Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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270 FXUS65 KCYS 271724 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1124 AM MDT Thu Jun 27 2024 .KEY MESSAGES... - Scattered showers and thunderstorms are expected across much of southeast Wyoming and western Nebraska this afternoon and evening. A conditional threat for severe thunderstorms will exist across extreme southeast Wyoming and western Nebraska, with large hail and damaging winds the primary hazards along with the potential for an isolated tornado. - A final round of strong to severe thunderstorms is expected Friday as a cold front sweeps across the CWA. At this time, the best chance for convection appears to be along and southeast of a line from Cheyenne to Alliance. && .SHORT TERM /THROUGH FRIDAY/... Issued at 215 AM MDT Thu Jun 27 2024 The primary focus over the next 12 to 24 hours continues to be a threat for strong to severe thunderstorms across southeast Wyo & the western Neb Panhandle during the mid/late afternoon into the early evening hours. SPC has highlighted much of the western Neb Panhandle in an ENH Risk for severe storms later today. However, this is not without uncertainty as CAMs seem to be having a very tough time handling the boundary layer moisture with significant HREF spread noted in surface dew points, as much as 10 to 15 deg F differences between 21z and 00z. This is likely the reason for such discrepancies among the CAMs with convective coverage, with the HRRR suggesting minimal activity today while the NAM Nest is rather aggressive w/ CI and multiple UH Tracks. Tds from the 06z NAM are around 5-7 deg F higher than the HRRR, contributing to a more favorable thermal profile for robust/deep convection by mid afternoon per forecast soundings. HREF probabilities are hinting at around a 40% chance of dew points remaining above 55 deg F as afternoon mixing commences this afternoon across the western Neb Panhandle w/ higher probs just east of the CWA. If we can manage to keep dew points in the mid to upper 50s, a conditional threat for severe thunderstorms will exist with CAPEs up to 1500 J/kg & 35 knot effective bulk shear favorable for supercells capable of very large hail, damaging winds, and perhaps a tornado. The best chance of this (right now) appears to be in western Nebraska for the time being, though if the dryline mixes less aggressively we could see some southeast Wyoming zones get in on the action. The big failure mode is a dryline which mixes rapidly eastward, with moisture quality effectively being cut off before CI occurs. Another round of strong/severe storms will be possible Friday as the axis of the shallow northern plains upper-level trough moves to the east. The primary focus here should be along & south east of a line from Cheyenne to Alliance, based on expected positions of the cold front. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Issued at 215 AM MDT Thu Jun 27 2024 Daily afternoon shower and thunderstorm chances continue through the long term forecast period. Friday evening, upper-level troughing will still be present across the region with a subtle 500mb shortwave pushing through. Most of the convection that forms will likely be earlier in the afternoon Friday, rather than Friday evening. However, there is a Slight Risk of severe weather for portions of the Panhandle. Upper-level troughing will slowly move out of the area Friday night into early Saturday morning as an upper- level ridge builds over the Intermountain West. With the cold front moving across the region Friday, Saturday will be much cooler as 700mb temperatures drop into the 6-8C range east of the Laramie Range, but west of the Laramie Range looks to be warmer as the upper- level ridge slowly moves closer to the region. Highs on Saturday will be in the mid-70s to low-80s east of the Laramie Range and mid- 70s to mid-80s west of the Laramie Range. Despite the strengthening ridge overhead, some 500mb vorticity maxima will traverse underneath, resulting in precipitation chances Saturday and most days throughout the long term forecast. The upper-level ridge will be at its maximum strength overhead on Sunday, leading to rapidly warming 700mb temperatures into the 15- 17C range. Surface temperatures will increase into the upper-80s to mid-90s east of the Laramie Range and upper-80s west, leading to a very warm end to the weekend. A slightly cooler and more active pattern returns for the work week as the upper-level ridge moves off to the east and troughing pushes in. Disagreement begins to appear in the long range models at this time. The GFS suggests zonal flow moving in after the trough pushes through, while the ECMWF keeps troughing overhead for Monday and Tuesday, with zonal flow arriving Wednesday. Disagreement continues throughout the remaining long term, but temperatures are likely to remain in the 80s and 90s, with warmer temperatures arrive for the 4th of July. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1123 AM MDT Thu Jun 27 2024 Scattered showers and thunderstorms are expected this afternoon and into the evening. Severe thunderstorms are possible east of the Laramie Range, mainly between 20Z and 01Z. Storms in these areas could contain strong winds with gusts over 60 MPH and large hail. The severe threat and storms in general will taper off in the Nebraska panhandle this evening. Rain showers, with the potential for heavy rain, will continue around southeast Wyoming terminals through roughly 06Z tonight. Any storm passing over a terminal in these areas will have the potential to reduce visibility. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...CLH LONG TERM...AM AVIATION...SF