107 FXUS65 KBOU 080252 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 852 PM MDT Thu Oct 7 2021 .UPDATE... Issued at 824 PM MDT Thu Oct 7 2021 Broad southwest flow aloft associated with the longwave trough currently off the West Coast continues to push mid and high level moisture into Colorado, with ample clouds tonight keeping temperatures a few degrees milder than last night. Still on track for an increase in moisture for Friday, which should bring some isolated showers in the afternoon primarily to portions of the high country. No significant forecast changes this evening. && .SHORT TERM...(This evening through Friday) Issued at 259 PM MDT Thu Oct 7 2021 An upper level trough currently off the Pacific West coast has dominating southwest flow over the state. There is slight ridging over the county warming area with weak QG subsidence in place bringing party cloudy skies and dry conditions. Conditions will be similar overnight and into early Friday with lows dipping into the upper 40s to lower 50s on the plains and mainly 30s in the high country. By Friday afternoon the upper trough will have moved onshore to CA keeping the region in southwest flow aloft. Through the morning and into the afternoon the jet circumventing the base of the trough will push into western CO with increasing moisture. This will start light showers over the higher terrain of mainly the northern mountains by the afternoon with areas above 11,000 ft seeing some light snow. As the trough and jet feature continues to push northeast the precipitation will become more widespread and continue into the overnight hours across the mountain region. At lower elevations for Friday conditions will remain dry with some increasing mid and upper level clouds by the afternoon. Temperatures will be similar to today. .LONG TERM...(Friday night through Thursday) Issued at 259 PM MDT Thu Oct 7 2021 ...STRONG AUTUMN STORM POTENTIAL TUESDAY - WEDNESDAY NEXT WEEK... We'll be watching next weeks strong storm system carefully over the next few days, more on that below. In the meantime, we'll have some unsettled weather to contend with, mainly over the mountains with rain and snow showers. Gusty winds Saturday, as well. For Friday night, a strong 140 knot upper level speed max and associated short wave will kick out of the Desert Southwest. There is enough moisture to provide more numerous showers in the mountains, especially along and north of a Rocky Mountain National Park to Vail line. Orographics are best suited for the Northern Gore and Park Ranges, while the rest of the forecast area is pretty strongly blocked by southwest flow aloft. Thus, we don't expect much more than a dusting to 2" in most of our high alpine areas as snow levels dip to around 10,500 feet. On Saturday, the upper level speed max sinks to our south, putting us in a slightly more unstable airmass with cold advection aloft and steepening lapse rates. Snow levels should continue to fall, dropping to around 9500 feet or so during the day. We should see at least a couple more inches of snow in the northern mountains, and perhaps slightly higher than that over the northern Gore and Park Ranges which again will be setting up better from an orographic perspective (more westerly flow). Winds will also be increasing with the cold advection, mixing, and downslope pressure gradient in place. We'll likely see wind gusts around 30-35 mph across most of the plains, with 40-50 mph peak gusts in the mountains and wind prone areas of the foothills. Most of the plains should stay dry given this downslope component, although not impossible for an isolated shower to survive. For Saturday night and Sunday, the upper level trough splits and weakens as it drops southeast across Colorado. As a result, we should see a gradual decrease to mountain snow showers and winds, while the plains remain on the dry side. Perhaps a few showers for the Palmer Divide area but overall appears the trend is for a mainly dry forecast for much of Sunday. Temperatures will be cooler, with highs by Sunday only being in the mid 60s across the plains, with 40s to lower 50s most of the high country. Now, onto the main forecast problem...the large storm dropping into the Southwest U.S. early next week, and then lifting northeast across the Central and Northern Rockies late Tuesday into Wednesday. The ensembles have been quite consistent with regard to the placement and timing of the upper level low and driving that into the Utah/Arizona border by 12Z Tuesday. From there, things become more uncertain. That said, about 60% of the ensemble members are leaning toward a deeper and slightly slower southern solution, while about 40% lean toward the north. The eventual track may actually end up dependent on the exact timing/amount of latent heat release that occurs as this storm taps into Gulf of Mexico moisture, and then heads northeast. We don't see much downstream blocking, but a high amplitude ridge should keep it on the slower side. Overall, the ECMWF and its ensemble members have remained fairly consistent, with that track showing a near miss for much lower elevation snow, but a bigger shot of accumulating snow and gusty winds/blowing snow for the mountains and even the northern foothills late Tuesday into Wednesday. It's certainly hard to get caught up in the details given just a small change in track will mean a big change in expected weather. It was interesting to note the large variations in the ensemble output, from the dry and windy conditions to a significant snow event (upwards of a foot) for parts of northeast Colorado. We have a feeling we won't know a whole lot more regarding the resultant weather for at least a couple more days given that a very small change in track would have a significant difference in expected weather. One thing is for sure, this will be a dynamic system with significant winter impacts somewhere across the Central/Northern Rockies. Stay tuned for the latest forecast updates! && .AVIATION...(For the 00Z TAFS through 00Z Friday evening) Issued at 824 PM MDT Thu Oct 7 2021 Southeast winds over the next few hours will transition to prevailing southwest drainage flow overnight. Friday morning, winds should be relatively light and variable, with a gradual shift to east/southeast winds in the afternoon and evening. SCT mid level clouds at/above FL080 likely for much of the period, along with abundant high clouds. VFR conditions likely to prevail through Friday evening at all terminals. && .FIRE WEATHER... Issued at 259 PM MDT Thu Oct 7 2021 Fire danger will increase Saturday as strong, gusty winds develop across most of the forecast area. Peak wind gusts will likely range between 25 and 35 mph across the plains, and between 30 and 45 mph over most of the mountains and foothills. However, humidity readings will be increasing. At this time, the lowest humidity readings and thus elevated fire danger is expected to occur over the Palmer Divide area eastward through Lincoln County. && .HYDROLOGY... Issued at 204 PM MDT Thu Oct 7 2021 Light showers will likely move over the burn areas late Friday afternoon through Friday evening, and then again Saturday, but rainfall amounts and rates should be rather light. Thus, no hydro impacts are expected for this period. Looking ahead, a large storm will move into the area next Tuesday and Wednesday. However, temperatures will turn cold enough to produce mostly snow over the burn areas. There would be just a small chance of heavier rain showers on the front end of this system late Monday or Tuesday. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Rodriguez SHORT TERM...Bowen LONG TERM...Barjenbruch AVIATION...Rodriguez FIRE WEATHER...Barjenbruch HYDROLOGY...Barjenbruch