Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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716 FXUS65 KCYS 210512 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1112 PM MDT Mon May 20 2024 .KEY MESSAGES... - The potential for strong to severe thunderstorms exists from 4PM and 11PM in far southeast WY and southwest NE. Large hail up to two inches in diameter, strong winds up to 60mph, and an isolated tornado can not be ruled out. The strongest storms will likely develop after 6pm. - Much cooler temperatures with more widespread to numerous showers and thunderstorms on Tuesday. Accumulating snow likely above 8000 feet, with a rain and snow mix possible down to 6000 feet late Tuesday night into early Wednesday morning. - Daily shower and thunderstorm chances continue through the long term. Could see some elevated winds in the wind prone regions throughout the week and weekend. && .UPDATE... Issued at 1142 AM MDT Mon May 20 2024 Noontime visible satellite imagery reveals a thick band of cloud cover over much of southeast Wyoming as well as the far northwest Nebraska panhandle. Pockets of lower-level clouds were observed streaming northward and eastward, providing some visual clues to an increase in lower level moisture in upslope flow. Surface observations across the southwest Nebraska panhandle and southeast Wyoming have shown a steady change in wind direction over the past 2-3 hours as northerly post-frontal flow shifts to easterly. However, of note is the rather paltry moisture in that surface easterly upslope flow. Td values in the Pine Bluff area remain in the low to mid-30s as of the noontime hour with obs farther east in the SNY vicinity showing Td values in the low 40s. A few pockets of elevated instability over the mountains of southeast WY and far northwest NE have resulted in some isolated weak thunderstorm activity, primarily tied to the differential heating boundary between the cloud shield to the west and clearer skies to the east. Over the next several hours, continued easterly upslope flow will slowly transport higher surface moisture values in from the east. This pool of deeper moisture is evident as a band of thicker near-surface stratus over northwest Kansas and the MCK area of Nebraska. Given the rather high T/Td spreads, initial thunderstorm development in southeast Wyoming will be high-based with a marginal hail and wind threat through around 22z. After around 23z or so, CAM guidance does remain consistent supporting the development of some deeper convection over far southeast Wyoming and moving into the NE panhandle. While deeper surface moisture and better low-level directional shear will likely remain south of the Colorado state line, RAP guidance does indicate long, straight hodographs over the NE Panhandle which would support an early supercell storm mode. With a deeply mixed boundary layer and plenty of cold air aloft, storms will likely tend to be outflow dominant and may quickly transition to bowing line segments with primarily a high wind threat. This is particularly evident in the past few runs of the HRRR over southwest Nebraska in the Scotts Bluff, Banner, and Morrill county corridor from around 6-9pm this evening. Areas along I-80 may ultimately remain shielded from stronger convection as any dominant supercells in northeast Colorado may block deeper moisture return. Regardless, it is looking like the 6-9pm timeframe will be the time window to watch today for storms, especially from the Wyoming state line eastward in the SW NE panhandle. && .SHORT TERM /THROUGH TUESDAY NIGHT/... Issued at 206 PM MDT Mon May 20 2024 Lingering rain showers are possible early Tuesday morning across much of the NE panhandle after Monday night`s convective activity with the initial leading shortwave continuing off towards the upper midwest. Additionally, ongoing precipitation will continue over east-central WY associated with the slow moving vort max over western WY. This feature will continue to drop 700/500mb temps across the area on Tuesday supporting of high elevation snowfall, mainly above 8000 feet elevation. This will lead to near advisory snow accumulations in the higher terrain of the Snowy and Sierra Madres throughout the day Tuesday. Cooler temperatures will continue into Tuesday night as skies begin to clear, especially after midnight, leading to much of south- central WY dropping near the freezing mark. East of I-25, lows will drop into the mid-30s with a few localized areas dropping near the freezing mark. Latest NBM probability of dropping to/below 32F degrees is 30-50%, especially extending eastwards off the Cheyenne and Pine Ridges. However, with the average date of the last spring freeze being in mid-to-late May across most areas east of I-25, no frost/freeze headlines are expected at this time. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Issued at 400 AM MDT Mon May 20 2024 Models continue to trend towards an active long term forecast for southeast Wyoming and western Nebraska. Several upper-level disturbances are progged to traverse across the region. Wednesday morning, an upper-level trough will move off to the east as another builds the coast of Washington and Oregon. Forecast soundings from the GFS for Wednesday morning no longer suggest a potential for snow across the region. Dry low-levels will likely not allow much precipitation to fall to the surface Wednesday morning. Therefore, removed the mention of snow from the Wednesday forecast. With the lack of snow expected now, temperatures should increase as 700mb increase to the 4-6C range. Surface temperatures should warm back into the low-60s to low-70s across the region. Could see some isolated showers throughout the day as residual energy remains overhead behind the departing trough, but widespread precipitation is not anticipated at this time. The second of many upper-level troughs digs through the region Thursday, with long range models in good agreement on the evolution. This trough is progged to quickly swing across the area with decent precipitation chances in the afternoon hours. At 700mb, a low is expected to develop across western Montana and move overhead throughout the day. A weak cold front will move through with the passage of this low, though the coldest air is expected to remain off to the north in eastern Montana. However, the cold front will act as a catalyst for more showers and potentially thunderstorms to develop in the afternoon hours before dropping temperatures for Friday. Thursday`s highs look to be in the low-60s to upper-70s again, with highs on Friday in the upper-50s to low-70s across the area. The trough will move out the region Friday with brief riding developing for Saturday before a third trough pushes towards the area. Behind the cold front, winds will pick up significantly and be further amplified by the 700mb low passing to the north. Height gradients will tighten throughout the day, favoring strong winds across the wind prone regions and likely spilling out into the adjacent plains. On Saturday, brief ridging is progged to develop aloft, allowing 700mb temperatures to increase back into the 4-6C range once more. Residual energy from the departing trough, as well as some energy ejecting out ahead of the incoming trough will be enough to spark some showers and thunderstorms during the afternoon hours. Temperatures will warm into the mid-60s to mid-70s, with isolated to scattered precipitation possible. With the passage of a 700mb low to the north of the CWA, height gradients will tighten across the region once more. Could see some elevated winds in the wind prone regions of southeast Wyoming and likely see some winds spill out into the adjacent plains. Daily shower and thunderstorm chances continue through the weekend, with the incoming upper-level trough. Some disagreement appears between long range models for Sunday evening, as the GFS has a closed upper-level low and the ECMWf has an open wave across the northern CONUS. However, both models still suggest precipitation chances so kept PoPs in the 20-30% range for the remaining long term forecast. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1106 PM MDT Mon May 20 2024 IFR/LIFR conditions expected east of the Laramie Range with northeast upslope winds. Conditions could persist through the morning Tuesday...before winds shift northwesterly and become downsloping. Showers and storms possible Tuesday afternoon...mainly across the northern Panhandle and here near Cheyenne. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...Dense Fog Advisory until 6 AM MDT Tuesday for WYZ116>119. NE...None. && $$ UPDATE...MAC SHORT TERM...MB LONG TERM...AM AVIATION...LEG