812 FXUS65 KTFX 170456 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 1055 PM MDT Tue Apr 16 2013 Updated Aviation .UPDATE... Tonight...A deep trof remains over the region but will slowly be moving east during the night. Moist, northeast flow at low levels will keep periods of light snow across mainly north and central Montana but additional snow accumulations will generally remain below an inch. Precipitation totals have stopped accumulating at the snotels over the Rockies and have cancelled the warning for the Northern Rockies with this update. Have also issued a winter weather advisory for passes in Lewis and Clark county where snow has caused poor driving conditions. An earlier update addressed changes to POPs and WX. Zelzer && .AVIATION... UPDATED 0500Z. Occasional light snow and low cigs will persist as moist north-northeasterly flow continues to reside over central and north-central Montana. VFR and MVFR will be predominant, however within bands of heavier snow, IFR conditions are possible for KBZN, KGTF and KLWT terminals. At KHLN, KCTB and KHVR, snow has tapered and cigs are gradually increasing, however the ample low-level moisture may lead to fog so have added VCFG and BR to these locations late tonight into early Wednesday morning. Mountains will remain obscured through the period. Conditions will gradually improve by Wednesday afternoon. MLV && .PREV DISCUSSION... /ISSUED 540 PM MDT Tue Apr 16 2013/ Tonight through Thursday...Broad upper trof remains over the area as well as a cold airmass. At the surface, north to northeast winds continue to bring periods of upslope light snow. A winter storm warning continues through the evening for the Northern Rocky Mountain Front while snow accumulations over the plains will remain light. The upper trof will begin to push slowly east late tonight with snow gradually coming to an end after midnight. Areas of fog may develop after midnight as well. As an upper ridge builds over the Pacific Northwest on Wednesday, drier air will begin to move across the area, though isolated snow showers will linger over the eastern portion of the county warning area as well as the southwest mountains. Dry and warmer conditions will grace the area Thursday as 500mb heights continue to rise. Westerly surface winds will become breezy Thursday afternoon. Temperatures will remain well below seasonal averages Wednesday but will rebound to near seasonal averages Thursday afternoon. Emanuel Thursday night through Tuesday... Models have continued to digress over the potential impact to the area from a series of shortwave troughs that will move along the periphery of the large scale upper level ridge located off the Pacific Coast. Have trended with the more consistent ECMWF concerning temperatures and pops from the shortwaves starting Friday Night and Saturday and continuing into the first part of the Work week. The cold front associated with the larger system late Sunday and early Monday could significantly cool temperatures in the long term. Have trended towards this in the forecast...instead of the warmer and milder GFS. This longer cooler situation appears to continue in the ECMWF solution...where the GFS quickly moves the system and the cooler air mass out of the area and has temperatures return to seasonal normals by Tuesday. Suk 12Z model runs have continued to trend toward the cooler/wetter solution that the ECMWF model began advertising yesterday, leading to greater confidence that another cold airmass will slide south into the forecast area late this weekend. Timing of the initial frontal passage is still not well agreed upon, but model trends indicate a somewhat quicker arrival sometime Saturday night or early Sunday. Have lowered temperatures Saturday night through Monday with the Sunday/Sunday night period appearing to be the coldest with temperatures again cooling to around 15-20 degrees below seasonal averages. Models also in general agreement on bringing a period of precipitation to much of north-central and central MT during the Saturday night and Sunday period and have raised pops considerably. Hoenisch && .PRELIMINARY POINT TEMPS/POPS... GTF 19 37 21 51 / 50 20 10 0 CTB 13 37 21 49 / 60 10 10 0 HLN 19 40 21 51 / 40 20 10 10 BZN 19 39 18 49 / 40 20 10 10 WEY 8 35 6 41 / 20 10 10 10 DLN 17 39 17 48 / 20 20 10 10 HVR 19 39 21 51 / 60 20 10 0 LWT 16 31 15 45 / 50 30 10 10 && .TFX WATCHES/WARNINGS/ADVISORIES... WINTER WEATHER ADVISORY until 6 AM MDT Wednesday Central and Southern Lewis and Clark. && $$ weather.gov/greatfalls