463 FXUS65 KBYZ 230249 AFDBYZ Area Forecast Discussion...Updated National Weather Service Billings MT 849 PM MDT Thu Oct 22 2020 .UPDATE... Light snow was tapering off over southeast Montana and all models continue that trend through the evening and early night time hours. Have cancelled the remaining Advisories for the area. Band of persistent light snow and low clouds were sitting over parts of south central Montana. Upslope was weakening quite a bit, so believe the snow will taper off shortly, but extended low PoPs through the rest of the evening for parts of south central and southeast Montana. High resolution models continue to show fog developing over south central Montana and spread it eastward through the night, with additional development for Fallon and Carter counties. Adjusted lows down a degree or two in spots to grab onto numbers a little closer to guidance, as should clouds break up late, readings could fall in short order. TWH && .DISCUSSION... Tonight through Friday night... Upper trof swinging through the area is still producing areas of lighter snow. Precip is quite shallow but with a moist dendritic layer there are flakes being generated by upslope flow. Have cancelled a few more of our winter highlights, but have left southern Big Horn and Sheridan counties eastward to go a bit longer. Sheridan area web cameras continue to show poor road conditions, and we have received similar reports for Lame Deer Divide. Wet roads will re-freeze quickly once we get past sunset as daytime temps are only in the 20s. As surface high pressure settles over eastern MT and the western Dakotas tonight, boundary layer moisture will remain elevated, with light upslope winds and at least partial clearing. Ingredients are in place for a night of fog/stratus especially for the western half of our cwa, and indeed this is exactly what the latest high models are suggesting. Localized valley fog is possible in our east as well, but air mass will be drier and thus confidence is lower. Friday will be mostly dry but with a chance of snow emerging by late afternoon in our west as we trend toward strong isentropic ascent ahead of digging British Columbia trof. Temps tomorrow will stay below freezing as the cold air mass stays locked on the east side of the mountains, so precip type is not in question. The emerging strong ascent and deepening upslope winds will bring widespread snowfall Friday night. Heaviest snow initially should be in our northwestern cwa (Melville, Harlowton, Ryegate) due to the very deep upslope flow and dendritic layer...but there are some model differences among the models with regard to how soon mid level winds turn easterly in our cwa. Tough to pinpoint exactly how the heaviest snow will evolve just yet, but this looks like a significant event for most of our cwa so have issued Winter Storm Watches. See extended discussion below for weekend details. Temperatures will remain unseasonably cold. Record lows are a very good bet at all of our main climate stations tonight, and will be close again Friday night. Billings record low for today (20F) will probably fall this evening. Look for high temps on Friday only in the 20s to around 30. Keep in mind that our normals are in the 50s. JKL Saturday through Thursday... We are still expecting widespread accumulating snow over the area on Saturday with plenty of moisture, good dynamics and upslope flow in place. Taking a look at model soundings, the dendritic growth zone will be quite deep on Saturday, favoring high snow ratios. Snow looks to end from the northwest to the southeast on Sunday, with most of the snow done by the afternoon as the forcing slides east and high pressure builds into the region. Along with the snow, gusty northerly winds will create some blowing snow over the area along with low wind chill values. Significant accumulations are expected across the area, with the greatest totals over central and western zones, where 6 to 12 inches of snow are possible. Higher totals are possible in the mountains, especially north and east facing aspects. Lesser, but still impactful totals are possible over the east. Along with the snow, record cold temperatures are forecast through Sunday. Highs will range from the teens and 20s on Saturday, with Sunday seeing temperatures only in the teens. Low temperatures in the single digits above and below zero are forecast for Saturday and Sunday nights. Drier conditions and moderating temperatures build into the region for the remainder of the long term as high pressure builds over the West. Downslope winds develop over the west Sunday night into Monday, which could cause areas of blowing snow. Will need to monitor for this potential in the coming days. Highs in the 20s and lower 30s on Monday will warm up into the 30s and 40s on Tuesday, with middle to upper 40s over the area by the middle of the week. STP && .AVIATION... Areas of light snow and flurries will dissipate late this evening. MVFR to local LIFR will gradually improve, and VFR should prevail in the east late tonight. There is a good chance of fog/stratus from KBIL-KSHR and westward beginning after 03z and continuing into early Friday. Fog/stratus could also form over far southeast Montana, from Baker to Ekalaka. Localized IFR or lower flight conditions are likely to occur where fog develops. JKL/TWH && .PRELIMINARY POINT TEMP/POPS... Fri Sat Sun Mon Tue Wed Thu ----------------------------------------------------------- BIL 010/027 018/022 004/015 901/025 014/037 028/043 029/045 21/B 9+/J 92/J 00/U 10/B 01/B 10/U LVM 009/031 016/023 902/017 904/034 014/044 026/050 028/052 12/S ++/J 61/B 00/U 10/N 11/B 10/U HDN 010/029 019/025 004/016 906/026 007/039 024/044 025/046 30/B 8+/J 92/J 00/U 11/B 10/B 10/U MLS 008/027 018/024 007/017 904/023 010/037 026/044 026/045 00/B 7+/J 81/B 00/U 11/B 10/B 10/B 4BQ 009/029 019/025 008/016 903/025 010/039 026/045 028/048 30/B 4+/J +2/J 00/U 01/B 10/B 00/U BHK 007/026 015/022 007/017 903/023 010/036 024/042 026/043 30/B 49/J 82/J 00/U 11/B 10/B 00/U SHR 006/032 017/029 003/016 907/029 007/042 023/049 025/052 30/B 3+/J +4/J 00/U 11/B 10/B 10/U && .BYZ WATCHES/WARNINGS/ADVISORIES... MT...Winter Storm Watch in effect from Friday evening through Sunday morning FOR ZONES 29>31-34-40-42-56-57-63>68-141-170-172-173-228-235. Winter Storm Watch in effect from Saturday morning through Sunday afternoon FOR ZONES 36-37-58-138-139-169. WY...Winter Storm Watch in effect from Saturday morning through Sunday afternoon FOR ZONE 199. && $$ weather.gov/billings