334 FXUS65 KGJT 111106 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 506 AM MDT Thu Oct 11 2018 .SHORT TERM...(Today through Friday) Issued at 302 AM MDT Thu Oct 11 2018 Precipitation has began to spread across the area ahead of a positively tiled midlevel trough over the Intermountain West. A subtropical jet is migrating northward within the warm advection regime, and will likely providing some large scale ascent in the left exit region that will move over the forecast area later today. In conjunction with the sub-tropical jet water vapor imagery is showing a vorticity maximum currently over the CA/NV state line. This feature is forecasted to lift out over Utah and Colorado later today producing some differential vorticity advection over the region. The primary focus for lift though will be a band of midlevel frontogenesis that should develop this morning. Model consensus places this band from Moab to into the central CO mountains. As a result valley rain and mountain snow increases in coverage and intensity over the next few hours. Snow levels currently range from 5,500 feet in northern Colorado to 8,500 feet in the San Juans, so some locations in the northern valleys could begin as snow before transitioning to a mix or all rain. Snow levels gradually lift through out the day keeping a majority of the snow accumulations above 9,000 feet. There is still a chance that dynamic cooling within the banded precipitation could prevent the temperatures from warming resulting in lower snow levels than anticipated. Another area of precipitation looks possible in the San Juans most likely associated with ascent from the approaching jet and orographic lift. Snow accumulations of 2 to 6 inches are possible mainly above 9000 feet. The main precipitation band will begin to dissipate this afternoon and early evening at the same time the shortwave trough lifts overhead. There will probably be a transition to more convective precipitation during this time frame with the increasing instability. In the evening another piece of shortwave energy swings through the western trough axis and clips northern Utah and Colorado potentially enhancing the orographics on the western slopes. Influence from the northern trough will result in a quick round of rain to the northern valleys and some mountain snow to the Uintas, Elkhead Mountains and Park Range. All of the precipitation comes to an end late this evening as dry air advects southward behind the northern shortwave, which is currently evident on the latest water vapor imagery. The highest snow totals by late this evening will occur in the band stretching from the Uncompahgre into the central mountains where 5 to 10 inches are possible with localized amounts up to 14 inches. Tomorrow appears to be dry as the western trough becomes cut off over the eastern Pacific. .LONG TERM...(Friday night through Wednesday) Issued at 302 AM MDT Thu Oct 11 2018 Cool, dry air will filter into the region from the northwest Friday night and Saturday. Skies will quickly clear leading to chilly overnight lows Friday night. Both the Lower Yampa and Central Colorado River Basins may need to be monitored closely over the coming shifts for any potential freeze highlights as temperatures drop into the upper 20s to low 30s. The upper level gradient will begin to tighten Saturday night into Sunday as the next Pacific trough digs into the west-central CONUS. Sergio continues to not be a threat for eastern Utah and western Colorado as the arrival of the elongated trough deflects its remnants to our south. Precipitation looks to begin as early as Saturday night with isolated showers developing across the northern tier of the forecast area, favoring the Continental Divide. Model solutions differ in regards to exactly how much moisture will accompany the passage of the trough on Sunday with the Euro being out the outlier at this time. Both the GFS and the Canadian keep the bulk of precipitation on the Front Range while the Euro digs the trough deeper into west-central Colorado, allowing showers to reach the southwest portion of the state. Regardless, a drastically cooler airmass will push into the region in the wake of the trough on Sunday night with 700 mb temperatures spanning from -7 to -11 degrees C from I-70 north and from -2 to -6 degrees C further south. Most, if not all, of the lower valleys stand a good chance of dropping below freezing during the overnight hours Sunday. The strong ridge in the Pacific looks to migrate slowly eastward through the first part of the work week which should bring a gradual warming trend through midweek. && .AVIATION...(For the 12Z TAFS through 12Z Friday morning) Issued at 502 AM MDT Thu Oct 11 2018 Showers will continue to spread from east-central Utah into western Colorado today as a cold front moves across the area. A band of rain has set up in a line extending from the La Sals northeast into the Elk Mountains. It is expected to last through the early afternoon with moderate rates at times. As has been the trend this week, TAF sites can experience MVFR to IFR CIGS and lower VIS in showers. Mountains will continue to be obscured at times due to snowfall and low clouds. Showers are expected later today, which will begin to taper off from west to east after 03-06Z tonight. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...Winter Weather Advisory until midnight MDT tonight for COZ009- 010-012-013. UT...None. && $$ SHORT TERM...KJS LONG TERM...MMS AVIATION...KJS