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Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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253 FXUS65 KCYS 201126 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 526 AM MDT Sat Jul 20 2024 .KEY MESSAGES... - Severe weather possible again Saturday, with damaging winds and isolated large hail possible. Expecting much cooler temperatures for Sunday. - Cooler weather expected this weekend into Monday ahead of a gradual warming trend to above average temperatures through next week. && .SHORT TERM /THROUGH SUNDAY NIGHT/... Issued at 116 AM MDT Sat Jul 20 2024 Current Weather... Thunderstorms have been moving across Converse County early this morning with a few cloud pulses now showing up in northern Carbon County. These storms are expected to remain weak, but should produce decent precipitation in regions they move over. Cloud cover has been increasing out in the Panhandle and across southern portions of the CWA, but there are still plenty areas with clear skies as of 08Z. Temperatures are pleasant across the region, with current temperatures in the mid-50s to mid-60s area-wide. Alliance and Chadron could see some fog begin to develop within the next hour or two, but looks to remain patchy in coverage. Discussion... Upper-level ridging continues in the western and southwestern CONUS. The CWA is still under the influence of northwesterly flow for today and Sunday. An upper-level shortwave trough is progged to slowly slide to the southeast and move just east of the CWA by early Sunday morning. This approaching shortwave will be the primary driver for convection on Saturday. While the 700mb low that produced convection on Friday will be off to the east over eastern Kansas and eastern Nebraska, the upper-level shortwave should provide enough lift across the region to initiate convection once again Saturday afternoon. Additionally, several 500mb vorticity maxima will eject out ahead of the approaching upper-level shortwave and further increase synoptic lift across the region. However, the 500mb high that had been over the Four Corners region will continue to shift to the west, cutting off a majority of the monsoonal moisture the CWA was seeing over the past couple of days. Northerly to northwesterly flow throughout the atmosphere will not be conducive for overly moist conditions across southeast Wyoming and western Nebraska, but current dewpoints suggest that there is still sufficient moisture left over from the moisture influx over the past several days. Forecast soundings from the 06Z HRRR suggest inverted- V soundings once again, with strong low-level lapse rates. MLCAPE values across western Nebraska will be modest, around 600-800 J/kg, which is still enough to get convective development this afternoon. The NAM suggests significantly more CAPE across the Panhandle, with values between 1000 and 1500 J/kg. The NAM is significantly more moist than the HRRR, suggesting PW values over an inch, while the HRRR show PW values between 0.75 and 0.8in. Looking at other short- range models, the HRRR appears to be the driest, with the RAP, SREF, and NAMNST all showing PW values more in line with that of the NAM. CAPE values vary significantly across the CAMs, so likely expecting between 800 and 1200 J/kg of CAPE across the Panhandle. Bulk shear values look lower today, ranging from 30-40kts across the region with an northwesterly orientation. Based on surface forecasts from the HRRR and NAMNST, a convergence zone may set up just south of the Black Hills, resulting in concentrated lift over the region. Additionally, northwesterly surface flow favors convection developing off the North Laramie Range and propagating to the southeast throughout the day. Therefore, expecting to see two groups of storms, those that develop south of the Black Hills and those that develop along the North Laramie Range. All will propagate slowly to the southeast due to the weak, northwesterly steering flow. With good CAPE and inverted-V soundings, damaging winds look to be the primary threat today, with an isolated large hail threat possible across the Panhandle. SPC has the region in a Marginal Risk again today. One failure mode again today could be the increasing cloud cover through the morning hours decreasing the surface temperatures and limiting surface heating and instability this afternoon. Based on events Friday, with weaker forcing today, this is a possibility, but some storms will likely develop. Their strength will be the biggest question and it is possible that most of the region just sees low-reflectivity virga bombs. Northerly upper-level flow remains for Sunday with the 500mb high remaining west of the Four Corners regions and preventing good monsoonal flow into the region. The northerly flow opens the door for several 500mb vorticity maxima traversing through the flow and favoring synoptic lift across the region once more. A similar setup is expected Sunday as the previous several days, but forcing looks significantly weaker at this time and surface flow is less favorable for terrain-induced convection. Cooler surface temperatures on Sunday will also help stabilize the atmosphere, with highs expected to be in the low-70s to mid-80s area-wide. PW values will be lower on Sunday as well, but remain in the 0.8-0.9in range, which still favors some convective development. Currently expecting afternoon showers and thunderstorms to develop as the vorticity maxima propagate over the region, but am not expecting severe weather at this time as MLCAPE remains low even in the most aggressive short- range models. && .LONG TERM /MONDAY THROUGH FRIDAY/... Issued at 116 AM MDT Sat Jul 20 2024 CWA stays under northwest flow through Wednesday in the long term with upper high firmly in place over Nevada and Idaho through most of the upcoming week. We continue to see monsoonal moisture streaming north into the CWA interacting with upper shortwaves in the northwest flow just about every day. So shower and storm chances expected each afternoon and evening. Wednesday begins the warm up as the high begins to shift eastward. Seeing 700mb temperatures up near +18C across western Wyoming into Carbon County Wednesday and by Friday...GFS advertising +20C over southeast Wyoming. Look for mid 90s Wednesday for highs to near 100 degrees Friday across the Panhandle. A pretty strong upper level low tracks across southern Canada Friday with a trough moving through Friday evening into Saturday. Could see fairly widespread rainfall Friday afternoon as the trough approaches. && .AVIATION /12Z TAFS THROUGH 12Z SUNDAY/... Issued at 523 AM MDT Sat Jul 20 2024 Some low stratus and fog at KSNY this morning will break up shortly after sunrise. Concern turns to this afternoon and chances for thunderstorms. Used latest HRRR on timing and locations for these storms. Showers and storms may continue into the overnight hours...especially in the Panhandle. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...AM LONG TERM...GCC AVIATION...GCC