452 FXUS63 KUNR 191035 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 435 AM MDT Tue Oct 19 2021 .DISCUSSION...(Today Through Monday) Issued at 304 AM MDT Tue Oct 19 2021 Upper low is currently spinning over Utah, with a 110- kt jet rounding its base. Clouds are spreading northward over northeastern WY, and rain is starting to push into southern Campbell County. Cold front is nearly through the CWA; nonetheless, temperatures at 2 AM are still in the 40s and 50s, and winds are relatively light. Models are in good agreement on the track and timing of the low pressure system, slowing it down slightly. Precipitation will spread from west to east today as the 700 mb low follows the SD/NE border, first as rain, then changing to snow most places overnight. Cooler areas (northeastern WY and BH) will see snow with some minor accumulations today. The period of highest snowfall rates is expected this evening and overnight, as temperatures cool and PWATs are still 150% of normal, and during which time there will be impressive lift within the dendritic growth zone. Convection is still a possibility during the afternoon/evening hours as well. Deterministic models and ensembles continue to depict the axis of greatest precip from central WY into southwestern SD and up through northern SD, where places could receive around and inch or more of QPF. Deterministic solutions, however, seem on the higher end of the ensembles, so have adjusted the QPF forecast more towards the ensembles. This resulted in a similar snowfall forecast as previously, whereas the deterministic solutions are showing higher snow amounts. Froude number less than one suggests blocked flow and that the upslope precipitation would fall along the northern/northeastern foothills, rather than the northern Black Hills. Have increased QPF/snow for the Spearfish/Sturgis areas and added them to the advisory; the Black Hills remain in an advisory as well. Also added northern Campbell County for the potential of a few inches of snow in Gillette, and adjusted headline times for the slower storm movement. Northerly/northeasterly winds will be breezy today, and strong just east/southeast of the Black Hills. Have issued a Wind Advisory for these areas, as winds are expected to decrease before rain changes to snow. If snow accumulates sooner/faster than expected, Winter Weather Advisories may be needed across portions of the SD plains; however, current forecast cross-checked with GFS/ECMWF/Canadian ensembles does not warrant additional headlines. Precip will shift out of the area Wednesday, but temperatures will be chilly, with highs around 40. Generally dry and mild weather is expected through the end of the week, as an upper ridge builds. A few sprinkles/flurries will be possible Thursday night as a clipper moves through the ridge. An active weather pattern returns next week. && .AVIATION...(For the 12Z TAFS Through 12Z Wednesday Morning) Issued At 429 AM MDT Tue Oct 19 2021 Widespread IFR conditions will develop over northeastern Wyoming, northwestern South Dakota and the northern Black Hills and adjacent foothills this morning, and then will continue spreading southward into southwestern South Dakota this afternoon and then into south central South Dakota tonight. Rain and snow will spread slowly eastward across the area today and tonight. A few thundestorms are possible over south central South Dakota. Gusty north winds will develop today, especially on the plains to the east of the Black Hills. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...Winter Weather Advisory from 3 PM this afternoon to 6 AM MDT Wednesday for SDZ024-025-028-029-072. Wind Advisory from 9 AM this morning to 6 PM MDT this evening for SDZ026-030-031-073-074. WY...Winter Weather Advisory from 3 PM this afternoon to 6 AM MDT Wednesday for WYZ054-057-058. Winter Storm Warning from noon today to 6 AM MDT Wednesday for WYZ055. && $$ DISCUSSION...Pojorlie AVIATION...10