Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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676 FXUS65 KBOU 232341 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 441 PM MST Sat Nov 23 2024 .KEY MESSAGES... - Mountain snow and light low-elevation snow late Sunday and Sunday night. - Heavier mountain snow late Monday night through Wednesday night with widespread and significant travel impacts likely. - Increasing potential for snow across the lower elevations Tuesday night and Wednesday. - Chilly but mostly dry Thanksgiving Day. && .SHORT TERM /Through Sunday/... Issued at 146 PM MST Sat Nov 23 2024 Current observations across METARs display afternoon temperatures on track to reach above normal temperatures through the early evening hours across the region. Gusty southwest winds produce wind gusts up 35 mph continue over the higher terrain this evening due to an upper level jet. Cross sections display incoming upper level moisture tonight thus partly cloud skies are expected for most the of the lower elevations and foothills. Additionally, tonight`s temperatures slightly above normal as the plains reach 25-36F and mountains reach 14-27F. Our upper level pattern quickly changes by Sunday afternoon as a shortwave trough brings Pacific moisture through northwest flow. Orographic flow should lead to 2-5 inches mainly for areas above 9 thousand feet north of the I-70 corridor. A few mountain valleys in Jackson and Grand counties could also receive 1-2 inches of snow through Sunday. There is potential for the incoming cold front to arrive late Sunday afternoon shifting surface winds northwest. With mid to upper level cloud cover increasing by Sunday afternoon, it is possible a few areas across the plains may struggle to reach the mid 50s. Thus, this update includes a decrease in afternoon highs across the eastern plains. A few areas along the I-25 corridor will be able to peak through the cloud cover but expect upper 40s to mid 50s for the lower elevations. High temperatures for the high country reach the low 30s to low 40s. && .LONG TERM /Sunday night through Saturday/... Issued at 146 PM MST Sat Nov 23 2024 Widespread snow is expected in the mountains Sunday evening as the tail end of an upper level trough and left exit region provide lift. Cross sections show saturated air up to roughly 450mb. However, lapse rates will only be 5-7 C/km and mountain top winds end up being relatively weak (less than 20 knots). Add this up and the orographic lift component of this system is weak. Going snowfall forecast of 2-6 inches and up to 3 inches in the valleys looks on track. For areas east of the mountains...the foothills, urban corridor and plains, there seems to be decent model agreement between the GFS and ECMWF models and their ensembles with a period of light snowfall Sunday night. However, the NAM/RAP/HRRR models don`t have a clue. NAM cross sections for Sunday night show barely enough moisture for clouds, let alone snowfall. The NAM/RAP/HRRR keep primarily northwest (downslope) winds for most of Sunday night. With surface high pressure building in from the north, we should see a period of northeast to east winds as the GFS and ECMWF are advertising. These east to northeast winds lead to convergence along the foothills and upslope flow. Temperatures appear cold enough for all snow, though can`t rule out a little rain at first. Plan to nudge snowfall amounts up, especially on the west side of the urban corridor, in and near the foothills. Snowfall amounts still look to be on the light side, less than 2 inches, but a better chance for slick roads for the Monday morning commute across western portions of the metro area. It quickly clears Monday morning as upper level ridging and subsidence moves bringing mostly sunny skies. Temperatures will be a little cool with highs in the mid to upper 40s across northeast Colorado. For Monday night through Wednesday night, westerly flow aloft will transport Pacific moisture across the intermountain west resulting in snowfall for the Colorado mountains. For the first part of the storm, orographic lift will produce most of the snowfall across the mountains through late Tuesday. Then an upper level trough moves into the region providing lifting across the mountains and also for the lower elevations Tuesday night through Wednesday night. Still plenty of model differences out there regarding this storm. The latest models move up the timing of the trough passage. The GFS and its ensemble members are on the lower end for snowfall with less than a foot of snow for the mountains. The ECMWF and Canadian ensemble members favor 1-2 feet of snow for the mountains. Will trend the forecast towards the heavier amounts given the amount of moisture being transported into the region and the duration of snowfall being 48-72 hours for the mountains. For the lower elevations east of mountains, the upper level trough should bring a decent shot of snow late Tuesday through Wednesday night. Amounts are still quite unclear this far out, but it`s very likely we will see snow covered roads Wednesday and Wednesday night, leading up to Thanksgiving. For Thanksgiving, the upper level trough and snowfall associated with it shifts southeast of the area, with only a few ensemble members hanging onto snow in the morning. However, even though the snow is expected to be finished, roads will likely be slippery Thanksgiving morning. Northwest flow aloft will prevail behind the exiting system Thanksgiving. This combined with fresh snow covered will produce cool temperatures with highs in the 30s across northeast Colorado. For Friday and next weekend, a longwave trough will reside over northeast North America with a high over low set up along the west coast of North America. This pattern will bring northwest flow aloft to Colorado resulting in cool and dry conditions. Temperatures are expected to be below normal with highs in the 30s to lower 40s during this period. && .AVIATION /00Z TAFS through 00Z Monday/... Issued at 436 PM MST Sat Nov 23 2024 Winds will gradually shift to the south by 01z and then more SSW by 05z. On Sun winds will become SW by 13z. Meanwhile, confidence in wind direction by midday is low at best. HRRR has winds becoming gusty from the west while other models have a Denver Cyclone SW of DIA which leads to a more E or NE direction in the aftn. For now have maintained continuty with previous fcst and kept them from the west thru 22z. After 22z the winds may swicth to a more NW component. Meanwhile VFR conditions will continue tonight thru Sun aftn. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 6 AM Sunday to 6 AM MST Monday for COZ031. && $$ SHORT TERM...AD LONG TERM...Meier AVIATION...RPK