Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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148 FXUS65 KCYS 291758 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1158 AM MDT Wed May 29 2024 .KEY MESSAGES... - Strong to severe thunderstorms mainly along and east of I-25 into the Nebraska Panhandle Wednesday afternoon with large hail and strong winds. - Active weather pattern will continue through the weekend with daily chances for showers and thunderstorms. - Summertime temperatures will be on the horizon next week. && .SHORT TERM /THROUGH THURSDAY NIGHT/... Issued at 352 AM MDT Wed May 29 2024 Latest GOES Nighttime Microphysics RGB shows localized low clouds and fog developing over the South Laramie Range early this morning with southerly low-level moisture advection ongoing ahead of the upper level trough approaching our area. Decided to issue a Dense Fog Advisory through 7 AM MDT this morning as localized dense fog could impact the morning commute along the I-80 corridor between Cheyenne and Laramie. Main forecast concern Wednesday remains the potential for strong to severe thunderstorms during the afternoon and evening east of I-25 into the NE panhandle. Steep mid-level lapse rates along with continuing southerly moisture advection east of the lee trough are supporting of MLCAPE values around 1500 J/kg overlapped with 0-6 km shear values 30-35 kt per the latest RAP. SPC continues to show a Slight Risk for severe thunderstorms with their latest Day 1 Outlook. Biggest uncertainties right now remain the placement of the dryline in eastern WY as that will likely impact how quickly these storms turn severe Wednesday afternoon once they move off the higher terrain of the Laramie Range. NAMNest shows a sharp moisture boundary with nearly a 20F degree dew point spread, while the HRRR is much more diffuse with this gradient as well as being positioned closer to the NE/WY border. Latest HREF guidance shows a 25th-75th percentile spread in SBCAPE values over 500 J/kg around the NE/WY border. Will need to continue to monitor forecast and observational trends throughout the day as these details will better refine location and timing of the strongest storms today. As of right now, CI in the higher terrain looks to occur around midday with storms strengthening later in the afternoon as they cross the moisture boundary with large hail and strong winds possible through the early evening. Hi-res guidance suggests upscale growth across the NE panhandle that will likely transition the main hazard threat to strong winds and heavy rainfall before storms move east of the CWA border around 8 PM. Farther west, additional showers and storms may develop closer to the cold front moving across Carbon Co with forecast soundings showing dry low-level and LCLs extend well above the freezing level supportive of dry microburst gusty outflow winds even with lighter showers/returns on radar. After high temperatures climb into the upper-70s to mid-80s Wednesday, Thursday will be cooler, but still near average for late May. Post-frontal upslope flow Thursday with moisture pooling up against the Front Range will support widely scattered thunderstorms in the afternoon despite the better environmental setup well to the east across the central plains. Daytime PoPs will be limited to the vicinity of the South Laramie Range during the afternoon ~20%, but looks to increase and expand into much of the NE panhandle as a secondary cold front associated with the same upper level trough now lifting into the north-central CONUS brings increasing moisture convergence into the CWA. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Issued at 352 AM MDT Wed May 29 2024 The pattern of mild to warm temperatures and near daily chances for thunderstorms will continue through the weekend, but a warming and drying trend is expected for the first part of next week. On Friday, a broad trough will be located to our north, and another piece of 500-mb vorticity will rotate through the area. As a result, Friday looks like the coolest day of the forecast period across the area with a sharp north to south gradient in 700-mb temperatures across the area from about 2 to 3C near US20 to 6 to 8C along I-80. Meanwhile, a surface high sliding down the east side of the Rockies Thursday night will push the dryline back west against the Laramie range and return low-level southerly to southeasterly flow over much of the High Plains. With some subtle added lift from the passing vorticity maximum, look for another round of thunderstorms to develop. While storms may just be isolated to widely scattered, they may have a slightly greater potential to become strong to severe on the Wyoming side. Veering wind profiles and more low-level moisture will be present. Forecast soundings show higher instability and sufficient wind shear over southeast Wyoming. However, a potent inversion around 700-mb will be stronger further north and east, which will limit the severe potential in the northern/eastern portions of the forecast area. The upper level trough quickly skirts to the east into the weekend, with a more quasi-zonal flow pattern setting up after this. The axis of a subtle ridge moves through the area on Saturday with another weak vort-max on its leading edge. Expect temperatures to climb back above normal as 700-mb temperatures return to around +10C. The dryline position is expected to be a little further east on Saturday compared to Friday, but its too early to say exactly where this will set up. While the vort-max aloft should help kick off scattered showers and thunderstorms, this will mainly be a virga and wind threat west of the dryline. However, once these storms reach a more moisture rich environment with southerly low-level flow, strong to severe convection could be on the table once again. There is still uncertainty regarding whether this will occur in our forecast area or to the east. Sunday looks similar, just a touch warmer and a touch drier as the ensemble mean dryline position edges further to the east. Thunderstorm activity should be a little bit more limited with drier air working into the area, but we`ll have to watch the exact dryline position as areas to the east of this will once again have a greater strong to severe storm potential. Right now, it looks like this area will be to our north/east, but it`s very close. Low level westerly downslope winds should support enhanced warming over the high plains, with highs expected in the mid 80s. Some of our hotter spots (Torrington/Scottsbluff, etc.) will also have a chance at the first 90F day of the year. Probabilities for a high over 90F are around 60% for Scottsbluff, 50% for Chadron, and 40% for Sidney at this time. While 700-mb temperatures may drop briefly behind a weak shortwave Sunday night into early Monday, they should rebound quickly. Ensemble mean values over KCYS climb tack to +15C by 00z Monday. Look at another warm to hot day. Temperatures aloft, around 500-mb, will warm faster than the low levels as the more potent upper-level ridge tries to work in across the area. The warmth aloft will limit instability and thunderstorm potential on Monday, and possibly into Tuesday as well. Almost all ensemble members keep most of the area dry on Monday, and most continue this through Tuesday and Wednesday. There is good consensus around another upper level shortwave pushing across the northern Rockies in the Tuesday/Wednesday timeframe, but the chance for precipitation with this is fairly low. However, this introduces some uncertainty in temperatures, with a more southerly track leading to a stronger shot of cool air, and a more northerly track leading to the hot temperatures continuing. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1147 AM MDT Wed May 29 2024 Latest satellite loop was showing some cumulus developing over the I-80 corridor between Cheyenne and Rawlins. This looks to be the breeding ground for our showers and thunderstorms early this afternoon. This cluster of showers and thunderstorms should push east through the course of the afternoon and become more linear and stronger as they approach the Nebraska panhandle. The bulk of these storms are expected to move east-northeast around 15 to 20kts, but any stronger storms may deviate to the east-southeast and slow down a bit. The majority of the storms should affect the Nebraska panhandle between 22z-01z with some of these storms possibly becoming severe especially in the southern panhandle. A frontal boundary is expected to push through the Wyoming TAF sites after midnight and the Nebraska panhandle Thursday morning and possibly bring some lower MVFR ceilings. Later shifts will need to keep an eye on that potential. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...MB LONG TERM...MN AVIATION...REC