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National Weather Service Forecast Discussion |
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000 FXUS65 KBOU 222130 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 230 PM MST Fri Jan 22 2021 .SHORT TERM...(This evening through Saturday) Issued at 230 PM MST Fri Jan 22 2021 Snowfall will increase slightly in intensity and areal coverage this evening and overnight as WNW flow aloft shifts more W then WSW ahead of the incoming trough, but should remain confined to the mountains and some parts of the northern foothills. Mild subsidence aloft will also be replaced by weak ascent ahead of the trough, though models are in good consensus about the best forcing missing northeast Colorado. The forecast area will be under the left exit region of a roughly 100 kt jet streak on Saturday night, which should also provide a dynamic boost by increasing mid level divergence and orographic flow. We have kept a Winter Weather Advisory in effect for the the Park Range and part of the Gore Range in Grand County from 5 PM this afternoon through 11 PM Saturday, expecting the greatest snow accumulations of 4 to 10 inches with gusts up to 35 mph in this area. Continue to expect less accumulations of 1 to 5 inches elsewhere above 10 kft. Expect lows tonight between 1 and 5 degrees warmer than last night across most of the area, putting much of the plains in the low to mid 20s F, foothills mid to upper 20s F, and mountains mid teens. Light snowfall will continue Saturday morning and probably through the day, intermittent except in the highest terrain. A weak front is expected in the early evening across the plains, which will likely not impact temperatures much being after the high but should bring some moderately gusty northwesterly winds. Highs Saturday are forecast to be similar to highs today though a few degrees warmer across much of the plains, reaching the low 50s F in much of the area adjacent to the Palmer Divide. Behind the front there is a chance of some light snowfall during the afternoon/evening hours spreading east from the foothills over the urban corridor to about I-25, north of I-70, and across the plains of northern Weld County as well, but looks to be only flurries at this time. .LONG TERM...(Saturday night through Friday) Issued at 230 PM MST Fri Jan 22 2021 The short wave cutoff trough is across SoCal late Saturday into Sunday. On Sunday the low becomes vertically stacked and then absorbed into the westerly flow as a weak open wave overnight into Monday. Besides being vertically stacked, it begins to get absorbed into a broader upstream trough as a stronger west coast trough digs southeast across Oregon and California. What that means is synoptic forcing and moisture advection across our area with the first tough will be weak. There will continue to be snow showers in the mountains into Sunday morning given SW flow and really good lapse rates, but without another source of lift the amounts will remain on the light side and only minor travel impacts are expected. There will be a lull in snow across most of the area during the day on Sunday until the weakening trough gets kicked east/absorbed and the first trough axis moves across Colorado late Sunday into early Monday. Snow showers should arrive mainly along and south of I-70 overnight into Monday morning, but again amounts will be very light, T-2" or so. The plains should remain dry other than maybe the Palmer Divide and some flurries for the eastern foothills and west/south suburbs of Denver late Sunday into early Monday. On Monday the stronger trough digs south across CA and builds a progressive ridge that moves across Colorado throughout the day. Thus, our area should remain dry on Monday during the daylight hours. Despite SW flow aloft afternoon, cold advection will occur and will keep temperatures in check. Across the plains highs should be in the 30s, but in and around Denver metro may top 40 degrees. The mountains will be in the 20s. Winds will be on the light side across the area with the ridge moving overhead. On Tuesday models in general are still having trouble agreeing on the evolution of the trough. The EC is more progressive and brings it ENE across southern Colorado, while the GFS is slower bringing it across central Colorado. The GFS solution would be a wetter solution for us with slower movement and better lift across our area. Neither solution would favor deep east/northeast upslope component, and QG lift is meager. Even the jet would be too far south as a source of lift. Orographics will be the primary mechanism for lift across the area with shallow upslope across the plains and WSW flow across the mountains. Looking at the ensemble output, the GEFS would favor snow across the plains, but the ensemble mean liquid equivalent is about 0.1-0.3" depending on location relative to terrain. The faster EC is about half that for the plains and I- 25 urban corridor. Both ensembles have little spread indicating confidence (for now) of light snow across the plains. In the mountains, relying again on just orographics and lapse rates means light snow as well. Models agree that Wednesday will be dry across the area with yet another west coast trough building a ridge that moves across Colorado throughout the day. 700 mb temps warm 10 degC by early Thursday but not soon enough for Wednesday`s high temperatures. It looks to remain a little below normal across our area for late January. Highs across the plains should be in the low 40s, 30s in the eastern foothills, and 20s in the mountain valleys. Thursday also looks dry as another storm system digs south across California and generates increasing southwest flow aloft. Should be enough warm advection in the flow to result in highs above normal. && .AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon) Issued at 230 PM MST Fri Jan 22 2021 Expect VFR conditions at all terminals through the TAF period. Mid to high level cloudiness should continue to gradually increase this afternoon and evening with broken to overcast skies overnight. No precipitation is expected east of the foothills today or tonight, and tomorrow only flurries are expected on the adjacent plains. Northeasterly winds this afternoon should persist through the evening until being replaced by drainage winds overnight. Tomorrow, expect a period of gusty northwesterly winds in the late morning or early afternoon preceding stronger, gusty northeasterly winds behind a cold front. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 5 PM this afternoon to 11 PM MST Saturday for COZ031. && $$ SHORT TERM...Direnzo LONG TERM...Schlatter AVIATION...Direnzo NWS BOU Office Area Forecast Discussion
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