Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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162 FXUS65 KCYS 211118 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 518 AM MDT Sat Sep 21 2024 .KEY MESSAGES... - Well below normal temperatures expected this weekend as we see the autumnal equinox occur, with some areas seeing a higher chance of rain showers along the I-80 corridor. && .SHORT TERM /THROUGH MONDAY/... Issued at 318 AM MDT Sat Sep 21 2024 Water vapor satellite imagery from the GOES-16 Channel 9 spectrum depicts the surface cold front quickly advancing southward towards the CO/WY state border as of 9Z this morning. The cold front aloft is still somewhat staggered in the lower boundary layer, as surface temperatures are slow to respond to the wind shift behind the FROPA. However, surface winds are responding much more quickly as wind gusts of 35-45mph along and behind the surface frontal boundary are being realized on mesonets across the cwa. Hi-res model guidance continues to ping the cold front being through the cwa southern border by 12-13Z today, with gusty winds behind it. These winds will subside in the subsequent hours after daybreak, with the majority of the strongest winds further south by 18Z today. 700mb temperatures are progged to be at approximately +5C to +10C at 12Z this morning, and adiabatically cool aloft from the incoming push of colder air advection to 0C to +5C by 0Z this evening. The dry cool front will not bring any rain showers, but we will see daytime highs on the order of 15-25 degrees cooler outdoors compared to yesterday`s high temperature. The lower elevations will struggle to reach the 60s for a majority of the day, with the North Platte River Valley in western NE seeing the highest likelihood to eclipse the 70 degree mark later today. Areas in the highest elevations will remain in the mid 40s to mid 50s, despite there being sunshine for the first half of the day. By this afternoon and evening, a quickly advancing upper level low (ULL) will eject out of the Four Corners toward the Central Rockies. Lee cyclogenesis on the eastern flank of the Continental Divide/Front Range of CO will support a surface low intensification. Pressure tendencies will fall, and a slightly increased pressure gradient will cause our mountain zones/gap areas to see slightly elevated wind gusts post-frontal passage. However, the most recent model data supports a further south trajectory of the surface low, and its deformation zone/precipitation TROWAL. What this means for our region is that the chances for rain showers will decrease. It is a good example of continually monitoring weather trends as the northern gradient of precipitation will have a steep cutoff, leaving the majority of our cwa dry. Nonetheless, we will have a 30-40% chance of rain showers along the I-80 corridor from the Summit to Sidney early Sunday morning. The pesky cloud cover, upslope flow, and chance of rain showers along this area will keep temperatures well below average. A light dusting of snowfall in elevations above 10,000 feet for Sunday morning remains for the Snowy/Sierra Madre Mtns. Daytime highs closest to the WY/NE/CO border will struggle to achieve a mid-50s F, but areas further to the north where clearing skies will become more likely towards late Sunday should see the highest likelihood of mid to upper 60s. The further south trajectory of the surface low will cause any lingering cloud cover to nudge out of our cwa by 6Z Monday for the most part. The surface high pressure that is progged at 1020mb by 6Z Monday will scoot towards the Central Plains overnight, leaving mostly clear skies in its wake. Lee-surface troughing is favored by daybreak Monday, so there will be an uptick in wind gusts on the foothills adjacent to the Central and South Laramie Range. This should allow areas due east to stay in the upper 30s to near 40 degrees for an overnight low. Western NE will see the mostly clear skies and lack of overnight mixing from winds, so morning lows are likely to be much cooler. Have nudged the morning lows another degree or two lower in the valley areas and far southern portions of the Nebraska Panhandle. Low to mid 30s for those regions and valley areas in southeast WY should be achieved, which will equate to some patchy frost in isolated areas. Weak upper level ridging is likely for Monday across our cwa, so we will see a nice uptick in daytime highs compared to Sunday. Overall, expect MaxT`s to hover in the mid to upper 70s for areas along and east of the Laramie Range, with slowly cooler temperatures in the mid to upper 60s for locations west of the Laramie Range. | && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... Issued at 318 AM MDT Sat Sep 21 2024 Headed into the middle of next week, the upper level pattern changes with 500mb height rises across much of the western CONUS as the upper low dives to our east. However, model solutions quickly begin to diverge with a portion of the upper low becoming cutoff from the flow setting up a rex block over the Great Lakes and Ohio River valley. This pattern will support above average temperatures across the area with 700mb temps climbing 12-14C and mostly dry conditions as any notable upper level systems lift over the Canadian border. At this time, Thursday looks to be the warmest day with afternoon highs in the mid-70s and 80s, but windy across the Laramie Range with a shortwave passing to the north. In-house guidance is not suggesting high winds, but could still see downsloping winds gusting up to 40 mph. The ridge over the western CONUS looks to stick around headed into next weekend and may be difficult to breakdown. Latest cluster analysis shows a solution of a stronger upper level trough digging through the northern Rockies late in the weekend with the potential return of precipitation and fall temperatures, but this is ~30% of the Grand Ensemble (mainly Canadian and EC members). && .AVIATION /12Z TAFS THROUGH 12Z SUNDAY/... Issued at 515 AM MDT Sat Sep 21 2024 A strong cold front will have moved through the area shortly after daybreak for most terminals across the region. Expect gusty winds of 25 to 40 knots this morning through afternoon, except for KLAR and KRWL. These terminals will see the wind gusts pick up closer to 18Z today. VFR is expected at all terminals through the late afternoon. A large area of showers will move from south to north in CO towards WY/NE. Expect VCSH and -SHRA for KCYS and KSNY this evening, with lower confidence of KLAR and KRWL seeing VCSH. Should expect possible brief fluctuations to MVFR if a stronger rain shower moves over the aforementioned terminals overnight through 12Z Sunday. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...BW LONG TERM...MB AVIATION...AM