textproduct: Cheyenne

KEY MESSAGES

- Intermittent, locally dense fog Saturday morning will clear out by midday. Another round of fog is possible Saturday night into Sunday morning.

- Mild conditions are expected for Halloween (Saturday), though a few showers will be possible along and west of I-25.

- A complex but disorganized storm system will move into the area Sunday, bringing cooler temperatures and a chance for rain and high elevation snow showers.

- A second round of precipitation is increasingly likely to develop over portions of the High Plains Monday into Monday evening.

- Widespread blustery conditions are anticipated Monday into Tuesday along and east of the Laramie range.

SHORT TERM /THROUGH SUNDAY NIGHT/

Issued at 340 AM MDT Sat Oct 31 2026

Spooky fog has settled in over portions of the High Plains to start this Halloween. This morning's fog is a bit more intermittent than yesterday, but at least patchy fog encompasses much of the High Plains. Expect fog to roll in and out a few times this morning. Dense Fog Advisories have been posted for much of the I-25 corridor and portions of the I-80 corridor, but may need to be expanded if fog pushes into the central/northern panhandle as well. Cloud cover and abundant low-level moisture are supporting very mild temperatures for this time of year, with today expected to start off in the upper 30s to mid 40s for all areas. The vort-max moving into the area out of the southwest is a bit ahead of schedule, as mid to upper level moisture is already pushing into southern Wyoming as indicated by GOES water vapor imagery. Expect shower activity to kick up over the next several hours, mainly in Carbon county, but rainfall amounts will be minimal. We actually have a fairly dry airmass in place west of the Laramie range, but expect dewpoints to rise as light rain falls into the dry boundary later. The shortwave trough axis should push through around midday, decreasing coverage of shower activity afterwards, though some shallow instability should keep isolated showers going along and west of the Laramie range through the early evening hours.

This year's trick-or-treat forecast is unusually mild. Sunset temperatures will be generally in the 50s, dropping to the mid 40s to around 50 by 9PM. Aside from a few light showers around, conditions should be pleasant with generally light winds. While portions of Carbon and Albany county will be breezy during the daytime hours, expect this to weaken by the evening.

Sunday will be a transition day as a complex storm system approaches from the west. Southwest flow aloft will support the advection of very deep tropical moisture. Precipitable water is expected to surge rapidly through the day Sunday pushing close to climatological maximums by the end of the day. With the aggressive moisture advection, we should see mostly cloudy skies over most of the area, and thus temperatures a few degrees cooler than Saturday. Forcing for ascent will push in Sunday morning in the form of positive theta- e advection and isentropic ascent. This looks most prominent along and west of the Laramie range as it will take a little longer to reach saturation further east. Expect numerous to widespread showers in Carbon County through much of the day, which will gradually push east into at least Albany and Converse counties later in the day. Further east, rainfall will be more spotty, with little to no rain expected in far southeast Wyoming and the central/southern Nebraska panhandle. With falling surface pressure over southwest Wyoming during this period, expect breezy south to southeast winds over much of the area during the day Sunday. Lift is expected to shut off sometime between sunset and midnight Sunday evening as the axis of the first mid-level shortwave trough swings through the area. This will put much of the area in a lull Sunday night into Monday morning.

LONG TERM /MONDAY THROUGH FRIDAY/

Issued at 340 AM MDT Sat Oct 31 2026

The approaching storm system continues to look rather messy. Multiple competing upper level lows will make for a muddy forcing picture across the region. A deep primary trough will be making its way across the western CONUS while a moisture laden upper level low riding the sub-tropical jet races in around it. The primary 700-mb low and the secondary center associated with the subtropical system will both pass through the area Sunday evening, leading to drier conditions over much of the area Monday morning. After passing our area, these two should consolidate into one system and stack on top of a rapidly developing surface cyclone. This should take over around daybreak Monday, setting up a strong pressure gradient over the High Plains and leading to increasing northwest winds Monday morning. Widespread gusty northwest winds can be expected Monday into Monday night over the High Plains. Confidence remains high in fairly widespread gusts of 45 to 55 mph, with still some potential to exceed High Wind criteria. Probabilities are generally around 30 to 40% for all along and east of the Laramie range, including the wind prone areas around the I-80 summit and I-25 between Wheatland and Glendo. The wind direction is not very favorable for high winds at Bordeaux. The lack of vertically stacked westerlies and potential for synoptic ascent rather than descent as surface winds increase decreases confidence in widespread high winds.

