094 FXUS65 KTFX 210359 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 859 PM MST Fri Jan 20 2017 .UPDATE... Freshened overnight lows per latest observations and consensus of short-term model guidance. With light winds tonight, patchy freezing fog may develop after midnight where skies remain at least relatively-clear. This includes the Helena Valley. While most will be dry tonight, occasional light snow is possible in far-southern portions of Southwest Montana due to weak lift/sufficient moisture within the 700-300 mb layer ahead of a shortwave trough approaching from the southwest. Any fresh snow accumulation will likely be an inch or less through daybreak. Skies will range from mainly clear in North-Central Montana to partly to mostly cloudy farther south. Jaszka && .AVIATION... Updated 2322Z. VFR conditions are mostly expected across Southwest and North Central Montana, with a few exceptions noted below. Scattered mid and high clouds will move over Southwest Montana overnight ahead of an approaching disturbance, as North Central Montana stays mostly clear. Decreasing winds should allow patchy fog to develop across much of the area between 10Z and 18Z, but recent history shows that the best chance for lower ceilings/visibilities at terminals will be at KHLN, where MVFR conditions. A few snow showers may then start to move into far Southwest Montana (including KEKS and KBZN) after 16Z, while areas north mainly see an increase in mid and high clouds. Coulston && .PREV DISCUSSION... /ISSUED 415 PM MST Fri Jan 20 2017/ This afternoon through Sunday...The period starts with a closed upper level low spinning just off the WA/OR coast. South of the low, water vapor imagery shows a very pronounced 140+ kt jet crashing into California. This jet will remain south of Montana through Saturday, but then it will briefly edge north by Sunday as the upper low opens up/weakens. Through Sunday, a couple of shortwaves rounding the base of the upper low will move NE through our CWA. For the rest of today and tonight, mostly dry conditions are expected with weak ridging aloft and weak high pressure at the SFC. The exception will be far SW MT along the MT/ID border where scattered snow showers will remain possible through tonight. With high pressure at the SFC and light winds tonight, fog will be possible tonight along the Hi-Line and some of the SW valleys. The first of the above-mentioned shortwaves moves through on Saturday. This wave should be weakening as it approaches the area with limited large-scale lifting expected. For this reason, the best chance of snow will be across SW MT, with little, if any, precip further north. Once again, the MT/ID border area will be favored for the greatest coverage of snow and highest snowfall amounts (albeit, fairly light). The next s/w moves through on Sunday and the models suggest this wave will be stronger with better large scale lift as the jet tries to nose into the area. Through Sunday afternoon, it appears the greatest chance of snow will still be across SW MT, coinciding with the approaching area of better lift. It's likely, though, that precip with this wave will continue on beyond Sunday afternoon and even reach areas further north (see the extended discussion for more). At this time, significant impacts from snow are not expected through the short-term period. But, along the MT/ID border, there will likely be enough snow (and maybe even some blowing snow) for at least somewhat slower travel, especially on Sunday and is an area we will keep an eye on for any possible future winter weather products. Temp-wise, no significant airmass change is expected, but a gradual cooling trend is likely through the weekend. Martin Sunday night through Friday...Medium range models are in reasonable agreement through most of next week. Period starts with a longwave upper level trough progressing inland across the western US early in the week followed by a building upper ridge from the middle through later portions of next week. Upper level jet and main shortwave energy associated with the upper trough early next week moves by well to the south of the region, however some weak shortwave energy does lift NE across the area on Monday ahead of the trough axis, followed by additional weak energy embedded within northerly flow behind the trough axis Monday night through Tuesday night. Best chance for a period of widespread precipitation will be with the energy ejecting NE across the region on Monday with southern portions of the forecast area having the best shot at some light snow accumulations. Light snow showers will linger Tuesday and Tuesday night in the northerly flow following the trough axis with drier conditions moving in Wednesday through the remainder of next week. Temperatures will cool to near or slightly below seasonal averages for the first half of next week under the upper level trough. Temperatures gradually moderate/warm late next week. Upper level ridge with surface high pressure over the interior western US late next week may lead to the redevelopment of strong temperature inversions in the valleys of SW MT, resulting in slower warming and possible redevelopment of air quality issues in some SW valley areas. Hoenisch && .PRELIMINARY POINT TEMPS/POPS... GTF 19 35 21 34 / 0 10 10 10 CTB 16 37 19 31 / 0 10 10 10 HLN 13 25 10 26 / 10 20 20 10 BZN 8 26 8 29 / 10 20 30 10 WEY 4 26 6 23 / 20 40 40 50 DLN 12 35 13 32 / 10 30 20 20 HVR 18 34 18 30 / 0 0 10 10 LWT 20 35 20 34 / 0 0 10 10 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ weather.gov/greatfalls