FXUS65 KBOU 301745 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1045 AM MST Thu Dec 30 2021 .UPDATE... Issued at 1004 AM MST Thu Dec 30 2021 Winds are nothing short of *howling* across our foothills and adjacent communities just to the east. Multiple gusts 80-90 mph have been recorded, with one exceptional gust to 105 mph reported in Rocky Flats shortly before 10 AM. Remarkably, there isn`t all that much surface support (via pressure gradients), but the intense cross-barrier flow and weak shear aloft appears to be enough for significant mountain wave amplification. Expect these winds will continue for a few more hours, before beginning to subside later this afternoon. In the mountains, snow showers are relatively few and far between at this hour, mainly driven by orographics and affecting the higher peaks and passes. That said, with winds as strong as they are, there is abundant blowing snow which is resulting in treacherous driving conditions. The forecast remains on track, with snow expected to increase in coverage this evening and overnight, and even more so on Friday. Front looks slightly delayed for the plains Friday, with snow showers looking unlikely for most areas before noon. && .SHORT TERM...(Today and tonight) Issued at 302 AM MST Thu Dec 30 2021 Nearly zonal flow aloft will be over the area today through tonight as sfc low pres intensifies from sern WY into ern CO. For the mtns, cross-sections show moisture depth rather limited today but does increase by tonight. Orographics certainly are favorable along with lapse rates in the 6-7 c/km. Overall, it appears most mtn areas will see periods of light snow through the day with heavier snowfall developing late this evening and overnight. Also with strong winds expected a lot of blowing snow and poor visibility will occur over the higher passes. Thus no changes have been made to warnings. However, Middle and North Park will need an advisory for snow/blowing snow for tonight which will continue through Friday. Meanwhile, biggest headache tonight has been trying to figure out high wind potential, in and near the foothills, later this morning through the aftn hours. Hi res data has been showing stronger winds, eventually mixing down to the base of the foothills, and possibly affecting the western areas of the I-25 Corridor from Denver north to the Wyoming border. Analysis of cross-sections still don`t show much mtn top stability for a mtn wave to develop. However, some of the higher resolution data does show some amplification late this morning thru mid aftn. Cross- barrier flow is in the 55-70 kt range, so if this does mix down, I certainly can`t rule out strong winds at lower elevations near the base of the foothills for a few hours. HRRR shows gusts up to 90 mph and has these winds spreading further east than some of the other Hi-Res data. Confidence in all of this isn`t high, however, I can`t ignore what I`m seeing so will issue a High Wind Warning for zones 36,38 and 39. Overall appears high wind threat will decrease by 23z. Finally, as far as highs today, will keep readings mainly in the upper 40s to lower 50s across the plains. .LONG TERM...(Friday through Wednesday) Issued at 302 AM MST Thu Dec 30 2021 A large trough will move from the Pacific Northwest towards the Four Corners region on friday as a small shortwave trough moves from Baja California towards El Paso. At the surface, there will be strong lee cyclogenesis across southeastern Colorado while a strong high pressure system slides eastward in Montana. A weak cold front will push across northeast Colorado early on during the day on Friday creating northeasterly winds behind it. Across the higher terrain, there will be ongoing snow showers mainly across the western slopes. As the trough nears, a jet stream aloft will strengthen and low to mid level frontogenesis will create a couple bands of moderate to heavy snowfall. Snowfall rates could approach 2 inches per hour in the heavier bands while snowfall rates around a half inch per hour are expected outside of the heavier bands. A couple of these bands may produce snowfall across the foothills and adjacent plains but snowfall should be rather light since the low to mid level flow will be westerly. During the afternoon, 700 mb flow begins to turn easterly across the northeastern plains creating upslope flow and strong frontogenesis which leads to the development of a few bands of snow showers on the plains. Models aren`t in full agreement yet on the placement of the heaviest bands with the ECMWF keeping these bands farther north than most other models. One thing to keep in mind is that these systems tend to arrive slower and depart much slower than models suggest a couple days before the storm. Therefore, PoPs and snow amounts were kept towards the lower side of guidance on the plains through Friday afternoon. The system will really ramp up Friday night as the depth of the easterly flow across the plains and foothills increases. PVA ahead of the trough will create strong QG ascent and there will be warm air advection from the surface to around 500 mb. As a result, there will be widespread moderate snowfall for the majority of our forecast area Friday night and PoPS in the 80s and 90s reflect that. Soundings show a deep dendritic growth zone during this time with strong vertical motion within that layer. Therefore, snow ratios will be on the higher side with ratios of between 14 to 17-1. Snowfall rates up to an inch per hour are possible from the mountains to the foothills and I-25 corridor. This will create slick and snow-covered roads leading to