period. First, an expansive upper level closed low is expected to slowly meander inland and become stretched out across the southwestern CONUS. Plentiful Pacific moisture will be pulled into the area, and then stretched out from west to east more or less along the I-80 corridor from California to Nebraska. Meanwhile, a very large and unseasonably strong upper level trough remains parked over the Hudson Bay region. Models are in good agreement showing this feature retrograding westward Monday into Tuesday, which will allow for a fairly potent vort-max to dive southward along the eastern side of the northern Rockies. In an unusual twist for this year, model guidance is trending towards the scenario in which the lift associated with the northern branch system arrives while plentiful moisture from the other system is still overhead. This scenario would have the potential to actually dent (but not even close to eliminate) the ongoing extreme drought conditions. The overall picture amongst the major ensemble members may be the best signal for widespread beneficial moisture we have seen since last fall, but the official forecast hedges on the conservative side. We'll first break down the overall evolution expected of this system, and then get into some QPF and snowfall probabilities below.

Monday will be a transition day across the area. There is fairly high confidence in areas along the I-80 corridor seeing warm temperatures around 10F above seasonal averages. Further north, Monday's highs have more uncertainty due to the expected daytime arrival of a strong surface cold front. Even further south, an earlier frontal arrival may mean we see a fairly early high temperature, with cooling occurring during the mid to late afternoon hours. Decent frontogenesis will be moving into a modestly unstable environment Monday afternoon, which should kick off the event with scattered showers and thunderstorms. Most model guidance shows the front slowing down as it pushes up against the mountains of southern Wyoming and northern Colorado, providing the potential for a longer duration window of lift Monday night through the day on Tuesday. Temperatures will continue to drop through this period ahead of another vort- max dropping down from the north which should arrive Tuesday afternoon or evening. Models are now in fairly good agreement showing the closed low over the southwest getting absorbed by the parent trough Tuesday night into Wednesday morning. In terms of our outcome on the ground, we should see snow levels starting off around 9 to 10 kft Monday afternoon, but this will rapidly drop overnight and through the day Tuesday. The latest LREF has 700-mb temperatures down to -4C over Cheyenne by daybreak Tuesday, with high confidence in values between -5 and -8C during most of the daytime hours Tuesday and into Wednesday morning. This would be cold enough to rain changing to or mixing with snow for the entire area. There is still a bit of uncertainty in the timing. If most of the snow falls during the day, the snow accumulation expectations will be considerably less due to the very strong early May sun, whereas snowfall at night would be much more likely to accumulate. As a result, we won't get into too much detail on the potential snowfall amounts. The current most likely scenario would mean accumulating snow above about 5500 ft in elevation.

With good low-level upslope east to northeasterly flow and the primary lifting mechanism tied to the frontal boundary stalling against the terrain, the area in the vicinity of the Snowy and Laramie Ranges will have the best chance at seeing drought-denting moisture from this system. Currently, Cheyenne has an 80% chance of seeing 0.5" or more of precipitation, and even a 50% chance for an inch of liquid! Laramie's probabilities are a little lower, since the area does not do well with easterly low-level flow. Probabilities also decrease rapidly along and north of the North Platte River. Current NBM probabilities for Scottsbluff are for a 75% chance for 0.25", a 50% chance for 0.5", and a 15% chance for 1 inch or more. Unfortunately, the Lusk to Chadron area is unlikely to see much moisture from this system with the best dynamics positioned south and west of the area, but the most likely scenario delivers between 0.05 and 0.25" to this area. NBM probabilities are actually conservative relative to the current ensemble suite, but that is a comfortable place to be at this time. This system is quite complicated and disorganized, and ensemble systems are generally under-dispersed. The current official forecast is near the ensemble 25th percentile due to these concerns, and the tendency of every storm system this season to fizzle once we get within a few days.

A few rain and snow showers may linger into Wednesday with temperatures remaining 10 to 15F below seasonal averages. Ridging will build back into the western CONUS for the end of the upcoming work week. We may see a period of windy conditions Wednesday night through Thursday as this transition occurs. Current probabilities for high winds in the wind prone areas are around 20 to 40%. Warm air advection will return quickly during this period as well. We should see temperatures climb back to near seasonal averages on Thursday, and then above average by Friday.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 1125 PM MDT Fri May 1 2026

VFR conditions expected for the forecast period. Northwest winds will occasionally gust around 20 knots Saturday afternoon and early evening. Winds will ease after sunset again, and may be variable at times.

CYS WATCHES/WARNINGS/ADVISORIES

WY...None. NE...None.