060 FXUS63 KUNR 190423 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 1023 PM MDT Mon Oct 18 2021 .DISCUSSION...(This Evening Through Monday) Issued at 238 PM MDT Mon Oct 18 2021 Satellite imagery this aftn shows an upper low over the Great Basin gradually shifting ewd toward the cntrl Rockies. Satellite-derived advected layer PWATs identify two plumes of low-to-midlevel moisture in the vicinity of this cyclone, one originating from the ern Pacific off of the CA coast and another from the tropical Pacific, emerging over MX/the swrn CONUS. Patches of cirrus are evident on IR/vis imagery crossing the nrn Plains/Dakotas in advance of this wave, with otherwise mostly clear skies across the region. Fairly warm and quiet day in general, with temps this hour in the 60s over the hills and 70s over the plains. Approaching system poses several forecasting challenges. First, as primary zone of Q-vector conv and deep fgen approaches the area late tonight into tomorrow AM, how far north will appreciable precip reach? For now, consensus suggests that at least the srn half of nern WY ewd into the Black Hills will experience moderate to heavy precip tomorrow AM into the aftn as PWATs of 150+ percent of normal overspread the area. Farther north, relative lack of forcing for high precip rates coupled with advection of dry air in the 750-650 mb layer by strong northeasterly winds will maintain lighter precip that may struggle to wet bulb entirely to produce heavy snow. Therefore, anticipating a sharp drop off in snow amounts from srn Campbell Co./Weston Co. nwd to the MT border, with locations along/north of I-90 (outside of higher elevations in the Bear Lodge/Black Hills) likely seeing little in the way of travel impacts. Note that if the shifts nwd, even slightly, amounts and impacts could be more significant. For now, have introduced a Winter Storm Warning over srn Campbell Co.--where amounts will likely be highest--and a Winter Weather Advisory from the Weston Co. plains onto the Black Hills. Advisory over portions of the hills may require an upgrade to a warning at some point once we have a better handle on potential for locally higher amounts. As low-to-midlevel cyclone shifts ewd and pivots tomorrow aftn into the evening, potential exists for heavy banded precip from the swrn to w cntrl SD plains. Deep fgen along the nose of a TROWAL extending from the Midwest into wrn SD should support at least one such SW-to- NE oriented band, with the potential for multiple bands of locally heavier precip. Model fcst soundings show thermal profiles capable of being very efficient snow producers, with deep isothermal layers near 0 deg C topped by steep lapse rates, suggesting that these bands could produce very heavy convectively driven snow in a short period of time. Following discussions with neighboring offices and given low confidence in the placement of these locally heavier bands, decided to hold off on any headlines for now. However, given the large percentage of members across EPS, GEFS, and GEPS suites suggesting such a solution (albeit with inconsistent placement), think the probability of at least one narrow band of heavy snow (with amounts over 6") over the wrn/cntrl SD plains is high tomorrow evening into the overnight. Farther southeast, drier midlevels will likely limit precip rates and potential for dynamical cooling; thus, anticipating that far s cntrl SD will remain liquid thru the event. System lingers thru around midday Wednesday over cntrl/s cntrl SD before departing to the east. A weak shortwave/clipper then crosses the n cntrl CONUS late Thursday into Friday, which could bring breezy conditions and a brief shot for precip. However, with fairly dry profiles, think potential for measurable precip is low for most. Predictability approaching the weekend decreases as a complex pattern with numerous shortwaves develops over the wrn CONUS. Guidance consensus suggests another, more amplified trof may approach late weekend/early next week. && .AVIATION...(For the 06Z TAFS Through 06Z Tuesday Night) Issued At 1019 PM MDT Mon Oct 18 2021 VFR conditions will continue thru tonight. After 12z, MVFR/IFR cigs in SHRA will begin to overspread the area from the west. Cigs/vsbys will further deteriorate from west to east during the day. As rain changes to snow across northeast WY and the Black Hills in the afternoon, more widespread IFR and even areas of LIFR are possible. These very poor conditions will then move into the far western SD foothills/plains in the evening. Gusty N/NW winds will also develop by mid-morning, especially over the wrn SD plains, with gusts near or above 35 kts possible. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...Winter Weather Advisory from noon Tuesday to 6 AM MDT Wednesday for SDZ024-028-029. WY...Winter Weather Advisory from noon Tuesday to 6 AM MDT Wednesday for WYZ057. Winter Storm Warning from 6 AM Tuesday to midnight MDT Tuesday night for WYZ055. Winter Weather Advisory from noon Tuesday to midnight MDT Tuesday night for WYZ058. && $$ DISCUSSION...Sherburn AVIATION...Johnson