862 FXUS65 KTFX 230348 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 845 PM MST Sun Jan 22 2017 .UPDATE... 845 pm...Freezing fog has become quite dense in spots this evening across North Central Montana, especially along the north and east facing mountain slopes due to the northeast winds. Light snow has already started to move into this area as well. Have therefore increased the chance of snow and issued a Freezing Fog Advisory until 11 am Monday. The combination of potentially icy roadways and bridges and light snow accumulation will make the Monday morning commute quite a bit more difficult. Have also added mention of patchy freezing fog to the valleys of Southwest Montana with the increasing potential for light snow. Remainder of the forecast appears to be on track. Coulston 455 pm...Quick update to add fog to all of North Central and Central Montana through Monday morning. Coulston && .AVIATION... Updated 2332Z. Pesky low clouds and occasional freezing fog are expected to persist, especially this evening, over much of North-Central MT, including KCTB/KHVR/KGTF. Expect IFR or worse at times in these areas. Light snow will overspread Southwest and North-Central MT from SW to NE this evening into early Monday morning and then likely persist through 00Z/Tue. Expect at least MVFR/IFR CIGS/VSBY in this snow. Mountains will tend to be obscured during the period. Jaszka && .PREV DISCUSSION... /ISSUED 432 PM MST Sun Jan 22 2017/ This afternoon through Tuesday...Water vapor imagery pretty much tells this story this afternoon. A well defined, deep upper level low continues to spin just off the WA/OR coast at this time. On the south side of the low, a strong upper level jet continues to run right into the state of California. On the leading edge of this jet, a mid-level warm front is steadily progressing NE through the Pac NW at this time. Broad, diffluent flow aloft combined with warm air advection along the warm front and large scale ascent associated with the jet appears to be driving the precip across the region at this time. At the SFC, high pressure has been nosing south into central Montana and has been associated with a very shallow, moist layer near the SFC. This has led to a persistent stratus deck from Cut Bank to Havre. It has shown very little movement this afternoon and will likely remain in place the rest of this afternoon, producing patchy freezing fog and reduced visibility at times. The 12Z TFX sounding shows quite a bit of dry air in the mid/upper levels at this time. However, continued moisture advection should continue to moisten the column areawide which, combined with the large scale ascent should allow light snow to break out CWA wide by tonight as the aforementioned warm front lifts NE through the area. Timing-wise, radar and webcams suggest snow is already beginning to reach the SFC across SW MT at and above pass level as of 2pm. The snow will take a bit longer to reach the valley floors, though, due to the drier air at the SFC. Here, this will likely limit the coverage of snow through tonight. From Great Falls to Lewistown, the snow should arrive this evening, then reach the Hi-line by Midnight. The warm front will stall over central Montana on Monday, then more or less return south as a cold front later Monday into Tuesday. This will allow a moist, northerly flow to keep light snow going across much of the area through Monday night. The snow should then begin to taper off from north to south on Tuesday as drier air filters in from the north. Regarding snowfall amounts, I expect a general 1-3" across much of the area, although for reasons mentioned above, the SW valleys may only see an inch or less. The greatest snowfall totals are expected across far SW MT along the MT/ID border where 4-8" is expected. Of note, there still exists the possibility of some mesoscale banding of the snow in this area on Monday, but the models continue to differ on the handling of the mid-level low as it tracks through southern ID and into Wyoming. Interestingly, most models have trended further south with the low track, but the Canadian insists on a more northerly track. For this reason, confidence in heavier snowfall rates/totals in this area remains low. Given the confidence issues, we'll continue to hold off on any winter weather highlights for now in this area until the models can come into better agreement. Regardless, travel impacts are likely over Monida and Raynolds Passes, but how significant remains to be determined. Elsewhere, lighter snowfall amounts/rates should keep impacts at a minimum. That said, coming after a relatively milder stretch, travel will certainly be impacted with the fresh coating of snow. One last note. Lingering low level moisture later Monday or Tuesday combined with drying aloft may actually allow some freezing drizzle to develop, mainly south of a Great Falls to Lewistown line. We'll continue to monitor this possibility, but for now opted to leave out of the forecast as not all models show this. Martin Tuesday night through Sunday...Northerly flow aloft will persist across the region through late this week with weak embedded shortwave energy bringing a slight chance for snow showers Tuesday night through Wednesday. Temperatures begin on the cool side, slightly below seasonal averages, but gradually warm late this week into next weekend as an upper level ridge amplifying along the west coast gradually shifts inland. Warming aloft will initially be realized across the plains of N-central MT first as a lee-side surface trough of low pressure brings breezy W/SW winds late this week into the weekend. Surface high pressure situated over the interior western US late this week will lead to temperature inversions in SW MT valleys where warming will be subdued with some air quality impacts also possible. By late next weekend, models indicate the upper ridge will flatten as energy moves into western Canada, resulting in strengthening winds and continued mild temperatures across N-central MT. Hoenisch && .PRELIMINARY POINT TEMPS/POPS... GTF 15 27 13 25 / 80 80 50 20 CTB 14 25 11 24 / 80 80 40 20 HLN 11 22 6 19 / 70 20 30 20 BZN 13 25 5 19 / 50 30 50 30 WEY 14 24 0 18 / 80 70 70 20 DLN 14 30 10 24 / 60 30 30 20 HVR 16 27 13 23 / 60 60 40 20 LWT 15 25 13 21 / 70 70 60 30 && .TFX WATCHES/WARNINGS/ADVISORIES... Freezing Fog Advisory until 11 AM MST Monday Cascade... Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton... Fergus...Judith Basin...Liberty...Northern Rocky Mountain Front...Southern Rocky Mountain Front...Toole. && $$ weather.gov/greatfalls