Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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273 FXUS65 KCYS 251128 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 528 AM MDT Thu Apr 25 2024 .KEY MESSAGES... - Scattered to numerous showers and thunderstorms are expected across southeast Wyoming and the western Nebraska Panhandle today and tonight. A few storms may be strong to locally severe, especially east of I-25. The primary hazards will be large hail and damaging wind gusts, although an isolated tornado or two will be possible. The potential will exist for cold-core funnel clouds as well. - An extremely active weather pattern persists through the weekend with widespread stratiform rain w/ embedded thunder likely for most areas from Friday through Sunday. - Accumulating snow is expected at elevations above 8000 feet, with the potential for 12+ inches in the Snowy and Sierra Madre Ranges from Friday through Sunday. The I-80 Summit between Laramie and Cheyenne will be very close to the main transition zone between rain and snow. Travel impacts remain possible, mainly from Friday night through Saturday. && .SHORT TERM /THROUGH SATURDAY/... Issued at 402 AM MDT Thu Apr 25 2024 A highly complex & extremely active mid & upper level pattern is expected to continue to evolve across the western and central US over the next 24 to 48 hours, giving way to chances for multiple hazards including severe convective storms, significant snowfall accumulations above 7500-8000 feet elevation, as well as chances for locally heavy rainfall for the lower elevations. The overall upper-level pattern will remain characterized by expansive long- wave troughing encompassing much of the western & central CONUS, with two distinct pieces of energy pivoting across the 4 corners region before ejecting northeastward across the southern/central high plains. Overall, models are in excellent agreement with the overall evolution of the pattern. However, there are a number of notable complexities with potential outcomes highly sensitive to the precise timing of disturbances and resulting interactions of boundaries which eventually take place. Nonetheless, it is quite likely that the majority of the CWA will see some sort of impact from these systems. The first notable short-wave disturbance will lift to the north- east across the 4 corners this afternoon, contributing to strong lee cyclogenesis over eastern Colorado with a sub-990-mb surface low expected to develop by 21z. The resulting southeasterly low- level flow should provide a sufficient fetch of moisture to help maintain appreciable surface dew points in the lower 50s, mainly across the western Nebraska Panhandle. The western extent of the deeper moisture remains highly uncertain with a sharp cut-off in the spread of HREF members along/southwest of a BFF-IBM line for progged dew points indicating uncertainty regarding the eastward progression of the dry line. The 10th %ile of the HREF ensembles are in the mid 40s to the east of this line, but quickly drop to the mid/upper 20s to the west. This may play a key role in storm severity today w/ better MLCAPEs (1000-1500 J/kg) possibly being focused more across our far eastern zones. Even so, there should be good instability today as thermal profiles rapidly cool later this afternoon as the mid-level cold pool approaches. Scattered/ numerous showers and thunderstorms should develop across western areas by early afternoon, with a slightly delayed initiation for the high plains closer to 21-00z given substantial inhibition w/ the initially warm, capped air mass aloft. High-res models would suggest initiation occurring in the vicinity of the dryline near the WY/NE border by 00z w/ numerous clusters developing through- out the evening. Vertical shear will be more than sufficient for organized, potentially rotating storms capable of large hail and perhaps locally damaging wind gusts. The last several iterations of the HRRR and NAMNest have suggested a few narrow UH tracks as well, suggesting potential for a low-topped supercell or two and the risk for a weak tornado or two. As thermal profiles cool, we would not be surprised to see at least several cold-core funnels develop in the early evening w/ numerous storm mergers likely to be taking place during this time. Dry-slotting on the back side of the rapidly maturing cyclone is expected to limit convective coverage across western zones after 03z, with the primary focus shifting toward stratiform rain and/ or embedded convection over the western NE Panhandle through mid day Friday. Brief short-wave dirty ridging should develop as the first low departs. A weak, slow-moving disturbance will uncercut this ridge over central Wyoming, resulting in a burst of notable 700-mb warm air advection across Carbon & Albany counties during the afternoon and evening hours on Friday. As such, rain & high- elevation snow should quickly re-develop as the next low deepens and pivots northeast across the 4 corners from Friday night thru Saturday. Yet again, lee cyclogenesis will result in substantial wrap-around moisture in the TROWAL/deformation axis on the back- side of a rapidly deepening cyclone over central KS. As a result we should see deep/moist northeasterly low-level upslope flow as well as favorable dynamics supporting widespread stratiform rain and embedded convection over much of the CWA. PWATs of 0.5+ inch will be near the 90th percentile of climatology, so would expect some pretty impressive rainfall totals through Sunday. Localized areas could see over 1.5 