Area Forecast Discussion
Issued by NWS Cheyenne, WY

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680 FXUS65 KCYS 132314 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 514 PM MDT Thu Jun 13 2024 .KEY MESSAGES... - Scattered to numerous showers and thunderstorms with small hail and gusty winds continue this afternoon into the early evening hours, especially along the I-80 corridor from Laramie to Kimball. - Strong to severe thunderstorms are possible across the high plains of southeast Wyoming and the western Nebraska Panhandle on Friday. Torrential rainfall and the potential for flash flooding will be the primary concern. However, large hail and damaging winds will be possible with any storms developing early in the afternoon prior to storm mergers. && .SHORT TERM /THROUGH FRIDAY NIGHT/... Issued at 315 PM MDT Thu Jun 13 2024 Latest satellite imagery shows scattered showers and thunderstorms over southeast WY extending into the southern NE panhandle. Latest SPC mesoanalysis shows around 500 J/kg of MLCAPE along and along the foothills of the South Laramie Range with a frontal boundary extending southeast into portions of CO. Storms will continue into this evening as a shortwave continues to pass along north of the upper level ridge in the southern Rockies. However, coverage should remain fairly isolated with storms weakening as they approach the NE border where greater convective inhibition is present. Additional shower development will be possible early Friday morning. Looking at increasing storm coverage headed into Friday afternoon associated with the previously closed low off the southern CA coast beginning to move across the central Rockies. Strong southerly moisture advection out ahead of the trough will support upper 50 to low 60 degree dew points for portions of the NE panhandle with moist vertical profiles and PWs near climatological 99th percentile values. This will lead to SBCAPE values exceeding 2000 J/kg east of the WY/NE border as indicated by the latest HREF guidance with 40 kt of 0-6 km shear. However, previous shifts noted 700mb WAA resulting in the quite a bit of capping that needs to be overcome for storms. While this may limit discrete convection early in the afternoon, larger scale lift with the approaching upper level trough will be supportive of widespread precipitation developing. Any discrete storms that could form early in the afternoon will be capable of producing large hail before upscale growth begins. It is important to note that latest model trends have been suggesting the best support may remain over eastern CO, as the upper level trough tries to dig farther south, potentially leaving southeastern portions of the CWA within the stratiform region of the convection in CO. While strong winds and hail will be possible with any stronger cells, heavy rainfall continues to be the main hazard with WPC highlighting the I-80 corridor east of Cheyenne with a marginal risk for excessive rainfall through early Friday evening. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Issued at 303 AM MDT Thu Jun 13 2024 The long term forecast will start off with relatively tranquil weather through the weekend with increasing chances of showers and thunderstorms early next week. Throughout the period, general southwesterly flow will be in place aloft, with stronger flow to the north and much weaker flow to the south. The outcome of Friday/Friday night`s convection will likely have a large impact on the forecast for Saturday, as a larger, more robust MCS/thunderstorm complex will likely clear a large majority of the surface moisture in place well to the east of southeast Wyoming/NE Panhandle. Global models likely do not yet have a handle on these impacts, however generally speaking surface moisture is currently progged to be much lower on Saturday afternoon. Still, with a bit of faster southwest flow aloft impinging on the CWA from the north, a few isolated high- based showers will be possible in the Laramie Range and through east- central Wyoming. A surface frontal boundary evident in both the GFS and ECMWF will dive southward on Sunday morning, squeezing surface moisture into a narrow band somewhere over the Nebraska Panhandle and perhaps as far west as southeast Wyoming. Typically, these post- frontal moisture-squeeze setups can produce severe weather in the high plains. However, we will once again be struggling to find much in the way of favorable wind shear as the bulk of faster flow aloft will remain to the north and west. Isolated multicell and outflow- dominant clusters will likely be the favored storm mode wherever the moisture corridor sets up given strong surface heating, inverted V soundings, and warm air in the mid-layers of soundings. By Monday and Tuesday, precipitation and thunderstorm chances look a bit more interesting across the CWA as a more robust trough attempts to swing through the northern Rockies embedded in the broad southwesterly flow regime. This system has seen quite a bit of spatiotemporal variability in the past few days of guidance cycles. As of early this morning, the main trough axis appears to be just a bit too far west to result in a more widespread thunderstorm threat on Monday afternoon. Still, moisture return in south southeasterly flow across the high plains is noted. Additionally on Monday afternoon into Tuesday, southwest surface flow west of the Laramie Range will also increase dramatically. Will have to watch the Monday evening and especially Tuesday timeframe for possible high winds over south-central Wyoming as this system approaches. As far as the thunderstorm threat goes, Tuesday looks a bit more favorable given the closer proximity of faster flow aloft associated with this trough. However, model guidance has shifted northward with the position of the center of the low, keeping cooler temperatures aloft and better wind shear/thermodynamic profiles farther north into northeast Wyoming and the northern Plains states. Again, with a plentiful reserve of surface moisture just off to the east and a potent trough in the vicinity, we will have to keep a close eye on this system over the next few days. Any shift farther south with the main low pressure system would certainly result in a more significant severe weather threat for eastern Wyoming and western Nebraska. Overall, temperatures will run above normal this weekend, and gradually cool off early next week. Forecast confidence is high in the temperature forecast, but only moderate with the precipitation forecast given the variable nature of model guidance from Monday onward. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
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Issued at 511 PM MDT Thu Jun 13 2024 VFR conditions expected through the 00Z TAF period. Primary aviation concerns this evening will be isolated showers producing gusty and erratic winds. KRWL gusted to 58mph near one of these showers, so gusty, erratic winds will be possible as showers move over or near the terminals. Winds expected to increase tomorrow, with showers and thunderstorms expected in the afternoon and evening hours, some of which may be severe.
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&& .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...MB LONG TERM...MAC AVIATION...AM