Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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056 FXUS65 KCYS 311751 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1151 AM MDT Fri May 31 2024 .KEY MESSAGES... - Daily chances for isolated to scattered showers and thunderstorms continue through the weekend. A few storms may have the potential to become strong to severe each day. - Warm and dry weather is expected most of next week, aside from an uncertain potential for a cold front which could bring some shower/storm activity on Tuesday or Wednesday. && .UPDATE... Issued at 632 AM MDT Fri May 31 2024 The stratus deck intersecting with the higher terrain of the Southern Laramie Range is leading to areas of dense fog near the I-80 summit. Opted to issue a Dense Fog Advisory until 9AM. This should start to break up by then, but can`t rule out it continuing a little later into the morning. && .DISCUSSION... Issued at 400 AM MDT Fri May 31 2024 It`s an active morning across the area with an unusual round of nocturnal thunderstorms pushing across the southern Nebraska panhandle. Low level moisture wrapping around the south side of a surface high near the MT/SD/ND triple point has been enough to bring in a narrow tongue of elevated instability into the southern panhandle. Overrunning isentropic lift over the surface frontal boundary associated with the leading edge of a shortwave trough has provided enough lift to get storms going in this area. The environment is marginal for severe weather, but can`t rule out another storm briefly producing severe hail or a quick wind gust. These storms are producing near constant lightning though, and a good dose of moderate to heavy rainfall. So far, Kimball has received over 0.80 inches of rain. As of now, these are progressive enough to reduce flooding concerns, but there is the potential for training storms which would increase the flash flood potential. Showers and storms should clear the area by mid morning. Elsewhere, a deck of low clouds is westward behind the surface cold front. This may lead to some areas of fog through mid morning near the I-80 summit and parts of Laramie county eastward along the Cheyenne ridge. Areas along and east of the Laramie range can expect a cooler day today in the wake of the overnight cold front. Low stratus will also keep things considerably cooler especially this morning. Expect clouds to start to break up from west to east starting late in the morning through the afternoon hours. The rate of clearing will set the stage for a conditional thunderstorm threat this afternoon. Coverage is not expected to be very high, but if the inversion erodes enough to break the cap, storms will have a decent amount of instability to tap into in the presence of sufficient wind shear. If this occurs, it will probably be around the Cheyenne area towards the southern Laramie range once again, but once formed may persist eastward along the I-80 corridor. Later this evening, a weak ripple in the flow aloft is expected to drive a brief period of isentropic lift once again, this time focused more on the northern NE panhandle. HiRes models are depicting a few showers and thunderstorms developing in a narrow tongue of instability after midnight and lasting through the early morning. As a result, expanded some low end PoPs for Sioux, Dawes, and Box Butte counties late tonight. The flow aloft transitions to more zonal flow this weekend, but a broad shortwave is expected to traverse across the west. A vorticity maximum on its leading edge will provide some enhanced lift Saturday afternoon. The main uncertainty for the thunderstorm threat is the dryline position once again. Some models, such as the GFS and HRRR, push this feature eastward rapidly, bringing dry air across the area. This would mean storms would mostly just be a gusty wind threat with inverted-v soundings in place. However, a slower departure of the surface high to the north may keep southeasterly moist flow further west, which would mean a chance for more organized convection. It`s too early to say which will end up verifying, but will need to monitor model trends over the next few cycles. Rinse and repeat for Sunday as the main axis of the upper level trough moves through our area during the afternoon. Again, the uncertainty is in the dryline position, but a few strong to severe storms could occur east of the dryline, with gusty winds to the west. Behind this shortwave, a stout but potent ridge starts to grow over the western CONUS. This ridge will bring fairly significant warming aloft, stabilizing the atmosphere into the early part of next week. Monday thus looks dry and warm to hot. The midweek trough will be the main forecast challenge for next week as another shortwave swings over the top of the ridge. The GFS continues to show a shallow and progressive trough that gives way back to the dominant ridge by Wednesday. The ECMWF has held with its solution of a much deeper and slower trough that would bring some shower and thunderstorm chances back into the picture for Tuesday and Wednesday. Overall, ensemble guidance has trended slightly towards the ECMWF solution over the last 24 hours, with about 45% of members now showing a much more pronounced cool down on Tuesday and Wednesday. The inter-quartile range in ensemble 700-mb temperature forecasts for Wednesday morning ranges from 3 to 10C, which is a pretty large spread for this lead time on the cusp of summer. The remaining 55% of members is mainly driven by the GEFS, which shows the ridge rebounding quickly, leading to just a brief pause in the hot temperatures. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 1151 AM MDT Thu May 31 2024 Westerly flow aloft will prevail. Wyoming TAFS...At Rawlins and Laramie, scattered clouds around 10000 feet will occur, otherwise clear skies will prevail. Winds will gust to 27 knots until 01Z, and to 25 knots at Rawlins after 15Z Saturday. At Cheyenne, ceilings near 1500 feet will improve to 5000 to 8000 feet through 12Z, then skies will be clear. Nebraska TAFS...At Chadron and Alliance, ceilings will range from 3500 to 7000 feet through the afternoon, then scattered clouds will be around 7000 feet until 06Z, then fog will reduce visibilities to 2 miles with ceilings near 1500 feet until 15Z, then ceilings will be around 3500 feet. At Scottsbluff and Sidney, ceilings will improve from 2500 feet to 3500 feet early this afternoon, then scattered to broken clouds from 3500 to 5000 feet will occur this evening, before fog reduces visibilities to 2 miles and ceilings to near 1500 feet from 06Z to 15Z, then scattered to broken clouds around 3500 feet will occur. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ UPDATE...MN DISCUSSION...MN AVIATION...Rubin