inches of rain over the next few days! High-elevation snow will also be likely, as 700-hpa temperatures fall to -2 to -4 deg C. A foot or more of accumulation will be a good possibility for both the Snowy and Sierra Madre ranges with this second wave, with more significant uncertainty below around 8000 feet elevation and potential impacts over the Interstate 80 Summit between Laramie and Cheyenne. Models continue to trend on the cooler side for Friday Night & Saturday, validating previous concerns of cold air damming along the front range favoring low- tier percentiles of the model spectrum for temperatures. Decided to populate snow levels with the 25th %ile of the NBM, which may bring a brief rain/snow mix into Cheyenne but would restrict the accumulations to the higher terrain to the west. The Summit will be right near the transitional bubble, with the worst-case being around 6-8 inches of heavy and wet snow. Ensemble means are near 2-3 inches using a 10:1 ratio. Will continue to hold off for now on any headlines given lingering uncertainty. Daytime highs will likely struggle to reach the middle 40s for many areas Saturday. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... Issued at 402 AM MDT Thu Apr 25 2024 We will have an active weather pattern continue for the extended forecast package. Near average temperatures for the weekend are forecast, and gradually increasing through the week to be approximately 5-15 degrees above climatological averages to round out the last few days of April and into early May. At this time of inspection, snow showers across the higher elevations will wind down on Sunday and Monday, with rain showers and thunderstorms anticipated daily for the entire cwa through the end of next week. Saturday night into early Sunday, we will have a slow-moving upper level low slowly propagating to the east-northeast, ultimately ejecting out toward the Upper Midwest and Great Lakes region by late Sunday. As this negatively tilted upper level system with an attendant mid-latitude surface low propagates away from our region, we will have wrap-around moisture coupled with CAA. 700mb temperatures ranging from 0C to -5C along with scattered rain showers in the lower elevations, and accumulation snowfall in the higher terrain will slowly dissipate from Saturday night to late Sunday afternoon. Isolated thundershowers are also possible, but sub- severe. Areas of highest confidence for additional rainfall accumulations in the lower terrain will be along and east of I-25 toward western Nebraska. Much needed rainfall is anticipated for these areas. The water faucet will be turned off for only a brief period as we transition to a quiet weather setup for approximately 24 hours. A shortwave trough will arrive from the Pacific Northwest by Monday evening, bringing a renewed chance for showers and isolated thunderstorms. This weak disturbance is anticipated to eject downstream toward the Northern Plains and Great Lakes region throughout the day on Tuesday, limiting our chances of rainfall from that. Tuesday is advertised by model guidance to transition to a more quasi-zonal flow regime aloft, with weak shortwave disturbances embedded within the H5 and H7 flow aloft. Diurnal convection is signaled to be present as mid-levels will adiabatically cool from a passing shortwave trough to our north, increasing the low-level and mid-level atmosphere lapse rates. Not expecting severe weather from this weak threshold of a disturbance, but lingering showers and thunderstorms could persist overnight into early Wednesday morning. Wednesday and Thursday will see an amplification of an upper level ridge, with it originating across the Mexican Plateau, and advecting warmer are northward across the Intermountain West. Weak shortwave disturbances are forecast to propagate off a much larger longwave trough from the Pacific Northwest on Wednesday, bringing another opportunity for showers and thunderstorms during the peak heating hours of the day. Thursday will have the aforementioned upper level ridge axis positioned over the Central Plains. The potent longwave trough will elongate and intensify across the western CONUS. This trough is modeled to become negatively tilted by Thursday of next week, with diffluent flow being coupled with moisture and instability across the Central Rockies. We will need to pay attention to this trend as it may be our next opportunity for stronger, organized convection across the cwa to round out the week. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1102 PM MDT Wed Apr 24 2024 Shower and thunderstorm activity is waning this evening, but we will still have a few more hours with the possibility for a brief downpour and a few lightning strikes. Southerly flow over the High Plains will usher in low-level moisture through the morning hours. Models have backed off slightly on the intensity of fog/low CIGs through Thursday morning, but a period of MVFR to LIFR conditions is still likely at KBFF, KAIA, KSNY, and KCYS. The latter two are the most likely to see periods of IFR, while MVFR is favored for the former. This should clear by the late morning hours as southeast winds increase again. Another round of shower and thunderstorm activity will develop after about 21z Thursday and gradually increase in coverage through the evening hours. These storms will have the potential to produce lighting, brief downpours, gusty winds, and hail possible. Expect a wind shift during the evening hours. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...CLH LONG TERM...BW AVIATION...MN