Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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876 FXUS65 KCYS 281153 CCA AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 545 AM MDT Sun Apr 28 2024 .KEY MESSAGES... -Lingering light snow showers will bring light snowfall accumulations through Sunday afternoon to the Sierra Madre and Snowy Range mountains above 8500 feet. Winter headlines remain in effect mainly due to those planning outdoor activities in the higher terrain during the weekend. -Strong winds are anticipated early Tuesday morning through Tuesday afternoon in the wind prone and gap areas of the higher terrain of southeast Wyoming. -Active weather pattern continues for the extended forecast as the next window for rain and accumulating snowfall exists for Wednesday night and Thursday. && .SHORT TERM /THROUGH TUESDAY/... Issued at 330 AM MDT Sun Apr 28 2024 A very slow moving upper level system responsible for multiple cloudy days in a row, and rain and snow showers continues to slowly trek to the east early this morning. Several locations are reporting light rain showers, especially the western Nebraska Panhandle, with chilly temperatures to boot. As of 9Z, observations depict southeast Wyoming and the Panhandle areas in the upper 20s to near 40 degrees, with the warmest temperatures residing along the North Platte River Valley. Foggy conditions and very low cloud ceilings from an upslope flow regime have made for low visibility readings. Will continue the Dense Fog headline through the morning hours of Sunday, and have the next shift assess whether to discontinue it or not. SNOTEL observations also showing light snow accumulations in the higher terrain, so a continuance to the winter headlines appeared prudent as we expect additional light snow in that area. Cold air intrusion from the mid-levels will keep temperatures chilly for the NE Panhandle today while combined with light rain showers. Expect partial clearing to occur for areas across the cwa, and with the higher sun angle in place despite the pesky cloud cover, daytime highs should recover the near seasonal normals for the southeast WY forecast zones, and the North Platte River Valley of the NE Panhandle. A few areas will reach the upper 50s to lower 60s where sunshine is present the longest today, but do expect the remaining tier of the cwa to see upper 40s to middle 50s. This evening and overnight, there will we a weak shortwave disturbance that is sheared off from a Pacific Northwest system, bringing another shot at light snow flurries and rain showers overnight to southeast WY. Not looking at anything impactful for weather conditions, except the mountain zones that may receive another inch or two of snow during the overnight hours.This will be a quick-moving shortwave on the heels of the departing cut-off upper level system making its way towards the Upper Mississippi Valley. Mostly sunny skies will become the main story on Monday. Daytime highs will make for a nice recovery after the chilly temperatures of late. Highs in the 60s and 70s for the lower terrain will occur, with cooler temperatures in the mountain zones. A new shortwave disturbance will propagate from the Canadian Rockies toward our cwa by Monday night. A surface low pressure over northern WY will move towards the Dakotas by Tuesday morning. This will bring another opportunity for light rain showers in the lower terrain and light snow in the higher terrain to be present. Additionally, model guidance continues to depict a potential mountain wave event across the Central Rockies, including our cwa for Tuesday. This could create some blowing snow issues for the higher terrain, but the higher sun angle may limit this potential during Tuesday. The gusty winds are favored to be strongest in the gap and wind prone areas. A high wind headline may be necessary to reflect this in the coming 24-36 hours. Downslope effects off the lee side of the Laramie Range will adiabatically warm areas, and dry out fuels for Tuesday afternoon. Have kept the forecast consistent with low afternoon humidity for Tuesday as we see slightly cooler daytime highs across the region, mainly upper 50s to lower 70s. With the recent moisture over the past couple of days, and spring green-up taking place, fire weather concerns should remain low to elevated. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... Issued at 330 AM MDT Sun Apr 28 2024 Primary forecast concern for the medium to long range will be the track and evolution of a Pacific storm system and associated cold front forecast to push across the central and northern Front Range late Wednesday and Thursday. Southeast Wyoming should begin to see the first round of activity with this system late Tuesday night as the initial frontal boundary stalls near the Nebraska/Wyoming border and a 110kt jet max aloft suggest nighttime showers and isolated thunder across the area through early Wednesday morning. Kept elevated POP across most of the area before the trough axis begins to accelerate eastward Wednesday. Increased min temperatures Tuesday night due to cloud cover and warm air advection just off the surface...which should aid in developing weak nocturnal convection. High temperatures on Wednesday should be seasonable and in the 50s to mid 60s before the cold front pushes into the region later in the day. For late Wednesday through Thursday, will be closely monitoring the position and evolution of the storm system forecast to track across the area. Models have continued to trend colder for this system, so nudged max and min temperatures downward a bit since all ensembles and deterministic models are showing much colder temperatures with 700mb temps between -5c to -10c Thursday afternoon. The 00z GFS solution is the most aggressive solution, showing broad WAA aloft and some frontogenesis as the upper level trough attempts to form a closed low across Colorado. The ECMWF and about half of the GEFs and Euro ensemble members show a much more progressive solution as the trough remains an open wave as it moves eastward into the Dakotas, mainly because the trailing system across the PAC NW is faster compared to the GFS. The 00z ECMWF solution is also a bit further north, but still shows widespread precipitation across the area although for a shorter duration. Increased POP into the likely category (55% to 70%), but will hold off on mentioning snow level, potential snow accumulations, etc until we get closer to the event due to the amount of uncertainty. Would like to note that all models show a cold enough airmass for widespread snow down to elevations of 5000 feet Thursday. Did not go that aggressive in the official forecast due to low confidence and the 00z GFS an outlier when compared to the middle "most probable" 25th to 75th percentile of all ensemble guidance. Lowered max and min temperatures below average for the end of the week. At this time, snow accumulations can`t be ruled out for elevations down to 5000 feet, with mainly rainfall for elevations below 5000 feet. Still dealing with a relatively large spread in forecast high temperatures for Thursday, but not as bad as yesterday. Highs will likely be well below normal with the GFS showing the coldest temperatures...generally in the 30s to near 40 early Thursday morning, dropping into the upper 20s through the day. For later this week and into next weekend, thankfully the cold airmass will be short-lived...which is typical for May as 700mb temperatures increase above 0c. All models show the front lifting northeast as a warm front, with a pleasant weekend possible as highs increase back to near average and possibly above average by next Sunday. However, can`t rule out a daily chance of showers and thunderstorms both Friday and Saturday afternoon. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 545 AM MDT Sun Apr 28 2024 Pacific storm system will lift northeast into Nebraska today and move out of the area tonight. Improving conditions are expected with rain and snow showers becoming more isolated by this afternoon. HAZARDS/WEATHER TRENDS: Steady snow/rain will come to an end later this morning across western Nebraska, with IFR or near IFR conditions improving to MVFR by this afternoon due to low CIGS. CIGS are expected to lift above 5000 feet AGL between 21z today and 00z Monday for all western Nebraska terminals except for KCDR...which may struggle to improve due to a lingering upslope northeast winds. Further west across southeast Wyoming, conditions will improve to VFR late this morning with another round of rain showers expected through this evening. Can`t rule out a few thunder-showers, especially around KRWL, but confidence/coverage is too low to include in the TAF at this time. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...Winter Weather Advisory until 6 PM MDT this evening for WYZ112. Winter Storm Warning until 6 PM MDT this evening for WYZ114. Dense Fog Advisory until 8 AM MDT this morning for WYZ116>118. NE...None. && $$ SHORT TERM...BW LONG TERM...TJT AVIATION...TJT