Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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532 FXUS65 KBOU 081154 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 454 AM MST Thu Dec 8 2016 .SHORT TERM...(Today and tonight) Issued at 454 AM MST Thu Dec 8 2016 Arctic cold is fully entrenched across the forecast area this morning with almost all locations below zero. There was just a narrow band of low clouds still keeping temps up above zero in the Boulder county area and southern Lincoln county. Meanwhile a couple spots near Fraser and Tabernash plunged to -30F or colder! The cold will remain in place across the plains through tonight with arctic high pressure only slowly retreating to the east. Mountain areas will be cold with increasing winds later today. Then warm advection will likely bring warming temps overnight to the high country but also a return of snow. A few showers may begin to develop as early as late afternoon, with more widespread snow expected to reach the mountains this evening as pacific moisture plume moves in. Despite warm advection, lapse rates gradually improve to greater than 5 C/km with further improvement into Friday morning. Winds also turn more westerly so even the I-70 corridor of the mountains should be impacted by accumulating snow. Amounts might be marginal for advisories there, but considering strength of winds with gusts 60 mph or more over the higher passes feel an advisory is warranted for all of the mountains. Highest totals in this pattern certainly shaping up for the mountains north of Rabbit Ears Pass. High temperatures on the plains today will struggle to push through the teens with only a couple spots from downtown Denver south possibly reaching 20F. High mountain valleys are tough with an extremely cold start and strong inversions, but should be some moderation late as clouds move in. The clouds will also make overnight lows difficult to forecast tonight as some areas across the higher elevations will begin warm air in earnest, while plains locations remain stuck in the arctic air but increasing clouds should result in some moderation. .LONG TERM...(Friday through Wednesday) Issued at 454 AM MST Thu Dec 8 2016 The active period of weather will continue into the medium range periods of the forecast. Moist and fast westerly flow will continue through Friday and into Saturday, generating more orographic snow in the mountains. Another 4 to 8 inches of snow will be possible by Friday evening. Blowing snow will also be possible over the higher peaks. Will continue the Winter Weather Advisory into early Friday morning, but it may need to be extended if the airmass remains moist enough. The core of the upper level jet will be centered over Wyoming, placing Colorado under the right exit region which is normally subsident. However, the moist nature of the flow pattern will interfere with the development of an amplified mountain wave. Therefore, gusty winds should remain over the ridges and high foothills. Cold air lingering at the surface will also keep gusty winds from moving further down the foothills. The westerly flow aloft will also result in dry conditions over the northeast Colorado plains. And that is just Friday and Friday night. For Saturday, snow will continue in the mountains as the moist westerly flow will continue. Cross sections still show a moist airmass up through 500 mb with drying downstream of the mountains. The continuing westerly flow will bring gradual warming along the i-25 corridor and eventuall out onto the plains. Warming should then continue through Sunday and into Monday. For Tuesday, there are differences between the GFS and ECMWF models related to how strong the next surge of Arctic air is going to be. The GFS run is much stronger, bringing very cold 850 temperatures all across Colorado. The ECMWF is more restrained and keeps the Arctic air over the eastern half of the state. All of the low level cold advection is going to take place under continuing westerly flow aloft. The MOS guidance from the GFS shows high temperatures Tuesday some 25 degrees colder than previous MOS guidance values. Until this scenario is confirmed by later runs, will stick closer to the ECMWF solutions. The chances for snow could continue in the mountains through next Wednesday, and the latter half of next week looks like yet another trough embedded in the westerly flow aloft moving over the state. It looks like the mountain snowpack will be in good shape for the near future. The plains are going to need to wait for the flow pattern to become a little more meridional. && .AVIATION...(For the 12Z TAFS through 12Z Friday morning) Issued at 454 AM MST Thu Dec 8 2016 Outside of the low clouds hanging around KBJC-KBDU-KEIK-KLMO...VFR conditions prevail and will remain in place through Friday morning. Don`t think the low clouds would reach KDEN as light drainage winds hold in place there, but could result in instrument approaches from the North through 16Z. There is also patchy fog in a couple low lying areas but again light drainage winds should help keep that out of the airport through 16Z. Winds should become more east/northeast after 18Z but remain under 8 knots. Then winds transition back to normal south/southwest winds shortly after 00z. && .BOU WATCHES/WARNINGS/ADVISORIES... Wind Chill Advisory until 8 AM MST this morning for COZ048>051. Winter Weather Advisory from 7 PM this evening to 5 PM MST Friday for COZ031-033-034. && $$ SHORT TERM...Barjenbruch LONG TERM...Dankers AVIATION...Barjenbruch

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