Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000 FXUS65 KCYS 272335 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 535 PM MDT Wed Mar 27 2024 .KEY MESSAGES... - Occasional wind gusts up to 60 MPH are possible near Arlington and Elk Mountain this evening and overnight. High Wind Warnings are in effect. - Warmer Thursday. Widespread wind gusts 45-55 MPH expected along and west of I-25 during the afternoon. - A slow moving weather system will bring an extended period with chances for precipitation from Friday through Monday. Specific details remain uncertain at this time. && .SHORT TERM /THROUGH FRIDAY NIGHT/... Issued at 221 PM MDT Wed Mar 27 2024 A High Wind Warning remains in effect for Arlington/Elk Mountain from 6 PM MDT this evening until 9 AM MDT Thursday. Winds should continue to increase into the late afternoon & early evening, as H7/H85 CAG-CPR gradients climb to 55 to 60 meters in response to surface pressure falls across eastern MT ahead of a weak clipper system. H7-H8 flow over ARL is still not particularly impressive through tonight. However, WAA aloft should generate a sufficient inversion near mountain top to support enhanced gap flows. Still seeing near equal probabilities of elevated (45-57 MPH) and high wind (58+ MPH) gusts portrayed by in-house random forest models, so a marginal high wind episode still looks probable. Winds will likely remain sub-warning for the other wind corridors (BRX/VDW) as the gradient orientation is less-than-optimal, despite decent flow aloft. The threat for 60+ MPH gusts is likely to come to an end by sunrise Thursday, but may actually persist beyond the 15z expiration as lapse rates steepen by late Thursday morning. Will need to consider potential for 55-60 MPH gusts impacting Rawlins as well on Thursday afternoon as surface pressure gradients seem to get quite strong across Carbon County per the GFS in response to a strengthening surface high over western Colorado. Generally would prefer to see stronger flow aloft and MAV/MET guidance at/ above 30 knots for sustained winds, but it is certainly possible to get a rogue gust or two. Will continue to max out gusts at 55 MPH for now, especially w/ the potential for greater coverage of low and mid-level cloud cover. A warming trend is likely through Friday as upper-level ridging builds into the region. Mainly dry on Thursday, with the exception of a few rain or snow showers in western areas, especially in the high country. Rain/snow chances should begin to increase by Friday afternoon given increasing SW flow aloft ahead of a deepening western US trough. Deterministic models vary greatly in the initial evolution of this wave, but a trend toward cooler and more unsettled weather appears likely as we head into the weekend. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Issued at 221 PM MDT Wed Mar 27 2024 A complex storm system is expected to traverse through the area this weekend as a split trough moves across the western CONUS. The first phase of the storm is described above in the short term section. As the strong closed low digs down just off of the California coast on Saturday, downstream amplification of the ridge over the high plains will push the stationary frontal boundary back to the north and usher in decent warm air advection. Southerly flow overrunning the frontal boundary should keep precipitation going over the boundary, but this will shift north through the day, decreasing PoPs across our area into Saturday, particularly further south. There may be some weak vorticity maximums ejecting along this boundary over the weekend, which reduces confidence somewhat in specific timing of precipitation. A boundary straddling or just north of the area also results in a fairly low confidence temperature forecast for Saturday and Sunday. For now, leaning that warm air will make it into the I- 80 corridor, so nudged to the NBM 75th percentile here, while nudged to the 25th percentile for the northern portion of the forecast area which may remain on the other side of the boundary. Additionally, the relative lull with the northward frontal shift will mainly apply to areas east of the Laramie range. Further west, moist westerly flow and frontogenesis will keep precipitation going across Carbon and Albany counties Saturday and Saturday night, with some models show fairly intense banded precipitation during this period. Thus, while the 18z Sat to 18z Sun is a relative minimum in 24-hour precipitation totals for Cheyenne, this is the maximum 24-hour period for Rawlins. On Sunday into Monday, the closed low will traverse across the desert southwest. Models differ somewhat still in the strength and degree of negative tilt of the southern branch trough, as well as its proximity to the northern branch trough. All of these will impact our precipitation outcome for Sunday into Monday. The overall setup is that the trough moving east of the Rockies will support lee cyclogenesis over east central Colorado. As the surface low strengthens, it will draw the frontal boundary back to the south and bring colder temperatures through our area, along with a change over back to snow, for areas of the high plains that warmed up enough for rain on Saturday or Sunday morning. Northeasterly upslope flow with some wrap-around moisture will lead to more widespread precipitation Sunday night into Monday over the high plains. However, even though we have yet another Colorado low, its disorganized nature is likely to cut into precipitation totals. The development of a secondary closed low somewhere over Wyoming or Colorado will also be key in determining if the band of heavier precipitation on Sunday/Monday will be across east central Wyoming into SD and northern NE, or more focused along the I-80 corridor. With uncertainty still quite considerable, trended QPF down somewhat below a fairly aggressive NBM, and closer to the ECMWF ensemble and GEFS mean. Ensemble mean 700-mb temperatures drop to around -8C late Monday in the wake of this system, so expect temperatures to drop back below normal, but not as cold as last week`s cold snap. Expect a fairly quick recover also, as the low moves out by Tuesday and strong ridging edges in on top of it from the northwest. Look for highs climbing back close to normal on Tuesday, and then above normal by Wednesday. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
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Issued at 530 PM MDT Wed Mar 27 2024 The flow aloft will turn westerly tonight, then west southwesterly on Thursday. Wyoming TAFS...Scattered to broken clouds from 7000 to 15000 feet will occur. Winds will gust to 40 knots until 03Z, then to 38 knots after 14Z Thursday. Nebraska TAFS...Scattered to broken clouds near 8000 feet will occur early this evening, then skies will be mostly clear with scattered clouds near 15000 feet after daybreak on Thursday. Winds will gust to 35 knots until 01Z, then to 25 knots at Scottsbluff and Sidney after 16Z Thursday.
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&& .CYS WATCHES/WARNINGS/ADVISORIES...
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WY...High Wind Warning until 9 AM MDT Thursday for WYZ110. NE...None.
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&& $$ SHORT TERM...CLH LONG TERM...MN AVIATION...RUBIN

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