difficult travel conditions. This is less than ideal with it being New Year`s Eve and our office will message the travel concerns heavily leading up to the event. Another impactful aspect of the forecast will be the chilly temperatures. A cold airmass from the northern plains will make it into our forecast area with temperatures falling into the single digits across the plains and below zero across the mountains. While cloud cover and snowfall will keep temperatures from bottoming out, the moderate winds speeds will create wind chills in the negative single digits across the plains and negative teens in the mountains. As mentioned before, these systems tend to move across the area slower than expected and it appears the model trend has already picked up on that. The latest guidance keeps light snowfall across most of the forecast area Saturday morning with it eventually tapers off to flurries by Saturday afternoon. This makes sense as the low level easterly, upslope flow turns northerly and then northwesterly throughout the day. There will be QG ascent in the mid to upper levels allowing snow showers to linger but that will change to QG descent by late Saturday afternoon. Highlights have been extended until 18Z/11AM Saturday. Overall, snowfall amounts will be aided by the good snow ratios and relatively slow moving nature of the system. The mountains will see the heaviest amounts with totals up to 2 feet. The foothills and adjacent plains will see values in the 5 to 12 inch range. Those values are close enough to Winter Storm Warning thresholds that a Winter Storm Watch was issued. From Denver eastward, the amounts are more uncertain but generally fall in the 3 to 6 inch range. Highlights will be needed in Denver and across the plains but none were issued at the moment due to the uncertainty in the amounts. The through moves east of our forecast area Saturday night with ridging building in over Colorado. With a fresh snowpack and clearing skies, there will be optimal radiational cooling which will allow for some very cold temperature readings. In the usual cold spots in the valleys like Greeley, Limon, and Kremmling, there is a good chance lows will reach the negative teens. Light drainage winds will keep the foothills and I-25 corridor from seeing values that cold but the low could reach below zero at DIA and other locations before the winds pickup overnight. Forecast low temperatures were brought closer to the NBM 25 percentile. There may need to be wind chill highlights as well. A large ridge will be over Colorado from Sunday through Tuesday leading to increasing temperatures and dry conditions. Depending on how fast the snow melts, high temperatures could reach the 50s across the plains as soon as Monday. By Wednesday and Thursday, a westerly jet stream will move over Colorado with snow possible in the mountains mainly. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1004 AM MST Thu Dec 30 2021 Strong W/SW winds at this time across the foothills and into western Denver, but gusts at terminals not as high. Still potential for brief gusts 30-40 kts for KDEN/KAPA, up to 65 kts for KBJC (low confidence) thru ~22Z before winds weaken. W/SW flow will prevail overnight into Fri AM. Front expected to move through the metro between 15-18Z Fri, bringing shift to NE winds for all terminals as well as lowering cigs. VFR conditions before 15Z likely for the terminals. && .FIRE WEATHER... Issued at 302 AM MST Thu Dec 30 2021 Elevated fire conditions will be in place across the plains this afternoon due to gusty west southwest winds and continued dry conditions. The fire danger will quickly subside early this evening as winds decrease. && .BOU WATCHES/WARNINGS/ADVISORIES... High Wind Warning until 11 PM MST this evening for COZ033-035. Winter Storm Watch from late tonight through Saturday morning for COZ035-036-038-039. High Wind Warning until 5 PM MST this afternoon for COZ036-038- 039. Winter Storm Warning until 11 AM MST Saturday for COZ031-033-034. Winter Weather Advisory from 5 PM this afternoon to 11 AM MST Saturday for COZ030-032. && $$ UPDATE...Rodriguez SHORT TERM...RPK LONG TERM...Danielson AVIATION...Rodriguez FIRE WEATHER...RPK ===== FXUS65 KBOU 302151 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 251 PM MST Thu Dec 30 2021 .SHORT TERM...(This evening through Friday) Issued at 128 PM MST Thu Dec 30 2021 We are in the midst of a truly historic windstorm across the Front Range, foothills and urban corridor. Despite much surface support, a strengthening stable layer a short distance above ridgetops, minimal shear aloft, and healthy cross-barrier flow have resulted in exceptional mountain wave amplification. Widespread wind gusts between 80-100 mph have been observed, and a handful of sites in Rocky Flats have seen several hours worth of gusts 100-115 mph. With relative humidity below 20%, the extreme winds are fueling rapid spread of the Marshall Fire, which has quickly become a December urban conflagration impacting Superior and Louisville. High-resolution models and forecast soundings provide some hope when it comes the weakening winds, indicating a rapid retreat of the strongest winds back into the foothills within the next ~2 hrs, with more widespread weakening a little after sunset. Wind is not the only forecast concern, with weather whiplash coming our way as we transition rapidly from fire to snow. A few terrain-enhanced snow showers (plus gusty winds) are already producing treacherous driving conditions across our higher elevations and mountain passes. With increasing moisture later this evening and tonight, expect snow intensity and coverage to increase in our mountains, first impacting the Park Range and subsequently spreading southeastward. By Friday, the long-awaited front will push south into northern Colorado, bringing a sharp drop in temperatures and increasing snow across the urban corridor and plains, and enhancing snowfall intensity in the mountains and high valleys. Snow may become heavy by Friday evening east of the Front Range. All of our mountain zones, including the foothills, are now included in the Winter Storm Warning. For the plains and urban corridor, a Winter Weather Advisory has been issued starting 11 AM Friday, with the exception of southern Lincoln County, with a broad swath of 2-6" expected. .LONG TERM...