Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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000 FXUS65 KBOU 210335 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 835 PM MST Fri Jan 20 2017 .UPDATE... Issued at 808 PM MST Fri Jan 20 2017 Weak short wave ridge axis over central CO with weak mid level qg descent over much of the cwa this evening. Some moisture increasing from the southwest but should not impact the north central mountains of CO until after 12z Saturday. The latest RAP13/HRRR runs show some showers developing primarily in zone 31 late tonight, but overall not much elsewhere across the cwa. The main area of showers should still be over west central and southwestern CO by 12z Saturday. As a result, no significant updates are planned to the forecast grids at this time. && .SHORT TERM...(This evening through Saturday) Issued at 350 PM MST Fri Jan 20 2017 Radars shows a few snow showers left over the plains. Models have an upper ridge over the CWA overnight with weak southwesterly flow aloft. The flow aloft stays weak on Saturday but is southwesterly, then westerly by afternoon. The QG Omega fields have downward motion over the forecast area tonight, then upward on Saturday. The boundary layer wind and surface pressure fields keep a weak gradient over the CWA the first two periods along with normal diurnal wind patterns. Models show a fairly dry night tonight, then moisture increases from west to east on Saturday. By afternoon, moisture is pretty deep over the mountains. There is nothing on the QPF fields tonight, then some moves into the western CWA by Saturday afternoon. For pops will go with 0-20%s in the high mountains tonight. Will increase pops through the day Saturday for the mountains with 40-60%s by afternoon. There should be some orographic enhancement late of Saturday. Will keep the plains dry both periods. For temperatures, Saturday`s highs will be a tad cooler than this afternoon`s highs. .LONG TERM...(Saturday night through Friday) Issued at 230 PM MST Fri Jan 20 2017 Sunday is a transition day in between storm systems. The Pacific storm impacting Colorado on Saturday moves east-southeast into the Red River Valley of Oklahoma by Sunday morning quickly drying out the atmosphere over Colorado. At the same time a strong long wave trough develops off the coast of California, which builds a ridge that moves from the Great Basin to over Colorado throughout the day Sunday. Despite the synoptic scale subsidence and advection of dry air, there could be enough orographic lift for light snow midday Sunday along the Park Range and other northern mountains. The rest of the area should see a dry and seasonably warm day. The ridge axis slides east and Pacific moisture returns to the mountains Sunday night into very early Monday. A small amount of synoptic lift from the approaching trough, combined the approach of a 150 kt southwestern jet streak should be enough to produce good snow across the mountains Sunday night through Tuesday. Highlights may be needed for some of the favored west and southwest upslope areas west of the Divide. Across the Plains there is enough lift generated despite a lack of easterly upslope that there is a slight chance of initially rain late Monday afternoon which would change over to snow by Monday evening. Amounts look light and confined to areas along and west of I-25. Monday night into Tuesday the trough axis moves across NE Colorado and there are chances of light precipitation across the northeast Plains. Cooler air filters into Colorado from the north and any precipitation that falls on Tuesday should be snow. Mountain snow should continue through Tuesday with generally lighter amounts compared to Monday. Best chances of accumulating snow on Tuesday in the mountains would be across the northern mountains north of I-70. Wednesday Through Friday: The progressive and unsettled pattern continues across the CONUS. A strongly positive tilted long wave trough extends from southern California east-northeast to the upper Mississippi Valley. A closed 500 mb circulation looks to exist across Iowa and Minnesota Wednesday morning and moves it into eastern Canada on Thursday. GFS and ECMWF has similar height fields over Colorado but handle the mid level flow across Colorado a little differently, with the GFS more northerly at mid and upper levels and the ECMWF more northwesterly. The model blend and ensembles have light mountain snow Wednesday morning so will keep low PoPs there. Overall, moisture is lacking given the mid- level flow originates from western Canada and the sub-tropical jet stream looks to be well south of CO across the U.S.-Mexico border. This is especially true Thursday and Friday where it should be dry across the CWA. With the northerly flow temperatures should remain below all the way through Friday. && .AVIATION...(For the 06Z TAFS through 06Z Saturday night) Issued at 808 PM MST Fri Jan 20 2017 VFR conditions with no significant impacts to the terminals through 18z Saturday morning. Typical diurnal winds through Saturday morning. There could be periods with ILS restrictions developing after 21Z Saturday afternoon, with bkn cigs 040-050 as a surface low develops near the OK/TX panhandles, so may address this in the 06z taf issuance. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Cooper SHORT TERM...RJK LONG TERM...Schlatter AVIATION...Cooper

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