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Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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445 FXUS65 KCYS 171004 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 404 AM MDT Wed Jul 17 2024 .KEY MESSAGES... - Increasing moisture today will support strong to briefly severe thunderstorms through this evening. Additionally, heavy rainfall could lead to localized flash flooding. - Severe potential late in the week followed by cooler weather for the weekend. Warm-up begins to start the week. && .SHORT TERM /THROUGH THURSDAY NIGHT/... Issued at 345 AM MDT Wed Jul 17 2024 Latest GOES WV imagery shows with an upper level ridge across the western CONUS with a large scale upper low across the Great Lakes region. More locally, scattered clouds have begun to develop just north of the analyzed surface low in northeast CO with isolated early morning showers. Latest observations show generally weak easterly winds across the NE panhandle with upper 50s to low 60s dew points working into the CWA. However, westerly winds across the South Laramie Range foothills could support addition shower development early this morning with additional convergence just east of Cheyenne. The active summertime convective pattern will continue once again today with showers and storms developing along the higher terrain late this morning. As a shortwave rounds the top of the ridge, better upper level lift will support more numerous coverage of storms today, with a few strong to severe storms. Rather marginal mid-level flow will keep bulk shear values around 30 kt leading to more multicellular storm mode that could briefly become severe with strong winds and hail. However, rather moist profiles with PW values near 0.9" will be supportive of heavy rain producing storms, especially along the South Laramie Range Foothills across the I-80 corridor through Kimball where CAMs have been most consistent with storm coverage. Latest 3-hr FFG for the southern NE panhandle ranges from 1.5-2". Storms will continue into the early evening before slowly dissipating and moving southeast out of the CWA. Headed into Thursday, the ridge axis over the western CONUS slightly shifts eastward keeping the best moisture along the eastern CWA border and extending into central NE. As another weak shortwave passes overtop of the ridge, afternoon storms will be possible (20-30%). Coverage will be much less than today, but a few stronger storms could still develop near areas of enhanced convergence from remnant outflow boundaries from previous night`s storms in an area of 1500-2000 J/kg MLCAPE per the latest RAP with HREF 25th percentile values still around 1000 J/kg. These will be in addition to storms developing over the higher terrain in southeast WY with daytime heating. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Issued at 345 AM MDT Wed Jul 17 2024 Northwesterly flow will continue across the region for much of the weekend into early next week. With a stout, 500mb high over the Four Corners region, monsoonal moisture will continue to advect into the region, increasing precipitation chances. Friday will have the best chance for precipitation as an upper-level shortwave traverses through the ridge and 500mb cyclonic vorticity advection increases. A 700mb low attempts to develop with this upper-level shortwave with a surface low trying to organize. The surface low will bring with it a cold front and strengthened winds at the surface. The approaching cold front will support lift across the region, leading to the development of showers and thunderstorms. The NAM currently shows around 1000-1100 J/kg of MLCAPE across the Panhandle for Friday afternoon. Forecast soundings suggest an inverted-V profile with strong mid-level lapse rates around 9 C/km. Relatively weak steering flow supports slower moving storms and 1.0in PWAT values indicate the potential for slow moving, heavy rain producing showers. However, the dry low-levels will likely lessen some of the heavy rain as a good portion will evaporate before hitting the surface. Nonetheless, strong to severe thunderstorms cannot be ruled out and the Storm Prediction Center currently highlights the Panhandle in a Marginal Risk for Friday. The active pattern will continue for the weekend as northwesterly, upper-level flow remains over the region. The 500mb high begins to retrograde to the west, which will decrease the monsoonal moisture across the region, evident by PWATs decreasing towards the 0.5-0.8in mark on Saturday. Northwesterly flow is favorable for precipitation across the region, as several vorticity maxima will traverse through the ridge and initiate afternoon showers and thunderstorms both days this weekend. With lower moisture in the area, these showers are expected to be more isolated in nature and likely not overly heavy rain producers. Temperatures will be pleasant this weekend in the mid-70s to mid-80s area-wide. Predominantly northerly upper-level flow will develop by Monday as the upper-level ridge amplifies to the west and a large fetch of northerly flow develops across the region. With this fetch of Canadian air, temperatures will be quite nice on Monday, with highs in the mid-70s to mid-80s area-wide once again. Afternoon showers and thunderstorm remain possible throughout the early portions of the week, typical for this time of year. Long range models are in agreement on increasing temperatures by mid-week, with highs returning to the mid-80s and mid-90s area-wide by Wednesday. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1136 PM MDT Tue Jul 16 2024 Light easterly winds overnight will help pull in more moisture ahead of Wednesday. This will lead to a slight chance for IFR CIGs at KSNY and KAIA between about 11z and 15z. The probability is too low to add as a prevailing group, but hinted at the possibility with FEW clouds below 1000 ft. Showers and thunderstorms will develop quickly Wednesday afternoon around 19z, and continue to around 02 to 03z. All terminals will have the potential to see nearby lightning, gusty winds, and brief MVFR to IFR VIS in locally heavy rainfall, but confidence is too low to go beyond VCTS at this time. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...MB LONG TERM...AM AVIATION...MN