Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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902 FXUS65 KCYS 271755 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1155 AM MDT Sat Apr 27 2024 .KEY MESSAGES... - Significant mountain snow is expected in the Snowy and Sierra Madre ranges, with total snowfall accumulations of 1-2 feet likely in the Snowys through Sunday morning. Winter Weather Advisories and Winter Storm Warnings have been issued. - Confidence is increasing in snow levels lowering sufficiently for heavy snow above 7500 feet over the South Laramie Range and I-80 Summit. However, uncertainty remains with the overall travel impacts due to the heaviest snow occurring during the daytime hours. A Winter Weather Advisory has been issued. - Widespread rain and high-elevation snow will impact a large portion of southeast Wyoming and the western Nebraska Panhandle through Sunday morning. Rain may change to snow as far east as the I-25 corridor into Cheyenne Saturday night, with minor accumulations possible on grassy surfaces. && .SHORT TERM /THROUGH MONDAY/... Issued at 400 AM MDT Sat Apr 27 2024 Light to moderate rain showers are slowly propagating northward early this morning. Areas across the higher elevations above 8000 feet are reporting snow showers. Expect this trend of light to moderate rain showers to persist through the remaining morning hours as chilly temperatures are also forecast to continue. Unfortunately, the cloud cover and upslope flow from the surface to upper levels of the atmosphere will keep it very cool outdoors today. Not expecting a large recovery in daytime highs as we remain largely in the 30s and 40s for the lower terrain, and below freezing in the mountain zones where winter headlines remain in effect through today. The forecast remains on track with a prolonged period of accumulating snowfall in the Snowy/Sierra Madre Mountains, so have not changed the headlines. The warmest areas that will see partly cloudy skies remain in our northern forecast zones, where temperatures may eclipse the 50 degree mark temporarily, before mostly cloudy skies become prevalent for the majority of the cwa. Several areas should see a wetting rainfall of 0.10 inches while the rain showers continue. Tonight and into early Sunday, this slow-moving weather system will become occluded for all intents and purposes. This will keep the window for rain and snow showers present across our eastern half of the cwa, with the orographic enhancement keeping accumulating snowfall in the mountain zones for another good while. Some of the model guidance members have the eastern tier of the south Laramie Range foothills and Central Laramie Range wet bulbing to near or below freezing, which could assist with light snow accumulations on the grass by daybreak Sunday morning. Have gone against this as the Kuchera ratio method for both the HRRR/NamNest have less than a half inch accumulating. As the day presses onward, expect the rain showers to change over to light snow showers for the NE Panhandle, primarily after sunset. Would expect up to perhaps a dusting of snow to accumulate on grassy surface total. Daytime highs will rebound slightly, with most locations see the lower to upper 50s, except the mountain zones where 30s will be present. We will have another weak shortwave bring the potential for light snow showers on Monday for the higher terrain. Additionally, upper level winds at 700mb are trending upwards by the late afternoon period. This coupled with subsidence/mountain wave signatures from model guidance are increasing confidence of gusty winds for southeast WY, especially the higher terrain. Have kept the breezy to gusty winds in the forecast. At this time of inspection, high wind headlines do not appear to be necessary, but 40-50mph wind gusts in the wind prone corridors by late Monday afternoon are highly favored. Partly sunny skies will become present by the afternoon for most of the cwa, outside of the higher terrain. This combined with the downsloping should assist with westerly winds warming the lee side of the Laramie Range and the western NE Panhandle nicely. Expect daytime highs in the 60s to lower 70s for the high plains. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... Issued at 400 AM MDT Sat Apr 27 2024 Long Range models start off in good agreement early to midweek, and then start to diverge by the end of the week as another Pacific storm system moves eastward into the Front Range and Intermountain West. Quasi-zonal flow aloft is expected to dominate the weather pattern late Monday through Wednesday with progressive upper level disturbances rapidly moving eastward every 18 to 24 hours near or colocated with the 100 to 120 knot jet across Wyoming and the Dakotas. This pattern typically results in near average temperatures for this time of the year with highs in the mid 50s to mid 60s and lows in the 20s to mid 30s. Some drier air will move into the area by Tuesday, limiting coverage of precipitation. However, can rule out isolated to widely scattered showers and a few thunder showers late Monday and Tuesday. Kept POP between 10 to 20 percent for now. Strong Pacific storm system will begin to move eastward and start impacting the region Wednesday with a higher coverage of showers and thunderstorms in the afternoon and evening hours. A brief period of strong gusty winds are possible late Monday night into Tuesday morning as a fast moving Pacific cold front brings some precipitation to southern Wyoming Monday evening. A Bora-like wind event is possible across southeast Wyoming with strong subsidence behind the cold front and 700mb winds of 55 to 60 knots early Tuesday morning. In-house wind models do show the potential for locally strong wind gusts, mainly along and west of the I-25 corridor. Increased winds near High Wind criteria and will need to continue to monitor this trend through the weekend for the possibility of High Wind Watches, especially in and near the wind prone areas. For later next week (Thursday and Friday), model solutions diverge quite a bit as the Pacific storm system moves over Wyoming. Ensemble guidance shows a large spread of max and min temperatures as a strong cold front will be located near the Wyoming and Colorado border at this time and models are having a hard time resolving timing and location of the front. Temperature spreads are as high as 15 to 25 degrees with closer to 40 degrees comparing the bottom 5th-10th percentile to the top 90th percentile. Most guidance, including the NBM, is showing this. This forecast period will likely become the primary forecast concern since all models show a cold enough airmass for widespread snow down to 5000 feet Thursday and Friday. Did not go that aggressive in the official forecast due to low confidence and uncertainty. Lowered max and min temperatures below average for the end of the week, but kept mainly rain and some thunderstorm activity below 7000 feet. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1139 AM MDT Sat Apr 27 2024 Latest surface analysis was showing the surface low in southeast Colorado with some strengthening taking place during the course of the morning. As a result we are seeing more upslope flow and precipitation developing along the Front Range. This has even resulted in some upslope/snow flow over near KLAR at the base of the Snowies. This trend should continue through the remainder of the afternoon as the surface low continues to deepen and lifts north towards the southern Nebraska panhandle. Not confident on how fast the back edge of the rain/snow mix will move out tonight, but we are more confident that the rain/snow mix will most likely linger through daybreak in the Nebraska panhandle. KAIA may even see some accumulating snowfall towards daybreak. MVFR conditions will be the rule through much of the afternoon and early evening across most of the TAF sites except for KRWL where they will remain mostly VFR. IFR conditions are more likely overnight across the majority of the TAF sites including KLAR where they may even see some radiation fog towards daybreak as the wind slackens over that area. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...Winter Weather Advisory until 6 PM MDT Sunday for WYZ112. Winter Storm Warning until 6 PM MDT Sunday for WYZ114. Winter Weather Advisory until midnight MDT tonight for WYZ116. NE...None. && $$ SHORT TERM...BW LONG TERM...TJT AVIATION...REC