Forcing will not really favor much lift during this period Monday AM, so expect fairly limited coverage around dawn, but this should change as the day progresses. Model guidance is now in fairly good agreement showing the mid-level low pressure elongating back to the southeast after it passes through our area. This will be forced by lee cyclogenesis ahead of the the primary upper level low swinging over the Four Corners which begins to slow down as it ejects to the east. This appears to be a very complex evolution, but the end result is to turn the wind direction in the 700 to 500-mb layer from northwest Monday morning to north or even northeast by Monday afternoon. Despite surface downslope northwesterly winds continuing, we may see support for ascent just above the surface layer. Where northwest flow is not as unfavorable (Douglas to Chadron corridor), seeing an increase in appreciable rainfall is more promising. The surface downslope will be tough to overcome around the Cheyenne to Sidney corridor. Thus, we appear to be finally converging on the scenario with moderate precipitation in our northern zones, and more limited precipitation further south.

The latest QPF probabilities reflect the increasing confidence in this scenario. LREF probabilities for 0.5" or more of liquid precipitation are around 50 to 60% in Rawlins, Douglas, and Chadron, which look like the most favorable areas at this time, though it is worth mentioning that at least for Rawlins some of this will come from the first phase Sunday and Sunday night, with a break early Monday morning, and then another round Monday late morning into the evening. Precipitation evolution may take on the character of typical northerly flow events across our area, with the subtle terrain features of the High Plains becoming unusually prominent. Specifically, look for more precipitation along the north sides of the Pine Ridge and Cheyenne ridge, with lower totals into the Wheatland to Bridgeport corridor and I-80 corridor (including Cheyenne) and points southward. Laramie and the I-80 summit will be an area to watch for greater uncertainty. This is a pretty unusual synoptic setup with veering winds above surface northwest winds, but that surface direction is favorable for increased precipitation around the city of Laramie.

The last piece of the puzzle with this storm system will be snow levels. Snow levels will be fairly high for most of the event, especially during the first phase where rain may fall as high as 9 to 10 kft. However, we can expect gradual advection of colder air during the day on Monday to reduce snow levels. By sunset, 700-mb temperatures should be below 0C over most of southeast Wyoming. As precipitation picks back up midday Monday into the evening, rain may change over to snow between 5000 ft and 8000 ft. The precise snow level will probably depend on the precipitation rates, with areas of heavier precipitation changing over faster, while lighter rates stay as rain. The problem with this is that there is considerable high end boom scenario potential on snow accumulation if a given area flips over faster than expected. The primary areas to watch right now are Douglas, Lusk, Harrison, and possibly Laramie and the Chugwater area. For now, no snow accumulation is in the official forecast because this is still a low-probability (30% chance) but high impact scenario. However, snow levels were blended to the NBM 10th percentile to ensure the mention of snow is included for these areas of concern.

A much drier airmass will rotate into the area after midnight Monday night, bringing an abrupt end to precipitation from north to south. Gusty northwest winds will continue into Tuesday, but temperatures should rebound to near seasonal averages. The remainder of the week looks to remain somewhat unsettled, with a series of moisture- starved upper level lows pushing across the region. Low-end chances for light precipitation will be present with temperatures near seasonal norms. If some of these lows track slightly to our north, we may have to watch for some gusty winds in the wind prone areas, but most ensemble members keep winds light during this period. By the end of the week into the weekend, we may have another closed low tracking across the west to keep an eye on. Uncertainty is extremely high at this lead-time, but about 1/4 of ensemble members have a closed upper level low tracking over the right area to bring widespread precipitation to the area. However, the remaining 3/4 members are either dry or with minimal precipitation.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1122 AM MDT Sat Oct 31 2026

Low CIGs will continue to lift over the next several hours for the Nebraska terminals, KAIA may hold onto the lower CIGs for slightly longer. A passing shower may come in the VC to KCYS and KLAR through this afternoon. Otherwise, not all that eventful for the remainder of the sites for most of the daytime hours. A return of FG is possible at KBFF, KSNY, and KAIA later this afternoon/evening, bringing IFR conditions back to those sites. This FG will plague these terminals through around 15Z Sunday, thereafter the FG should lift and disperse with a return of gusty winds on the order of 25 knots. These breezy winds will also return to KCYS and KLAR by 16Z Sunday with gusts 20 to 25 knots.

CYS WATCHES/WARNINGS/ADVISORIES

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