(Friday night through Thursday) Issued at 128 PM MST Thu Dec 30 2021 Snow is still on track for Friday night and Saturday morning. An upper level trough will dive south across the Great Basin Friday night and the Central Rockies on Saturday. Lift ahead of the trough will produce widespread snow. Expect a band of heavier snow, associated with frontogenesis, to track east-southeast across the area Friday night. Another ingredient for snowfall will be northeast upslope winds behind a arctic cold across the Front Range. Total snowfall looks to be 2 to 5 inches east of I-25. Amounts should be a little higher west of I-25 across the Urban Corridor with 4-8 inches expected. Snow begins to decrease Saturday morning and ends from northwest to southeast. By noon Saturday, snow should be finished north of I-70 with just light snow lingering a couple hours longer to the south. Coldest air of the season will accompany this system. Temperatures Saturday morning will be in the single digits with near zero readings over the eastern plains as well in the mountains and foothills. Highs will be chilly as well with temperatures struggling to make it into the teens. Northwest flow aloft will prevail Saturday night and into Sunday ahead of an upper level ridge moving across the Great Basin. Strong surface high pressure will also be over the Great Basin with lower pressure over Colorado and off to the east. This should result in gusty west winds Saturday night and into Sunday, especially in the foothills. Could see issues with blowing snow in and near the foothills. These gusty winds should flush out the arctic air, but it will continue to be chilly Sunday with highs around 40 degrees and gusty west winds. The ridge flattens Monday as it tracks across the Central Rockies. Westerly flow will usher in warmer air with highs climbing into the 40s to lower 50s over northeast Colorado. The airmass will remain dry with no precipitation expected. For Tuesday through Thursday, a ridge off the west coast and a trough over Northern Rockies, Pacific Northwest, and western Canada will bring westerly flow aloft Tuesday. As the ridge and trough progress eastward, flow aloft will turn northwesterly. Could see snow return to the northern mountains Wednesday under this pattern. Temperatures should be mild Tuesday, and possibly into Wednesday. Models indicate a cold front will back into northeast Colorado sometime Wednesday or Thursday. We likely see a chilly day or two until this modified arctic air is flushed out. May also see some light precipitation accompany this colder air as well and will have low PoPs in the forecast for Wednesday and Thursday. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1004 AM MST Thu Dec 30 2021 Gusty W/SW winds may continue over the next few hours, before weakening and transitioning to SW drainage flow overnight and into Friday morning. Smoke and reduced visibility, at times below 2 mi, is likely for KDEN and KBJC through this evening at least due a large wildfire NW of Denver. Front expected to move through the metro between 15-18Z Fri, bringing shift to NE winds for all terminals as well as lowering cigs. VFR conditions before 15Z likely for the terminals. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 11 AM Friday to 11 AM MST Saturday for COZ040>046-048>051. High Wind Warning until 11 PM MST this evening for COZ033-035. Winter Storm Warning from 5 AM Friday to 11 AM MST Saturday for COZ035-036-038-039. Winter Storm Warning until 11 AM MST Saturday for COZ031-033-034. Winter Storm Warning from 11 PM this evening to 11 AM MST Saturday for COZ030-032. High Wind Warning until 5 PM MST this afternoon for COZ036-038- 039. && $$ SHORT TERM...Rodriguez LONG TERM...Meier AVIATION...Rodriguez ====== 000 FXUS65 KBOU 310349 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 849 PM MST Thu Dec 30 2021 .UPDATE... Issued at 840 PM MST Thu Dec 30 2021 The strong winds have subsided somewhat this evening, but not as quickly as this afternoon`s models indicted, including the hi- resolution models. The Marshall Fire in southeastern Boulder County, has decreased somewhat, but it is not out at this time. There is still considerable glow out the windows of the office looking east-southeastward. Models suggest weakening westerly and southwesterly low level winds overnight. With the radar information and some observations, the best guess on the winds over the fire currently are west-southwesterly at 25 to 40 mph still. A cold front and upslope flow is not expected to get into and across the CWA until about 15Z Friday morning, and around 14Z at the Marshall Fire itself. The fire will likely not be totally out until late Friday afternoon/early evening when the upslope snowfall kicks in. Will make some adjustments, based on real data, to the wind, sky, temperature, wind gust, weather and pop GFE grids this update. &&