Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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310 FXUS65 KCYS 181720 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1120 AM MDT Thu Jul 18 2024 .KEY MESSAGES... - Severe weather potential for Thursday and Friday, with Friday looking to be the better day. - Cooler weather expected this weekend with daily chances of storms through early next week. && .SHORT TERM /THROUGH FRIDAY NIGHT/... Issued at 316 AM MDT Thu Jul 18 2024 Current Weather... Mostly clear skies as of 08Z for most of the region this morning. Some clouds remain over northern portions of the CWA and southern Albany and western Laramie counties. Precipitation in the northern regions has come to an end over the past few hours, leaving fairly quiet conditions overnight. Fairly light winds are ongoing across southeast Wyoming and western Nebraska with temperatures in the mid-50s to low-60s at this time. Clear skies, light winds, and lowering dewpoint depressions over the southern Panhandle should lead to some patchy fog developing within the next few hours. Discussion... Upper-level ridging continues over the western CONUS for Thursday with northwesterly flow over the region. Several 500mb vorticity maxima are progged to traverse through the ridge during the day Thursday, promoting localized, but weak, synoptic lift over southeast Wyoming. The stout 500mb high over the Four Corners region will remain in place throughout the day, ushering monsoonal moisture into the region and increasing PW values into the 90th percentile for western Nebraska. PW values approaching 1 inch are expected for much of the Panhandle, with some higher values creeping into far southeast Wyoming/Laramie County. A convergence boundary looks to set up along the Laramie Range, with easterly winds to the east of the mountains and westerly to southwesterly winds west of the mountains. With increased moisture in the region and sufficient surface lift provided by the surface convergence zone and terrain- induced lift across the southern Laramie range, showers and thunderstorms are expected to develop across the higher terrain and propagate into the adjacent zones. With daytime heating across across the region and increasing moisture from monsoonal flow, MLCAPE values are progged to be between 500 and 1000 J/kg across southeast Wyoming and closer to 1000 to 1500 J/kg in the Nebraska Panhandle. While surface moisture will be on the rise throughout the day, mostly dry low-level sounding profiles are expected with inverted-V HRRR forecast soundings across southeast Wyoming. Panhandle soundings are more moist in the lower-level, but still suggesting strong mid-level lapse rates. Bulk shear values will be fairly weak, ranging from about 30-35kts with weak steering flow. Overall, isolated showers and thunderstorms are expected to develop over the higher terrain this afternoon and propagate easterly into the Panhandle where the environment will be more favorable for severe weather. Some of these showers and thunderstorms will likely be heavy rain producers due to the high PW values and the weak steering flow aloft. SPC has the CWA in a Marginal Risk for severe weather, with gusty winds and isolated large hail being the primary threats. Right now, looks most likely that the gusty wind threat will be dominant over southeast Wyoming, with the hail threat increasing into the Panhandle where the environment is more conducive to more severe weather types. Friday will feature another day with predominantly northwesterly upper-level flow across the region. Several 500mb vorticity maxima will traverse through the ridge, with the 500mb high over the Four Corners region retrograding back to the west. While this shift to the west will lead to weaker monsoonal moisture advection into the region, lingering moisture from Friday will allow ample moisture to remain across the CWA. The main instigator for convection on Friday will be a 700mb low that is progged to eject to the southeast out of the Dakotas and across central Nebraska. This 700mb low will be accompanied by a surface low and attendant cold front. As the cold front is dragged across southeast Wyoming and western Nebraska it is expected to initiate convection across our region. MLCAPE values over 1000 J/kg are expected across western Nebraska. Bulk shear will increase with the surface low to the east and the cold front moving towards the south, leading to shear values in the 35 to 45kt range. Steering flow will be stronger Friday, so while PW values wil be around an inch, heavy rain producers will be more difficult to get as storms will not remain over the same area for a long time. Forecast HRRR soundings suggest dry low-level across the Panhandle with strong low-level to mid-level lapse rates. However, storms should still be surface based and SBCAPE values are well over 1500 J/kg. Overall, expecting storms to develop along the advancing cold front with isolated storms possible initially before likely conglomerating into a multi-cellular structure as the evening goes on. SPC has the CWA in a Marginal Risk for severe weather with strong winds and large hail as the primary threat. Minimal turning with height will limit any tornado potential on Friday. Temperatures for Thursday and Friday will be warm towards hot. Thursday will be the warmest day, with highs in the mid-80s to mid- 90s. Friday will drop a few degrees as the 500mb high shifts slightly further west. Highs will be in the low-80s to low-90s area- wide. Winds will be fairly calm for the end of the week, with no strong gusts expected across the region. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Issued at 316 AM MDT Thu Jul 18 2024 Overall, looking at cooler weather this weekend with afternoon highs around 80 on Saturday and potentially dropping into the mid-70s for Sunday. This will be all while much of the western CONUS is expecting above average temperatures developing under the vast upper level ridge, however much of our CWA will be positioned well east of the ridge axis and more under the influence of the large scale trough across the central and eastern CONUS allowing for cooler weather. Daily chances for showers and thunderstorms continue with multiple disturbances passing over and along the eastern periphery of the ridge. Saturday and Sunday look to have the best chance for more widespread coverage as stronger upper level support will be present associated with the upper low over the central CONUS mixed with post-frontal east/northeasterly flow up against the Laramie Range. Looking ahead to the middle of next week, a large scale trough approaches the PacNW attempting to breakdown this stale upper level pattern. Latest cluster analysis shows growing uncertainty with the timing of the trough passage to the north. One cluster mainly comprised of GEFS members shows a quicker trough progression across the Canadian border that shifts the thermal ridge overhead while other clusters more comprised of EC members are on the slower side. Will need to continue to monitor model trends as this will impact how quickly we return to above normal temperatures for the later portions of next week. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1111 AM MDT Thu Jul 18 2024 VFR is expected for all terminals through the forecast period. VCTS and VCSH is anticipated for portions of southeast WY and the lower NE Panhandle. Wind gusts 20-25 knots outside of gusty and erratic winds from nearby VCTS. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...AM LONG TERM...MB AVIATION...BW