340 FXUS65 KTFX 311728 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 1730 AM MST Mon Dec 31 2012 ...UPDATE TO AVIATION... .DISCUSSION... TODAY THROUGH WEDNESDAY...Shortwave troughs embedded in a northerly flow aloft will keep at least a slight chance of light snow across the TFX CWA through Tuesday. The northerly flow aloft is due to a sharp upper level ridge of high pressure just off the Pacific coast. One shortwave is moving through western Montana this morning, which will keep a good chance of light snow along and west of the Continental Divide today. Another shortwave will move south out of Canada this afternoon and through southern Montana overnight. Forecast models develop little if any precipitation with these disturbances, so am expecting mostly flurries if anything. The last shortwave to affect the area during this period will move more so over eastern Montana on Tuesday, but it will have stronger lift and more moisture associated with it than the previous ones. This will bring a good chance of light snow (with little if any accumulation expected) to the eastern portion of the TFX CWA, with less of a chance for the western half. The upper level ridge off the Pacific coast will then move onshore on Wednesday, forcing the energy in the northerly flow east of the area and bringing drier conditions to the TFX CWA. With high pressure at the surface over the Pacific Northwest and lower pressure east of the Rockies, breezy westerly surface winds will generally prevail along the east slopes of the Rockies. The best chance of widespread breezy winds will be on Tuesday with the passage of a very weak cold front associated with the aforementioned stronger shortwave. Across the northern half of the area, temperatures will not vary much through Tuesday night as highs remain below seasonal averages and lows stay near normal. However, the more wind-protected southwest valleys will generally remain a few degrees cooler than areas to the north. Overall, high temperatures will then warm closer to normal on Wednesday as the upper level ridge approaches the area. Coulston Wednesday night through Monday...A rather quiet weather pattern will continue to reside over the region from Wednesday through Monday. The GFS and EC models do differ a bit through the period...thus leaned more towards the warmer and drier EC model. With an upper level ridge building over the region mostly dry conditions are expected through Friday. Then the flow aloft becomes a bit unstable and a small chance for a passing light snow shower can not be ruled out for Sat thru Mon especially over the Western and Southwestern mountains. In terms of temperatures, expect mild temperatures for Thursday and Friday, with slightly cooler temps for Sat thru Monday. Brusda && .AVIATION... Updated 1730Z. A weak shortwave will sweep from north to south across the forecast area today and tonight. Mid level clouds have already moved over the Rocky Mountain Front and should reach southwest Montana shortly after 00Z. MVFR/VFR conditions should prevail with only isolated to scattered snow showers but widespread mountain obscurations. Cloud cover will diminish from north to south after 12z Tuesday. mpj && .PRELIMINARY POINT TEMPS/POPS... GTF 31 17 31 13 / 0 20 10 10 CTB 30 16 31 12 / 0 10 10 0 HLN 26 12 26 3 / 10 20 10 10 BZN 19 6 21 -4 / 10 20 10 10 WEY 18 -5 18 -8 / 20 20 20 10 DLN 22 7 24 0 / 10 20 10 0 HVR 26 11 28 8 / 20 10 20 20 LWT 28 14 30 10 / 20 20 30 20 && .TFX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...COULSTON LONG TERM...BRUSDA AVIATION...MPJ weather.gov/greatfalls