889 FXUS65 KTFX 020434 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 934 PM MST Sun Dec 1 2013 .UPDATE...Significant winter storm system still on track to move into Montana on Monday. Ahead of this system, a moist west to northwest flow aloft will continue to bring precipitation to the western mountains overnight. Going forecast for tonight appears to be generally on track and have not made any changes to temperatures or pops/weather. However, models continue to struggle with the arrival time of the Canadian cold front and have once again slowed it down by several hours. I have updated wind direction (but not speed) to show a slightly slower shift to north winds across North Central Montana on Monday morning. I am somewhat skeptical of the High Wind Warning for tonight and Monday morning and considered dropping it and lowering wind speeds since there has (so far) not been any strong winds. However, the latest model guidance continues to suggest that winds will increase late tonight so have opted to leave it in place for now. The remainder of the forecast looks to be on track. mpj && .AVIATION... UPDATED 2355Z. A moist and unsettled westerly flow aloft currently over the forecast area will slide south overnight with partial clearing expected. However, a strong upper level low and associated Arctic cold front will sweep southward into Montana by 12Z. Behind the front there will be a strong wind shift to the north with rapidly lowering ceilings and visibility in developing snow and blowing snow. By late afternoon on Monday LIFR/IFR conditions should prevail over North Central Montana with IFR/MVFR conditions in Southwest Montana. mpj && .PREV DISCUSSION... /ISSUED 455 PM MST Sun Dec 1 2013/ Tonight through Tuesday...Strong upper level low currently centered near Queen Charlotte Island along the BC coast will descend south into the Pacific NW tonight and into the northern Rockies Mon Night. Strong/moist westerly flow will continue to produce snow across the mtns tonight with lighter rain/snow showers at lower elevations. Winds aloft from the west peak around midnight tonight and the set of high wind/winter weather warnings already in effect look good through tonight. Arctic frontal boundary currently pushing south into central Alberta should reach the MT/AB border around 08-10UTC and continue south through n-central MT Monday morning, continuing south through the remainder of the forecast area Monday afternoon. Shift to north winds and will end the threat for wind gusts in excess of 50kts, but north winds behind the front will remain fairly strong at 20-30kts. As the upper low center reaches the region on Monday except a band of frontogenetic precipitation to develop over southwest MT, where marginal instability could also create some localized bursts of heavier snow. to north, low clouds and light snow will develop behind the Arctic front Monday afternoon then intensify late Monday afternoon and evening as the upper level energy move across the area and upslope northerly flow deepens. snow will decrease in intensity on Tuesday as a very cold airmass deepens over the region and continued breezy north winds create wind chill values of -20 to 30. Will consolidate the winter weather highlights across north central MT into a warning for all elevations as even areas that receive lesser accumulations will see impacts from blowing snow and very low low wind chills. for the southwest will maintain the winter storm warning in the mtns and add a winter weather advisory for the valleys. Hoenisch Wednesday through Monday...Coldest air of the season and coldest temperatures in several years, at least for an extended duration, are poised to impact Montana during this time. The last time Great Falls reached -20 F or colder was January 2012. A deep 500mb trough axis will remain over the Northern Rockies through this period. Mid-level flow is directed from north of the Arctic Circle into the Western U.S. which will keep the bitterly cold temperatures entrenched in our area into the upcoming weekend. Highs on Thursday and Friday will be near zero to slightly below. However, for parts of north-central Montana, north/northeast of Great Falls, highs may struggle to reach -10 F. Models in agreement with 850mb temperatures in the low to mid 20s C below zero on Friday. The coldest overnight lows are forecasted for Thursday night with widespread -15 to -25 F readings possible. 1000-500mb thickness values start out around 500 dm on Wednesday, potentially dropping to 492 dm near the Canadian border by Saturday morning. The 12z/Sunday ECMWF brings an upper level disturbance out of Northern Canada on Friday/Saturday. An associated surface low pressure system then tracks to the southwest of Montana. If the ECMWF verifies, even colder temperatures would be possible on Saturday and Sunday as strong cold air advection on the northern side of the surface low drops 850mb temperatures to near -30 C. Trended temperatures down below consensus for next weekend as the ECMWF has been consistent in bringing in the reinforcing shot of arctic air and the GFS has been trending colder. Additionally, confidence is high that an extensive snow cover will exist at this time from the system early in the week leading to strong radiational cooling at night. Uttech && .PRELIMINARY POINT TEMPS/POPS... GTF 31 31 1 2 / 20 70 100 90 CTB 23 23 -6 0 / 20 100 100 70 HLN 33 36 3 10 / 30 50 80 50 BZN 32 39 3 9 / 30 70 70 50 WEY 26 34 -9 7 / 100 100 90 50 DLN 34 38 5 11 / 30 60 60 40 HVR 26 30 4 8 / 60 80 100 90 LWT 31 35 -1 3 / 30 60 90 90 && .TFX WATCHES/WARNINGS/ADVISORIES... HIGH WIND WARNING until 5 AM MST Monday Cascade...Central and Southern Lewis and Clark...Judith Basin...Northern Rocky Mountain Front...Southern Rocky Mountain Front. WINTER WEATHER ADVISORY from 11 AM Monday to 11 PM MST Tuesday FOR ELEVATIONS BELOW 6000 FEET FOR Beaverhead...Broadwater... Gallatin...Jefferson...Madison. WINTER STORM WARNING until 11 PM MST Tuesday FOR ELEVATIONS ABOVE 6000 FEET FOR Cascade...Central and Southern Lewis and Clark...Judith Basin...Meagher...Southern Rocky Mountain Front. WINTER STORM WARNING from 3 AM Monday to 11 PM MST Tuesday Blaine...Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton...Fergus...Hill...Liberty...Toole. WINTER STORM WARNING until 11 PM MST Tuesday FOR ELEVATIONS ABOVE 6000 FEET FOR Beaverhead...Broadwater...Gallatin... Jefferson...Madison. HIGH WIND WARNING until 11 AM MST Monday Gallatin...Madison. WINTER STORM WARNING until 11 PM MST Tuesday Northern Rocky Mountain Front. && $$ weather.gov/greatfalls www.twitter.com/NWSGreatFalls www.facebook.com/US.NationalWeatherService.GreatFalls.gov