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Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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888 FXUS65 KCYS 172108 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 308 PM MDT Sat Aug 17 2024 .KEY MESSAGES... - Hot temperatures will persist through the weekend with widespread daytime highs in the 90s for areas along and east of I-25. - Daily chances for showers and thunderstorms will exist across the region over the next several days as several shots of monsoonal moisture ride along the western periphery of the upper level ridge. && .SHORT TERM /THROUGH MONDAY NIGHT/... Issued at 300 PM MDT Sat Aug 17 2024 Quiet weather today with hot temperatures across most of the region with readings in the low to mid 90s along and east of I-25, and mid to upper 80s west of I-25. Visible satellite loop shows mostly sunny skies with some fair weather cumulus clouds and intervals of midlevel cloudiness moving to the east. Satellite also shows the next potent upper level shortwave moving northeast across Utah this afternoon. Models show this shortwave continuing to lift northeast into western Wyoming tonight as some cooler air aloft works it`s way into south central Wyoming. Thankfully, the timing of this shortwave will be too late in the day to trigger widespread thunderstorm activity across southeast Wyoming as we lose instability around sunset. Kept POP below 30 percent across the lower elevations for scattered showers and high-based thunderstorms. Current CAMs and high res guidance are showing isolated to widely scattered coverage at best...mainly west of the I-25 corridor through midnight tonight. The main concern will be gusty outflow winds with a few of these cells. Otherwise, should be pretty quiet and mild tonight. Weather will be similar on Sunday (compared to today) even as the upper level shortwave moves east across the area. Current model timing suggests this feature will be dissipating while eject east into the central plains just in time for Sunday afternoon peak heating. The upper level ridge axis is forecast to quickly restrengthen behind this shortwave with increasing upper level subsidence through the afternoon. Expect the dry air and lower lapse rates aloft to suppress most convection through the afternoon hours. Continued the trend of lowering POP Sunday afternoon and left probability of thunder below 20 percent for most locations...with a slight trend upward towards the evening hours as low level moisture really starts to ramp up. For Monday, SREF guidance is beginning to show higher probabilities of low stratus and fog in the morning. Surface winds are forecast to veer into the east and southeast due to a stronger than average surface high pressure pushing into the Great Plains. With increasing low level moisture through the day, starting to get concerned with the potential for heavy rainfall and flooding concerns as NAEFS is showing 95th to 99th percentile PWATs across the eastern plains and even around 90th percentile for all of central Wyoming. Storm motions look pretty weak, especially through the mid afternoon hours with winds aloft increasing a bit into Monday evening. Kept POP around 55 to 75 percent for most of the area for numerous showers and thunderstorms. May need to watch for strong to severe thunderstorms, but with the lack of 0-6km shear through much of the day...confidence is pretty low at this time. Slightly cooler temperatures are expected on Monday due to increasing cloud cover and upslope east to southeast winds, but highs should stay close to average for this time of the year (80s to near 90). && .LONG TERM /TUESDAY THROUGH FRIDAY/... Issued at 210 PM MDT Sat Aug 17 2024 A fairly typical late summer weather pattern will carry us through most of the long term forecast period. Overall, it looks like a fairly active monsoon pattern, but with a few break periods in between days of active monsoon surges. Picking up on Tuesday, most model guidance shows a break period in the monsoonal flow. The strong upper level ridge centered over New Mexico will broaden westward into southern California, which will deflect the monsoon moisture into the eastern Pacific and instead draw dry subtropical air into our area. Precipitable water drops to near or slightly below average for this time of year over our area. Meanwhile, the shortwave passing through Monday will have passed to the area, and we will see some subtle synoptic scale subsidence. All together, this should lead to a hotter and drier day on Tuesday with highs running perhaps 5-10F above average and just a few isolated showers and thunderstorms, mainly confined to the vicinity of the higher terrain. This break period looks short-lived though, as most models indicate the western flank of the upper level ridge over Arizona breaking down on Wednesday, allowing for the monsoon moisture to push through once again. Some subtle vort-maxes will also be pulled out of the southwest, helping to enhance shower and thunderstorm activity. This overall pattern looks to mainly hold through the second half of next week with a series of weak shortwaves locally enhancing lift for thunderstorms. While the timing of those features is uncertain at this lead time, there is moderate confidence in a fairly active pattern of afternoon and evening showers and thunderstorms beginning again on Wednesday (possibly late) and continuing into Friday. Most model guidance shows the monsoon pattern at least weakening by Saturday, though it`s a little early to say if we`ll completely dry out or just see lower storm coverage. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1139 AM MDT Sat Aug 17 2024 VFR conditions expected for the forecast period. Mostly clear skies will continue this afternoon, but mid to high level cloud cover will increase from southwest to northeast late this afternoon through the evening. Scattered showers and isolated thunderstorms will develop tonight in WY, mainly between about 01z and 08z. These showers may produce gusty winds. For now, just have VCSH, but may need to add thunder and/or TEMPO groups for wind in the next update. Gusty west to northwest winds will develop Sunday morning. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...TJT LONG TERM...MN AVIATION...MN