Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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000 FXUS65 KBOU 280441 CCA AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 936 PM MDT Thu Apr 27 2017 .UPDATE... Issued at 930 PM MDT Thu Apr 27 2017 Wrap around showers in far northeast Colorado have yet to exit the area. At their present forward progress, should see the bulk of these showers exit the area by around midnight. Otherwise the few remaining snow showers dotting the high country and spotty rain showers on the plains should continue to weaken and on the plains, dissipate in the next hour or two. No additional snow accumulation from the high country showers for the remainder of the night. Therefore have adjusted the current forecast to reflect this reduction in clouds and shower activity. Also made only minor adjustments to temperatures and winds. Lastly...no change to the current winter storm watch at this time. && .SHORT TERM...(This evening through Friday) Issued at 227 PM MDT Thu Apr 27 2017 GOES-16 Water Vapor and Visible imagery continues to show in great detail the short wave trough that continues to generate convective snow showers across the high country and rain showers along with a few thunderstorms, across the Plains this afternoon. The main activity is currently well east of the I-25 corridor, across Morgan, Lincoln, and Washington counties and will continue to move east through this evening. A fair amount of convective instability (300-600 J/kg ML CAPE) is available to the leading line/cluster of showers and thunderstorms, which would be favorable for sustaining the thunderstorms and generating small hail. The rain showers and thunderstorms should exit the Plains into Kansas by late this evening. Across the mountains, with the departure of the shortwave trough and loss of diurnal heating, the snow shower activity should diminish this evening. Until this evening however, conditions are still favorable for brief, moderate to heavy snow showers/squalls that adversely impact travel across the higher mountain roads, especially along the I-70 along corridor across Summit county. Will keep the advisory in place for the mountains above 9000 feet until 6 PM given the expected potential travel disruptions with the snow squalls. Tonight the lead short wave trough moves into W. Nebraska taking most of the synoptic scale lift with it. The exception will be far NE corner of Colorado where light rain may continue tonight as the short wave trough stalls over W. Nebraska. Westerly upslope flow will keep light snow chances going tonight across the northern and central mountains but any additional accumulation will be light. Friday morning, in the wake of the short wave trough with a surface reflection, a strong surface low pressure area across NW Kansas, will drive a cold front across the Plains of Colorado Friday morning. Strong and gusty north/northeast winds will accompany the frontal passage and another round of precipitation is possible across the northeast Plains. Cloud cover and the cold front will keep high temps in check across the Plains, with mid and upper 40s across the southern part of the area including the Denver metro, and struggling to reach 40 across our northern border with Wyoming and Nebraska. The front will also generate upslope snow showers across the eastern mountains and foothills of the Front range and Palmer Divide Friday afternoon. Snow levels Friday afternoon behind the front will be around 6500 feet MSL but drop below 6000 feet by Friday evening. Focus then turns to what may happen Friday night and Saturday, and even 30 hours prior to the potential start of the snow, the global models differ quite a bit on the synoptic scale setup. This of course affects snow amounts across the I-25 corridor. A 140 kt northerly jet digs south out of the Pacific northwest, located across the Great Basin by Friday evening. The strong trough downstream of the jet will then dig south into the 4 corners region by Friday evening, setting the stage for strong dynamic lift, a developing surface low, and upslope flow across northeast Colorado. However, how the models handle the lead short wave trough impact how much lift can be generated and for how long across our area. GFS/NAM have the short wave trough deepen and cut off over the Nebraska panhandle at 00Z, resulting in a much broader, elongated 500mb trough from the 4 corners into the Nebraska panhandle. The EC and Canadian models weaken the lead short wave trough and move it quickly northeast, resulting in a stronger, much less elongated 500mb low across the 4 corners at 00Z. Implications on the sensible weather at the ground will be covered below by the long term forecaster. .LONG TERM...(Friday night through Thursday) Issued at 227 PM MDT Thu Apr 27 2017 Minor adjustments to the Friday night/Saturday forecast based on accepting the trend toward a more elongated low and nudging the forecast in that direction. This keeps the best lift roughly along and south of Interstate 76 and limits the duration of the strong upslope in areas north of Denver. This could result in less precipitation, but the more northerly winds behind the stronger development aloft over northeastern Colorado will also bring cooling faster Friday evening. The result of this would be more snow along the Front Range during the most intense part of the storm Friday night. This trade off results in not much change to our snow forecast for the Denver area. With the faster cooling, there is less chance of getting skunked altogether, but we should still wind up with a significant gradient across the city due to colder temperatures and stronger and more prolonged upslope over the southern part of the metro area. Impact will also depend greatly on temperature. In the cities the roads should be warm enough to restrain accumulation, though there could be some slushy times later Friday night and early Saturday morning. Bigger impact for the cities could be snow accumulation on trees and resulting tree/power line damage. The higher end of our accumulation range could be a significant tree-breaker and power outage event. For areas above 6000 feet there is much more chance of some accumulation on the roads. The system moves out Saturday afternoon, with the mountain showers fading in the evening. One big question is the amount of clearing and wind speeds to affect low temperatures Sunday morning. At this point it looks like there should be pretty good clearing, though there will likely still be some wind in areas that aren`t sheltered. Guidance has slightly cooler temperatures Sunday morning and this looks good. This makes for mid 20s in a lot of places that would impact tender vegetation and sprinkler systems. The rest of the forecast features cool northerly flow with a couple embedded disturbances. Unstable enough for some low shower chances, but the best chance will be Tuesday and Tuesday night. The ridge eventually builds upstream with a little warming and drying trend toward the end of the week. && .AVIATION...(For the 06Z TAFS through 06Z Friday night) Issued at 930 PM MDT Thu Apr 27 2017 VFR conditions at Denver area airports for the reminder of the night. Cannot rule out a stray light rain shower overnight, mainly before midnight. Otherwise gradual clearing with light drainage winds in place. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Storm Watch from Friday evening through Saturday afternoon for COZ035-036-041. && $$ UPDATE...Baker SHORT TERM...Schlatter LONG TERM...Schlatter AVIATION...Baker

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