Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000 FXUS65 KCYS 191124 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 524 AM MDT Mon Apr 19 2021 .SHORT TERM...
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(Today through Wednesday) Issued at 330 AM MDT Mon Apr 19 2021 Primary forecast concern will be snow accumulations and associated impacts over the next 24 hours across southeast Wyoming and western Nebraska. Strong Canadian cold front is currently positioned near Rawlins and stretches east to just north of Cheyenne at this hour. Whitaker, about 15 miles north of Cheyenne, is reporting north winds and temperatures lowering into the 20s while Cheyenne is still hanging on to a gusty west wind and a temperature of 42 degrees. The front has pushed south through the Nebraska panhandle with Sidney and Kimball reporting northeast winds and lowering temperatures. Snow has developed north of the North Platte River Valley, but accumulations so far have been light from near Douglas over towards Chadron Nebraska. This front will continue to move southwest through Cheyenne and eventually Laramie over the next 3 hours, with snow developing along the I-80 corridor sometime between 800 AM and noon local time. Added some fog across the Pine Ridge and Cheyenne Ridge due to the potential for advection fog behind the front. No major changes to the forecast during this winter weather event. Although the system remains on the progressive side, moving south of the area by late this evening, there are some impressive dynamics including strong frontogenesis, upper level diffluence, WAA aloft, and upslope winds/convergence at the surface. 0.5 to 1.0 inch snowfall rates look like a good bet along and west of the I-25 corridor for a few hours this afternoon and into this evening. Current headlines look good, with maybe a slight adjustment in Advisories needed for the central and southern Nebraska panhandle. East winds do not appear quite as strong compared to 24 to 48 hours ago, so do not expect blowing snow to be a major issue. Drifting snow will likely be the primary concern besides any falling snow, with drifting possibly on area roadways across southeast Wyoming, especially the higher elevation mountain passes. Snow accumulations and road impacts will be tricky with this system, mainly because of the time of the day most of the snow occurs. In theory, snow would accumulate more efficiently at night this time of the year compared to the daytime. On the other hand, pavement temperatures should remain relatively cold due to the much colder than normal temperatures the last 7 days. Snowfall rates as little as a half inch per hour may be enough to overcome pavement temperatures during the day. Kept snowfall accumulations somewhat below consensus and ensemble guidance, but even then, still coming up with a solid 3 to 5 inches across the Advisory area, and 5 to 9 inches in the mountains including the I-80 Summit. For Tuesday and Wednesday, overnight lows will be very cold for this time of the year and generally in the teens to low 20s. Some single digits are likely west of I-25 where skies clear out and a decent snowpack remains. Longwave trough will remain over the northern/central plains and the Great Lakes region through the middle of the week, resulting in cold northerly flow for the Front Range. Daytime high temperatures will struggle to increase into the upper 30s to low 40s as 700mb temperatures remain near -10 to -15c across most of the region. A secondary disturbance will dig south on the backside of the main upper level low across the Great Lakes Region. This disturbance will likely enhance snow shower activity across the eastern plains and in the mountains on Tuesday into Tuesday evening. Lower chances for snow showers expected on Wednesday as the low to midlevels stabilize. However, a weak shortwave trough will approach from the west, which may result in an increasing threat of snowfall near the Colorado border through the afternoon hours. .LONG TERM...(Wednesday Night through next Monday) Issued at 243 AM MDT Mon Apr 19 2021 Deepening trough over Nevada and developing closed 500 mb low pressure will continue moving eastward with vorticity advection ahead of the centered pressure and coupled moisture advection bringing in another round of precipitation to mainly along the I-80 corridor east of Rawlins into the NE Panhandle. Accumulations are fairly lacking in both the GFS and EURO 00z runs to be too excited for but certainly any moisture right now is appreciated. Trended a touch more to the EURO for the WED-Thu system after 00z Thu as GFS limits moisture more on the CO side. With some of the precipitation occurring in the overnight in sub-freezing temperatures, snowfall accumulation possible but given the liquid amounts available, any accumulation will remain around a tenth to a dusting especially on grassy surfaces, roadways likely to be only wet though some minor glazing could be a concern. With the main trough axis moving eastward through the region Friday, another ejected pocket of vorticity and mositure via another shortwave for a better chance of some additional moisture to more of the forecast region. Model solutions showing upwards of around 0.10" of liquid for the event for many with southern mountains seeing it mainly in snow. Ridging sets in ahead of shortwave Saturday night into Sunday with precipitation chances extremely limited based on model solutions as main ridging again persists ahead of our next system that will approach CA coast Monday afternoon with following round of precipitation middle of next week. Otherwise, temperatures and conditions will return to more of late April with daytime highs into the 50s and 60s by Friday and into the 70s for the Panhandle and North Platte River Valley on Sunday. Overnight lows Thursday through Saturday will remain near to below freezing for many.
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&& .AVIATION...
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(For the 12Z TAFS through 12Z Tuesday morning) Issued at 515 AM MDT Mon Apr 19 2021 Latest satellite and surface observations have captured the movement of the cold front through the region this morning with advection freezing fog in the wake of the front. Contrary to some of the observations such as KCYS, the wind and freezing fog are contributing to light riming and not really freezing rain as shown by area sensors. Further deteriorating conditions for all sites today as snowfall chances pickup later this morning through the afternoon being heavy at times with best chance for accumulating in afternoon and evening. Flight conditions will remain at least at MVFR to LIFR for all sites into the evening as conditions lift slowly back to VFR as snow ends from north to south.
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&& .FIRE WEATHER... Issued at 234 AM MDT Mon Apr 19 2021 No fire weather concerns this week due to widespread snowfall today, and cold temperatures through early Thursday morning. Models are indicating a warming and drying trend for next weekend, which may result in increasing fire weather concerns across portions of the region. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...Winter Weather Advisory until 6 PM MDT this evening for WYZ101- 102. Winter Weather Advisory from 9 AM this morning to 11 PM MDT this evening for WYZ105>108-110-114-115-117>119. Winter Weather Advisory until 11 PM MDT this evening for WYZ103. Winter Storm Warning from 9 AM this morning to 3 AM MDT Tuesday for WYZ116. NE...Winter Weather Advisory until 6 PM MDT this evening for NEZ002- 095. Winter Weather Advisory from 9 AM this morning to 11 PM MDT this evening for NEZ020-054. && $$ SHORT TERM...TJT LONG TERM...WM AVIATION...WM FIRE WEATHER...TJT

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