textproduct: Cheyenne

KEY MESSAGES

- Warmer this weekend with temperatures around 5 to 10 degrees above normal. Slight Chance for Thunderstorms on Sunday as the next Pacific storm system moves onshore.

- A strong Pacific storm system will bring scattered showers and storms along with seasonably cool temperatures for the start to middle of next week, with a slow warming trend into the later part of the week.

SHORT TERM /THROUGH SUNDAY NIGHT/

Issued at 155 AM MDT Sat Sep 26 2026

Pretty quiet weather expected this weekend with pleasant conditions and temperatures around 10 degrees above seasonal averages. There are some changes to Sunday afternoon as models are trending towards a more unstable boundary layer with increasing south to southeast winds...resulting in decent moisture advection across the High Plains.

For today, models all in agreement showing mid to upper level flow veering into the west. Surface winds are already starting to respond early this morning with westerly winds observed along and west of the I-25 corridor. This trend will continue through the day with dewpoints lowering into the 30s to mid 40s. High resolution models are showing some sprinkles or light rain showers developing near the mountains. A quick look at the soundings show a strong temperature inversion near 500mb with poor lapse rates through most of the day. There is no forcing present today with upper level ridging becoming dominant. Expect mainly afternoon cumulus this afternoon with maybe a few light sprinkles at times. Kept POP between 5 to 10 percent into this evening. Otherwise, warm for late September with highs in the mid 70s to mid 80s, warmest below elevations of 5000 feet.

Drying trend will be short-lived as all models showing return flow off the Great Plains later in the day. Expect light and variable winds in the morning to shift into the south to southeast through the afternoon and evening hours. There is also a Low Level Jet signature present across the eastern high plains as 700mb winds climb above 35 knots Sunday evening. This will result in moisture advection along and east of the I-25 corridor through the day. There is even some midlevel moisture advection over central Wyoming, ahead of the Pacific storm system moving onshore. Continued to increase POP between 20 to 40 percent for most of the forecast area starting Sunday mid afternoon and continuing through the late evening hours as a midlevel shortwave ejects northeast across the area colocated with an 80 knot jet max. Some thunderstorm activity is expected with CAPE of 500 to 1000 j/kg east of the I-25 corridor.

LONG TERM /MONDAY THROUGH FRIDAY/

Update issued at 110 AM MDT Sat Sep 26 2026

Will continue to monitor the Pacific storm system and remnants of an eastern Pacific Hurricane on Monday and Tuesday. Current 00z models continue to trend towards split flow with the 00z GFS particularly dry for our area and showing little to no precipitation. The 00z Canadian does show the upper level low tracking a bit more ENE with some decent rainfall possible along the I-80 corridor and the mountains. Models are starting to trend warmer for Tuesday and Wednesday, which is common with split flow. 00z ECMWF is just arriving, and shows a solution in between the Canadian and the GFS. Will continue to keep an eye on this event, but as of right now, a much needed widespread rainfall event looks unlikely at this time given the overall pattern and the lack of a strong upper level ridge across eastern half of the United States. Lowered POP, especially north of the North Platte River Valley and tweaked temperatures towards the 50th percentile of the NBM, GEFS, and ENS.

Previous discussion... Issued at 220 PM MDT Fri Sep 25 2026

After a warm and largely dry weekend, the weather will turn more unsettled once again as we head into the early part of next week. We will have an interesting synoptic setup across the area, with the typical upper level ridge over the southern Plains, combined with an approaching Pacific trough. The wild-card factor here will be the potential for an infusion of an exceptionally moist airmass ahead of and associated with Hurricane Polo as it moves into Baja California early next week. Unfortunately for our area which remains in drought conditions, these factors do not look likely to align and produce heavy rainfall for our area. Instead, the Pacific trough is trending towards a split trough scenario. The northern branch will move through on Monday, leaving behind a closed low in the southwest, which eventually absorbs the remnants of Hurricane Polo. Moisture will be abundant by Monday, with PW values around the 90th to 99th percentile of climatology, but forcing and instability look unimpressive at this time as the core of the northern branch of the split trough passes to our north. Still, we should have enough to at least support scattered showers and possibly a few thunderstorms. Look for increased cloud cover and slightly cooler temperatures as a result, though highs should still be a touch above seasonal averages. A cool front with the northern branch system will knock temperatures down by about 5-10 degrees for Tuesday. A chance for showers remains in the forecast, but the most likely scenario at this time leaves the closed low, and thus the potential for widespread heavy rainfall, mainly to our south.

Heading into the middle of the week, the most likely scenario supported by about 80+% of ensemble members features seasonable temperatures and dry conditions as the upper level closed low spins to our south. Cloud cover may spill into our southern zones associated with the system to the south. While the probability of a different scenario is low, it is still worth a mention. In about 20% of ensemble members (and the AI-GFS), the closed low over the southwest lifts more to the north instead of the east, which would bring abundant wrap-around moisture into our High Plains zones Wednesday into early Thursday. Regardless of the precipitation potential, there is also a chance for some strong winds behind this system. The official forecast reflects the former scenario, which is far more likely at this time, but we can't quite rule out the latter just yet. After the closed low moves out, expect another warm-up beginning Thursday, and peaking on Friday.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 1109 PM MDT Fri Sep 25 2026

Most of the shower activity this evening has dissipated, however lower ceilings at KAIA and KSNY are expected to continue through about 12z. The latest high resolution guidance suggests that visibilities stay above six statue miles, but CIGs below 1000 feet are likely. Elsewhere skies will continue to clear through the morning hours. The shortwave trough will push east and with it drier continental air will follow. A change in winds out of the west to northwest with gusts between 20 and 30 knots are possible during the morning hours. VFR conditions for most of tomorrow at terminals. A few showers could develop near KCYS, KLAR, and KBFF in the evening.

CYS WATCHES/WARNINGS/ADVISORIES

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