Expires:No;;318847 FXUS65 KCYS 242348 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 548 PM MDT Wed Apr 24 2024 .KEY MESSAGES... - Above average temperatures with isolated to scattered showers and thunderstorms continuing into this evening. Patchy dense fog expected near the I-80 corridor from Cheyenne to Sidney late tonight and early Thursday. - Scattered to numerous showers and scattered thunderstorms expected Thursday afternoon and evening, with some thunderstorms possibly becoming severe east of the I-25 corridor. Large hail and damaging wind are the primary hazards, but an isolated tornado can not be ruled out. - Localized moderate to heavy rainfall possible on Friday morning across the Nebraska Panhandle. && .SHORT TERM /THROUGH FRIDAY/... Issued at 243 PM MDT Wed Apr 24 2024 Latest radar and satellite shows numerous showers and thunderstorms that have developed early this afternoon mainly along the I-80 corridor from Rawlins through Cheyenne with leading shortwave energy ahead of the larger scale upper level trough approaching the CA coast. Marginal instability (250-500 J/kg MLCAPE) and shear (25-30 kt) are currently present over the area that have resulted in a few brief 50 dBZ cores developing quickly and then collapsing leading to gusty winds. A recent storm that passed over KCYS gusted to 41 kt. Main hazards will continue to be gusty winds and occasional lightning extending eastward into the southern NE panhandle through early this evening. Looking at another warm day on Thursday before increasing cloud cover and storm development occurs in the afternoon with the large scale trough moving into the four corners region. Lee cyclogenesis over eastern CO will enhance south/southeastward into the southern NE panhandle. SPC has portions of the CWA under a marginal risk for severe thunderstorms. Latest CAM guidance shows numerous strong thunderstorms developing mainly over the NE panhandle and portions of eastern WY with the potential for a few to turn severe, which is right on track for this time of the year. The the first Severe Thunderstorm Warning in our CWA is on average April 25. MLCAPE values around 1000 J/kg with 30-40 kt of 0-6 km shear present will be sufficient for severe storms to develop with strong low-level shear possible just north or the surface low. Areas of uncertainty include portions of extreme southeast WY and the placement of the moisture boundary impacting location of CI in the afternoon. While mean MLCAPE entering Kimball Co is around 1000 J/kg as mentioned, HREF 10th percentile values drop below 200 J/kg suggesting a potential failure mode with the lack of westward moisture transport/eastward dryline propagation with downsloping winds off the Laramie Range. Additionally, strong ascent with the approaching trough will lead to numerous storms with the potential for embedded stronger cores rather than discrete cells. Storms will likely continue through the mid- evening with upscale growth developing off the east. Headed into Friday, this deepening surface low will continue to propagate into central NE with colder air wrapping around the back side of the surface cyclone into our CWA. 700mb temps will begin to dip below 0C with wrap around moisture continuing over the NE panhandle early Friday. While temperatures will be cooler with afternoon highs dropping down into the 50s, the colder air mass behind this system does not arrive until Friday night, so ptype will likely remain as rainfall through Friday afternoon. Additionally, gusty northwest winds with the strong mslp gradient are likely. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Issued at 400 AM MDT Wed Apr 24 2024 Medium-range deterministic models and various ensembles continue to display excellent agreement w/ regard to an energetic mid and upper-level short wave over the Four Corners region early in the forecast period, pivoting around the base of larger scale upper- level troughing encompassing much of the western & central CONUS late in the week. The GFS/ECM/GEM all suggest similar timing and placement of this disturbance ejecting north and east across the southern and central high plains on Saturday, giving way to very strong/robust lee cyclogenesis over southeastern Colorado with a sub-998 millibar surface cyclone tracking across central/eastern Kansas from Saturday night through Sunday. Widespread stratiform precipitation is likely to develop on the back side of this low, as an impressive TROWAL & deformation axis evolves by mid-day on Saturday. Latest ensemble cluster analysis indicates substantial model agreement with QPF exceeding 0.75 inch across a large part of the CWA from Saturday through early Sunday. Perhaps this will come as no surprise, given widespread PWs of 0.5+ inch, near the 90th percentile of climatological norms for late April and early May. Overall, this appears to be a fairly warm system w/ most of the lower elevation areas likely to see rain. However, as 700-mb temperatures fall to near -2 to -4 deg C, elevations above about 7500 feet could see notable accumulations of heavy and wet snow, including the I-80 Summit between Laramie & Cheyenne. The Snowy/ Sierra Madre ranges could see over 12 inches for the event. Will need to watch areas into the I-25 corridor in central Laramie Co as the CAD signature along the front range could suggest current models are too warm w/ their thermal profiles. Although it seems to be an unlikely scenario at this time, a change over from rain to snow cannot be entirely ruled out as far east as Cheyenne. No headlines at this time given we are still looking at 5th period/ beyond. Temperatures should be seasonably cool due to the clouds and precipitation, so have trended toward lower %iles of the NBM spectrum through Sunday. Temperatures quickly rebound by Monday/ Tuesday w/ the return of upper-level ridging. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/... Issued at 540 PM MDT Wed Apr 24 2024 The main aviation concerns with this package are ongoing shower and thunderstorm activity, followed by low CIGs/fog tonight into Thursday morning for some High Plains terminals, and then the next round of showers and thunderstorms Thursday afternoon. Scattered showers and thunderstorms continue to move across the area this evening, though activity is beginning to show signs of weakening. KSNY and KLAR will see thunderstorms close by in the next hour. After that, thunder is expected to become more isolated but can't be ruled out through the end of activity around 06z. The most likely impact of this activity is gusty and/or erratic winds. Gusty SSE winds over the High Plains will continue through the evening, before waning after midnight. Increased low-level moisture will lead to a deck of low clouds creeping in from the south. This is expected to produce a period of MVFR to LIFR conditions at KCYS, KSNY, KBFF, and KAIA. The KCYS terminal may be right on the edge of the cloud deck, but also has the potential to see dense fog if inside the fog area. This should manifest as low CIGs for Nebraska terminals, though some fog is still possible at KSNY. This should break up between 15z and 18z Thursday. Then, the next round of showers and thunderstorms will begin to develop after about 21z Thursday with lightning, brief downpours, gusty winds, and hail possible. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...MB LONG TERM...CLH AVIATION...MN