719 FXUS65 KBOU 190438 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 938 PM MST Thu Feb 18 2021 .UPDATE... Issued at 935 PM MST Thu Feb 18 2021 Widespread fog has developed along the low lying areas near Limon, affecting I-70 and down US 40/287 down to Hugo. With clear skies and winds that will keep the moisture pushed into that low lying area, will issue a Dense Fog Advisory overnight. Motorists should anticipate poor visibility in that area throughout the night, with improvement not expected until after sunrise. UPDATE Issued at 812 PM MST Thu Feb 18 2021 With the fresh snow, clear skies, a few observations of fog, and some hints of continued reduced visibility in the high resolution models, felt confident enough to put in mention of fog along the South Platte River Valley (SPRV) and southern Lincoln County tonight after midnight. Even though winds will be increasing tonight, enough decoupling in the SPRV should lead to light winds, and southerly winds over southern Lincoln County will lead to slight upslope winds to help moisten up the airmass. Also increased windspeeds across most of the area for tomorrow as the jet moves in, and with warmer temperatures expected, those winds will be able to mix down. Still not a great mountain wave profile, and with only 60 kt perpendicular winds at mountain top, not expecting winds high enough and widespread enough to reach the warning criteria of 75 mph in the high country. Will definitely still see some 60+mph there, with gusts 30 to 40 across the foothills, urban corridor and adjacent plains. && .SHORT TERM...(This evening through Friday) Issued at 215 PM MST Thu Feb 18 2021 Today and tonight, there may be a few light snow showers on the high western slopes of the Front Range but otherwise the area should remain dry. Winds will increase in the mountains and high foothills late tonight and early tomorrow morning as a stable layer builds over the Front Range due to subsidence behind the exiting trough. There is a decent mountain wave signature in cross sections across the Front Range. Forecast lapse rates above the mountains are very stable but ridgetop flow is only around 50 kts and not yet zonal enough to produce extreme wind. However, gusts in the mountains should reach up to 50 mph overnight and some of the higher foothills will probably see gusts up to 35 mph as well. Overnight lows will be in the single digits in the mountains and most of the plains, while downsloping winds will warm the foothills and adjacent plains. Tomorrow, winds and gusts will continue to increase as flow aloft increases and turns more zonal with the ridgetop stability still in place, though maybe slightly weaker. Gusts may reach up to around 60 mph in the mountains and 45-50 mph in the higher foothills by late morning or early afternoon. Snowfall will likely increase in the mountains west of the Continental Divide again tomorrow morning and afternoon due to increasing northwest orographic flow and a broad jet to stream some moisture from the Pacific. The mountain peaks may accumulate a few more inches by tomorrow evening, but elsewhere very little accumulation should be expected. Highs tomorrow will be warmer than today, in the mid to high 20s F in the mountains and high 30s to mid 40s F on the plains. .LONG TERM...(Friday night through Thursday) Issued at 215 PM MST Thu Feb 18 2021 Will start the period Friday night under a strong (140+kt at 250mb) upper jet streak. Snow showers over the mountains should initially decrease in coverage Friday evening and into the early overnight hours Saturday as the first subtle shortwave departs and a brief period of QG subsidence spreads over the northern mountains. Still enough moisture near the surface along with favorable upslope winds to promote at least a chance of continued light snow showers over the higher mountains. The lull in activity over the mountains should be short-lived, as a more notable shortwave trough enters the region by Saturday morning. Should see much better lift across the region, combined with fairly steep lapse rates and a deepening moisture field, and POPs increase over the mountains during the day Saturday. Meanwhile, strong winds at ridgetop will promote the potential for wind gusts of 55 to 65 mph over the higher mountains and mountain passes. While snow accumulations won't be particularly impressive, still could see some blowing snow from Friday night through Saturday night. Forecast gets a little more challenging by late Saturday. Mountain snow will gradually wind down through the evening hours as the best forcing shifts east towards the plains. Meanwhile, a fairly strong surface cold front will push south across the plains by late Saturday evening. There appears to be decent agreement in the GFS/ECM/UKMET/GEM that there will be a brief period of snow along the urban corridor behind the frontal push. Models show weak upslope flow (though not nearly as deep as yesterday's snow event) with good moisture and continued synoptic scale ascent overnight. Have bumped up chances for precipitation up a touch, but it's worth noting that both the higher resolution NAM and RGEM show much in the way of precipitation. Confidence in the current forecast is low, and it is worth monitoring the setup for a round of light snow across the I-25 corridor. By Sunday, we transition to northwesterly flow aloft. Cool 700mb temperatures will keep highs near or below 40 in most places on Sunday, but temperatures will rebound to the 50s, and perhaps even 60, on Monday and Tuesday. The next storm system arrives sometime by mid to late week. Model solutions continue to vary from run to run, but an cooler and unsettled weather pattern should be setting up for the second half of the work week. && .AVIATION...(For the 00Z TAFS through 00Z Friday evening) Issued at 812 PM MST Thu Feb 18 2021 Expect VFR conditions at all terminals through the TAF period. A jet will be pushing into the area overnight and into Friday. This will increase mid to high level cloudiness tonight and tomorrow, with wave clouds expected to the lee of the Rockies. Between 12z-18z, this wave cloud may deepen and extend all the way down to near 10kft MSL to the lee of the Front Range Mountains. Drainage winds tonight, slightly enhanced by a deepening lee surface trough, will then become gusty and out of the northwest starting late tomorrow morning through the late afternoon. Peak gusts are currently expected up to around 35 kts, strongest closer to the foothills such as at BJC. && .BOU WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until 7 AM MST Friday for COZ046. && $$ UPDATE...Kriederman SHORT TERM...Direnzo LONG TERM...Hiris AVIATION...Kriederman