FXUS65 KCYS 241004 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 404 AM MDT Wed Apr 24 2024 .KEY MESSAGES... - Above average temperatures with isolated to scattered showers and thunderstorms this afternoon and evening. Patchy dense fog expected near the I-80 corridor from Cheyenne, WY to Sidney, NE 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 across the area due to persistent rain showers. && .SHORT TERM /THROUGH FRIDAY/... Issued at 400 AM MDT Wed Apr 24 2024 An active weather forecast pattern is taking shape across the area in the upcoming period of the short term forecast package. Our region will be under the influence of an amplifying upper level ridge today. This will lead to a quick surge in daytime highs compared to yesterday. We will see partly cloudy skies once the light rain showers push further to the east later this morning. Expect high temperatures approximately 7-12 degrees warmer than yesterday, topping out in the upper 60s to upper 70s for most of the lower elevations. As the ridge amplifies, and we have subsequent WAA occurring in the lower levels of the atmosphere today, shortwave disturbances embedded within the upper flow aloft will propagate across the region. Convective temperatures will be achieved for several areas, and light rain showers and isolated thunderstorms will be the result by the late afternoon hours. The rain shower activity will move from west to east. At this time of inspection, our convection today looks disorganized. The main weather event to discuss for the short term forecast will begin to take shape early Thursday morning. An upper level longwave trough will propagate over the Four Corners of the desert southwest by Thursday morning. Further downstream, a deep fetch of moisture will advect northward from the Gulf of Mexico and southern CONUS early Thursday toward the Central Plains, courtesy of a 30-40 knot nighttime LLJ. The weather setup for our cwa east of the Laramie Range will include a deep swath of upslope flow. Dense fog and low-level cloud ceilings are expected to ensue, which may evolve into a short-fused Dense Fog headline for Wednesday night and Thursday morning. PWATs are expected to surge to 90-99th percentile per climatological sounding data on Thursday/Thursday night (between 0.5 and 0.8 inches). This will happen due to a surface low over Colorado that is anticipated to undergo rapid lee- side cyclogenesis. The rapid deepening of the surface low over northeast CO will approach near-bombogenesis thresholds: 15-18mb decrease in just over 12 hours on Thursday. The aforementioned upper level longwave trough will eject toward the Intermountain West/Central Plains during this same time period, becoming negatively tilted. Model guidance is beginning to come into solid agreement for convective initiation to occur in several locations, including our cwa. Steep mid-level lapse rates, SBCAPE and MUCAPE values in excess of 1000-1500 J/kg for areas east of the Laramie Range are progged, with even some members showing readings as high as 2000-2500 J/kg by 0Z Friday along the WY/NE state line. This weather system will shift from a mesoscale to synoptic-induced convection Thursday evening and overnight into Friday morning. Before it does so, the potential for a couple of discrete supercells exists. However, there are a couple of discrepancies to note. Model soundings for southeast Wyoming show DCAPE values of 750-1000 J/kg, with slightly less readings in the NE Panhandle. 0-6km effective shear parameters are not the strongest, but there will be a small and narrow corridor east of I-25 near the WY/NE state line that could be just the right amount thanks in part to lobes of vorticity maximums propagating through the region. After taking a look at sounding analogs, there is a slight concern for a tornado or two to occur. The primary reason for this is because of isolated pockets of storm-relative helicity in excess of 200 m2/s2, and very low cloud bases near or slightly below 1 km being possible east of I-25. Overall, damaging straight line winds in excess of 60-70mph, hail between one and two inches in diameter, and an isolated tornado can not be ruled out. This is further evidenced by the Storm Prediction Center expanding the Marginal Risk for isolated severe weather for the majority of our cwa east of I-25 during the Day 2 overnight forecast update. Thursday night, the aforementioned upper level trough is anticipated to become cut-off or "stacked". The attendant 700mb low and surface low will be semi-stationary in approximately in the same position for several hours. This will create a narrow window of elevated to potentially strong wind gusts along the Interstate 80 corridor east of the Laramie Range. At this time of inspection, it does not appear we will need any high wind headlines. Fortunately, temperatures will be much warmer in comparison to a previous weather system earlier this month that resulted in widespread wind gusts near or above 60mph for several hours. We will see several rounds of scattered to numerous rain showers and thunderstorms from Thursday night into Friday behind a cold front passage. The net result will be cool, cloudy and rainy conditions through Friday afternoon. Areas of highest focus for moderate to heavier rainfall totals will be across the Nebraska Panhandle. This will assist with the spring green-up across the area as several locations are anticipated to see a range of 0.1 to 0.5+ inches of rainfall from Thursday afternoon to Friday afternoon. The surface low will slowly propagate to the east by Friday evening, further away from our cwa. However, the break in between precipitation will be short-lived as the next weather disturbance will be on its way by this weekend. Stay tuned for updates on that in the long-term discussion. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... 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 /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1102 PM MDT Tue Apr 23 2024 VFR conditions expected through the 06Z TAF period. Primary aviation concern will be lingering shower activity along I-80 overnight. Intermittent showers have been impacting both KLAR and KCYS, with VCSH near KRWL. Rain showers expected to continue around KLAR and KCYS overnight. Additionally, could see some LLWS at KBFF from 09- 14Z tonight. Winds expected to increase tomorrow morning and continue through the remaining TAF period. Showers move towards the Nebraska terminals late in the TAF period. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...BW LONG TERM...CLH AVIATION...AM