Cheyenne, Wyoming

• Locally dense fog is possible Saturday night into Sunday morning for areas east of the Laramie Range.
• 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.

Looking at a not-so-spooky rest of Halloween with favorable conditions for trick or treaters. Clouds are gradually decreasing early this afternoon, leading to mild temperatures across the forecast area. Highs in the 50s and 60s are expected, with temperatures falling into the 40s and 50s this evening for the candy seekers that will be out and about. Cannot rule out a stray convective shower this afternoon and the early part of the evening as sunshine helps build weak low-level instability. However, these will be short-lived and will have little precipitation in them. The rest of the night looks relatively uneventful aside from another round of fog for areas east of the Laramie Range. Weak southerly winds will once again increase low-level moisture, leading to the potential for dense fog late tonight and into Sunday morning.

The following 48 hours will be much more active with widespread precipitation chances. This two part system will bring precipitation, wind, and cooler temperatures to the region. A potent upper-level trough will begin to push into western Wyoming on Sunday. Southwest flow ahead of this trough will advect ample tropical moisture into the area, allowing PWATs to reach climatological max per NAEFS guidance. Broad and weak forcing will lead to predominately stratiform precipitation through most of the day. Precipitation chances will be highest for areas along and west of the Laramie Range, and along a line from Douglas to Chadron northward. Areas outside of the aforementioned areas could still see precipitation, however chances appear to be lower based on current Hi-Res models. Precipitation during this "first wave" is expected to fall as rain as it will arrive ahead of the cold front. Southwest flow will actually help advect fairly mild 700 mb air into the area ahead of the front. So despite the abundant cloud cover, temperatures are still expected to reach the 50s and low 60s. There will be a brief "lull" in precipitation early Sunday evening before the cold front arrives.

The cold front will arrive Sunday night, gradually ushering in cooler air. Simultaneously, leeside cyclogenesis will be occurring in northeast Colorado, forming a surface low. Cluster analysis from both the GEFS and ECMWF ensemble are in good agreement regarding surface low placement and track into the day Monday. Once the surface low forms, precipitation will once again become pretty widespread across the CWA overnight. Still likely looking at mostly rain Sunday night as the cold air is slower to arrive.

Monday, expect a colder day behind the front with temperatures in the 40s for southeast Wyoming and low 50s for the Nebraska panhandle. Temperatures could be cold enough to see some snow mix with rain, but this will be dependent on the snow levels which are still uncertain. However, at this point, cannot rule out snow levels down to about 6000 feet. Heavier precipitation rates will also be more likely to see the full switch from rain to snow. Higher rates does come with the potential for some minor accumulations. The mountains and higher elevation areas are still the areas with the highest confidence in seeing accumulations. Lower elevations west of the Laramie Range could see some minor accumulations but uncertainty is high. Ensemble guidance from both the GFS and ECMWF still have very few members latching on to any accumulations for these valley locations. However, did force some snow into the forecast for places like Laramie and Rawlins since there is at least a decent chance of these cities seeing a trace of snow. As the surface low tracks into central Nebraska during the day Monday, the CWA will get into some good wrap around moisture, leading to widespread precipitation throughout the day. The location of the surface low will also lead to a very strong MSLP gradient across the forecast area. The strong gradient will lead to a very windy day Monday, with blustery northwest winds. 50 to 55 MPH wind gusts are expected for the Interstate 25 corridor and the eastern plains. Cannot rule out high winds at this juncture, however, northwest winds are typically not a favorable direction for high winds in the CWA. Strong winds and precipitation will continue into Monday evening.

As we start off the long term period, the region will be sitting firmly under a trough aloft with a northerly flow regime overhead. Shifting gears a bit to the surface, there will be a deep surface low located to our northeast across southeast South Dakota. The result of the placement of this surface feature will be a tight northwest pressure gradient over the CWA causing surface winds to become northerly to northwesterly along with residual moisture being fed into the CWA. This will keep chances of precipitation, possibly a few snow showers mixing in to our west across the Laramie Range and later in the evening across the Nebraska Panhandle with little to no snow accumulations expected through Monday night at lower elevations. Higher elevations, north of 9,000 feet could see some accumulating snowfall on the order of 1 to 3 additional inches Monday afternoon/evening, especially across the Sierra Madre and Snowy Ranges. Overall, QPF totals through Monday night look to be on the order of a tenth to two tenths, highest across northern portions of the Nebraska Panhandle, of an inch primarily north of I-80 and east of I-25. As mentioned briefly above, the surface gradient will tighten and this will cause the potential for strong winds Monday night in the wake of the surface low and associated cold front. Ensemble guidance supports to a degree wind gusts approaching or exceeding 50 mph, primarily a swath along the I-80 corridor in the Nebraska Panhandle. Due to weak subsidence, Omega (GFS), these strong winds will be gradient driven at the surface due to the tight pressure gradient that sets up over the area.

As we progress in this forecast period, the stronger winds will diminish through Tuesday morning into the early afternoon hours as the surface low slides off to the east, causing the surface gradient to lax and winds to ease. Not only this, but drier conditions return with weak ridging building in aloft by Tuesday afternoon. By Wednesday, some model differences arise with a trough that moves to our west and how that feature evolves. One solution has it a bit faster and sliding across the CWA through Wednesday while the EC has it a bit slower and weaker. Beyond Wednesday, model consensus lacks with the placement and amplitude of synoptic features, so stay tuned to the forecast for future refinements to the forecast. Now, taking a closer look at temperatures, expect highs to climb into the lower to mid 40s west of I-25 and lower to mid 50s east of I-25 through the remainder of the work week into next weekend. While lows dip into the lower to mid 30s across the CWA, coldest west of the I-25 corridor.

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.

WY

None.

NE

None.