FXUS65 KCYS 232321 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 521 PM MDT Tue Apr 23 2024 .KEY MESSAGES... - Expect warmer temperatures and an increase in shower and thunderstorm coverage for Wednesday. - A progressive disturbance aloft and at the surface will help produce scattered to numerous showers and scattered thunderstorms Thursday afternoon, with some thunderstorms possibly becoming severe east of a Cheyenne to Torrington to Alliance line. - Cooler, wet and unsettled weather is likely for Friday and the weekend. && .SHORT TERM /THROUGH THURSDAY/... Issued at 255 PM MDT Tue Apr 23 2024 Tonight...Satellite imagery and WSR-88D reflectivities indicating showers developing across southern portions of Carbon and Albany Counties as a weak disturbance moves across in the low amplitude ridge aloft. Will paint isolated to widely scattered showers and a few thunderstorms this evening south of a Rawlins to Wheatland to Scottsbluff line. A few showers may linger past midnight along and just north of the Colorado state line. Not as cold tonight as the airmass slowly moderates. Wednesday...Expecting an increase in afternoon and evening shower and thunderstorm coverage with an attendant increase in low and mid level moisture availability. Low level convergence on southeast winds will aid in lift for precipitation development. Warmer temperatures with 700 mb temperatures near 7 Celsius. Wednesday night...As the next energetic shortwave trough aloft moves into southern Nevada, and the flow aloft backs to southwest, the models indicate the low level southeast winds will help low level moisture increase after midnight across our eastern counties, and especially for far southeast Wyoming and the southern Nebraska Panhandle, where boundary layer progs suggest a decent chance for fog and low cloud development, so will add fog to our gridded forecast through the late night hours. Thursday...The negatively tilted and potent shortwave trough aloft moves into central Colorado in the afternoon, with low level convergence maximized along a surface trough from northeast Wyoming to northeast Colorado. Although bulk shear values are not too impressive, steep mid level lapse rates and adequate low and mid level moisture along with mesoscale and synoptic lift suggests scattered to numerous showers, and scattered thunderstorm development in the afternoon, especially along and east of Interstate 25. The Storm Prediction Center forecasts a marginal risk of severe thunderstorms east of a Cheyenne to Torrington to Alliance line, and thus we have added the mention of a few severe storms to the gridded forecast. Maximum temperatures will be somewhat limited by the widespread cloud cover and precipitation coverage. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Issued at 255 PM MDT Tue Apr 23 2024 Unsettled weather is expected to continue through the weekend, but another warming trend will begin early next week. Two potent shortwaves will eject out of a longwave trough positioned over the Western US between Friday and Sunday, keeping daily chances for showers, a few thunderstorms, and high elevation snow in the forecast each day. Beginning on Thursday night, the first upper level shortwave trough axis will pass across the area. Initially this disturbance will cross the Rockies as an open wave, but most guidance has the upper level low rapidly deepening as it moves into the High Plains overnight. This will allow the system to establish some wrap-around precipitation as the night progresses, even though this will not be present initially. As a result, the best chance for more appreciable stratiform precipitation in the wrap-around area late Thursday night into Friday will be further north and east in the forecast area. This will be a fairly warm system, with limited access to cold air. Thus, expecting rain for most areas, but areas above 9000 ft are still expected to see snow, and snow levels could creep down towards 8000 feet Friday morning. As with the last several events like this one, expect a period of gusty northwest winds in the wake of this system across the area on Friday. Most ensemble members have 700-mb winds around 40 to 50 knots along the I- 80 corridor from the summit east towards Sidney, supporting confidence is a windy day. At this time, it looks like winds should remain below High Wind criteria for the plains, but the southern Laramie range and foothills could be closer. In-house guidance based on the GFS has been fairly consistent in showing about a 40-50% chance of high winds for this area beginning early Friday morning and continuing through midday. Expect a cooler day Friday as well, but highs will still be fairly close to average or just a few degrees below. The first low departs Friday night, leaving our area sandwiched in between the two lobes of upper level vorticity on Saturday. During this period, a surface high pressure is expected to slide down the east side of the Rockies and turn the flow to northeasterly over the High Plains. The cold air is again fairly lackluster here, with this front only knocking 700-mb temperatures down to -2 to 0C or so. Plenty of cloud cover will lead to cooler highs on Saturday, probably struggling to get out of the 40s to mid 50s. As the second shortwave begins to traverse across the Rockies, look for another round of cyclogenesis across Colorado Saturday afternoon into the overnight hours. This is expected to develop further south than the first round, and also close off and develop a solid wrap-around flow a little earlier in its evolution. There are some timing differences between models, but after a brief lull, precipitation is expected to fill back in sometime between the late morning hours and sunset, with the highest confidence in widespread precipitation occurring overnight. Subtle differences also still exist in the exact track of the upper level low, which lead to disparate rainfall amounts across the I-80 corridor. It's a little early to get into rainfall amounts, but high totals are favored further south and east in the forecast area for the second event. Precipitation type should be mainly rain once again, but this system will have a little more cold air than the first. With 700-mb temperatures dropping closer to -4C by sunrise Sunday, snow is expected to at least mix in above 6500 ft, and we could see some snow down to about 5000 ft. For now, just introduced a mention of rain/snow mix for the 5000 to 6500 ft elevation range, though confidence in any snow impacts here is low. Winter weather headlines could be needed again for the mountains and I-80 summit area. Look for a gradual end to precipitation and clearing Sunday and Sunday night with another cool day expected. Once the second upper level low departs, we abruptly return to a zonal flow pattern by Monday, allowing temperatures to trend back above normal. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 517 PM MDT Tue Apr 23 2024 VFR conditions expected through the 00Z TAF period. Primary aviation concern will be showers developing along I-80 this evening and potentially impacting KCYS, KLAR, and KRWL through the evening hours. A second band of showers is expected to develop in western Nebraska. Included VCSH for most western Nebraska sites and all Wyoming sites. Winds expected to increase tomorrow morning through the remaining TAF period. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...RUBIN LONG TERM...MN AVIATION...AM