264 FXUS65 KBOU 031845 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1145 AM MST Wed Feb 3 2021 .UPDATE... Issued at 1145 AM MST Wed Feb 3 2021 A couple changes to make at this hour though the forecast looks good in general. We're outperforming on temperatures today, so we'll raise highs by a couple more degrees. This will bring Denver close to the record for the date of 70. As far as the snow, we're seeing a leading band (really a band of bands) develop from north of Grand Junction to Rocky Mountain National Park, with the main band trailing it from south of Salt Lake City to near Rawlins. There's plenty of instability and shear, with quite a bit of lightning where this band has intersected the Wasatch mountains this morning. Most of the models show a complex banded structure like this with quite intense snow in the bands, though I wouldn't bet too much on the minor details. There's a general emphasis from southern Grand and Summit counties mid to late afternoon to areas south of Denver by late evening. Main impacts would be I-70/Berthoud Pass this afternoon, foothills W/SW of Denver for rush hour, and possibly I-25 south of Denver mid to late evening. For now, we'll raise PoPs and snowfall in these areas a bit more. Probably not a lot outside of these areas though everyone could wind up with a quick rain/snow shower this evening. && .SHORT TERM...(Today and tonight) Issued at 340 AM MST Wed Feb 3 2021 An upper level trough will move towards the area from the northwest today with WSW flow aloft. Moisture will increase ahead of this system with rather steep lapse rates. With position of upper level jet this should lead to areas of heavier banded snowfall across the mtns by midday thru the aftn hours. Will likely see snow squall activity in some mtn areas this aftn into the early evening hours with rapidly changing road conditions. Across nern CO, sfc low pres will be in place with downslope WSW low level flow. This will allow for another mild day with highs in the upper 50s to mid 60s across the plains. In addition a few light showers may move off the higher terrain late this aftn which may briefly enhance wind gusts. Wouldn't be surprised to see a few brief gusts up to 50 mph across portions of the plains. For tonight, the main upper level trough will move across the area with the flow aloft becoming more WNW. Cross-sections continue to show decent lapse rates over the area with favorable orographics in the mtns. Thus should continue to see additional snow in the mtns with amounts by 12z Thu in the 6 to 12 inch range. Across nern CO, a cold front will move across the area by mid to late evening. The low level flow will be mainly northerly behind the front, however, some of the high res data is turning the winds more NNE. Meanwhile, as the upper level trough moves across, there could be an enhanced area of precip behind the front. Right now it looks like areas along and south of a line from Denver to Akron could see a brief period of heavier snow late this evening thru 2 am or so. In addition gusty north winds may cause areas of blowing snow and poor visibility over the plains as well. .LONG TERM...(Thursday through Tuesday) Issued at 340 AM MST Wed Feb 3 2021 By Thursday the center of the trough is across North Dakota and the ridge axis extends southward across the Central Plains. The core of the cold air aloft thus gets shifted east throughout the day and drier WNW flow will likely put an end to the orographic snow in the mountains for all but the highest ridges of the Park Range and Continental Divide north of I-70, where just enough moisture hangs around for light snow. No significant travel impacts are expected and additional accumulation should be 0-2" through the evening hours. Across the plains, the surface pressure gradient picks up and with some clearing resulting in vertical mixing, northwest winds could gust over 35 mph across the far northeast plains and along the Wyoming border. Despite some modest warming aloft temperatures will remain below normal with highest in the upper 30s to around 40 across the plains, upper 20s in the foothills, and low 20s in the mountain valleys. There may be a few flurries before midday across the eastern foothills, Palmer Divide and to a lesser extent south of I-76, but no additional accumulation is expected. Strong Northwest flow continues Friday and moisture advection begins in earnest early Friday ahead of the next embedded trough aided by a 120 kt jet. More favorable WNW flow will result in snow picking up across the west slopes and the 700 mb flow becomes saturated west of the Divide by Friday morning. Lapse rates just above the mountains are poor until later in the day on Friday. Nevertheless, light snow is expected along favored WNW facing slopes throughout most of Friday and all of Saturday with persistent, fairly moist northwest flow aloft. On Saturday afternoon the left exit region of a 120 kt jet streak passes over northern Colorado, but the atmosphere is drying out by then so the vertical motion boost may not make much of a difference in snow amounts. Upslope flow becomes westerly late Saturday into Sunday, but dries out significantly which will bring an end to the snow in the mountains by early Sunday. Synoptic support for lift is lacking because even the weak troughs are missing us to the north Friday and Saturday. Thus, snowfall amounts may stay below advisory levels and travel impacts minor, though the Park Range (zone 31) may see a decent snow over a 36 hour period. Much of the high country along and west of the Divide should see accumulating snow. Across the plains, downslope flow combined with no synoptic or mesoscale support for lift means just a slight chance of snow on Friday, with the best chance along the Wyoming border and far northeast corner. The rest of the area including the I-25 urban corridor may just see a few flurries, if that. Highs Friday will be near 40 across the plains, with 20s in the mountain valleys. We've been advertising an arctic front on Saturday for a few days now and not surprising, the models are having a hard time figuring out how to handle it. One thing is certain: the coldest air of the year, by FAR, will be across the central and northern plains this weekend, driven south by a long wave trough over Hudson Bay Canada. The trough axis is over 1000 miles east of us, making it a challenge to get the real deal arctic air into Colorado. The EC and Canadian models keep the arctic air mostly east of us, while the NAM and GFS slosh the cold air most of the way to Denver but just the northeast corner gets the really cold air under that solution. Even the ensemble solutions are downplaying how far west the coldest air gets on Saturday, with just a handful of members of both the GEFS and Euro ensemble bring the coldest air west of I-25. Given all this, the cold front will move across eastern Colorado Saturday but may not bring the true arctic air much further than the far northeast corner. Will keep highs well below normal but aside from the northeast corner, highs for now are forecast to be in the upper 20s to low 30s, other than the low 20s for the northeast corner. With a weaker front PoPs are not significant for most of the plains, though the far northeast corner is close enough to frontogenetic forcing early Saturday that they could see up to inch of snow or so. Sunday and Monday northwest flow continues and any embedded moisture should stay northeast of Colorado. Can't rule out very light snow across the mountains given orographics but lapse rates are poor. Sunday morning a jet max moves across the state and there is a pretty good inversion between 500-600 mb. With 75 kts of flow in that layer, a mountain wave could develop which forces winds down the east slopes. Too far out to say how far east the strong winds get, but the mountains and eastern foothills could see gusts over 70 mph if this trend holds. Temperatures both days will remain below normal with the coldest temps across the northeast corner of Colorado, closest to the arctic airmass. Tuesday and beyond the pattern looks cold with embedded moisture in WNW flow resulting in chances for mountain snow, but so far not seeing anything significant snow-wise for the plains and I-25 corridor. && .AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon) Issued at 1145 AM MST Wed Feb 3 2021 West winds will develop across the Denver area this afternoon, between 20z and 00z, with gusts to around 25 knots causing potential crosswind issues for KDEN/KAPA for a few hours. Winds will become northerly this evening, with gusts to around 30 knots. Scattered rain and snow showers will develop early this evening, with a few hours of snow showers likely 04z-08z and areas of IFR visibilities in the showers. Most places will have an inch or less, though locally heavier showers are expected south and west of Denver. Quick clearing is expected between 08z and 12z with lighter winds. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 9 AM MST Thursday for COZ031-034. && $$ UPDATE...Gimmestad SHORT TERM...RPK LONG TERM...Schlatter AVIATION...Gimmestad