Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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973 FXUS65 KBOU 182324 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 524 PM MDT Wed Sep 18 2024 .KEY MESSAGES... - Dry and warm through Friday with elevated fire weather conditions. - Storm system will impact the area this weekend. 70-80% chance of precipitation, but considerable uncertainty regarding how much. There is a chance of significant rainfall, and several inches of mountain snow depending on track/evolution. - Turning much cooler by late Saturday and Sunday. Below normal temperatures early next week, then likely (60% chance) back to drier and warmer weather toward the middle of next week. && .SHORT TERM /Through Thursday/... Issued at 245 PM MDT Wed Sep 18 2024 Water vapor imagery shows very dry air within southwesterly flow aloft. Subsident flow over our CWA has lead to a quiet weather day with mostly sunny skies. There are gusty winds especially across the high country but winds have been rather weak across the plains. Tonight and into tomorrow, a ridge aloft over Texas will strengthen and there will be ridging developing over Colorado within the southwesterly flow aloft. This ridging will increase subsidence and dry weather is expected to continue. There will be weak lee cyclogenesis in northeast Colorado tomorrow which will lead to light southeast winds. Without any downslope winds, highs will be similar to today. Highs will top out in the low to mid 80s across the plains. With the weak winds, critical fire weather conditions are not expected. && .LONG TERM /Thursday night through Wednesday/... Issued at 245 PM MDT Wed Sep 18 2024 We`ll start this forecast period with upper level ridging across Colorado, resulting in dry and unseasonably warm conditions through Friday. For now, it appears enough mid level ridging will occur for lighter flow aloft and weaker pressure gradients, thus marginal breezes for fire weather concerns. High temperatures Friday will reach the mid to upper 80s across the plains with the warm mid level temperatures and bountiful sunshine. The main concern for this forecast period will be this weekend`s potential storm system and resultant weather. While we`d like to say confidence is growing as we approach this weekend, there is still a considerable amount of uncertainty with regard to the track and speed. This uncertainty is rather typical for a storm system in this synoptic setup (strong upper level ridging over Texas and then a rather compact low getting ejected from the Desert Southwest). Thus, we expect to gain only slow confidence one way or another on the deterministic QPF forecast. One can see the significant differences between each and every operational run (e.g. 00Z and 06Z GFS having a whopping ~3" precip for Denver metro, but then the 12Z only showing ~0.1"!). Therefore, we`ll need to keep an eye on the big picture for now and generally align our forecasts with ensemble averages. Speaking of ensembles, it was interesting to note the 00Z GEFS output favored areas to the north of Denver, while EC ENS showed generally the heaviest QPF swath from the Front Range Foothills due east across the plains, while the CMCE favored south of Denver. Now the latest 12Z GEFS had also shifted south toward the CMCE, so we`re even seeing some gyrations in the ensemble averages. Individual ensemble ranges were impressive to say the least, showing anywhere between nothing and 4 inches of liquid precipitation. There`s even a large spread in the 25-75th percentiles - which suggest roughly 0.25 - 1.5 inches of precipitation. One thing we are more certain about is temperatures turning cooler, with highs likely struggling to the lower 60s across the plains by Sunday. The main message is this...Considerable uncertainty exists, but there is a very good chance (70-80% chance) of precipitation this weekend. We have a good chance (50-60%) of seeing a moderate amounts of precipitation, and low (~20% chance) of seeing light precipitation, and a similar ~20% chance of seeing a high end precipitation event. If the moderate to high end event unfolds, then we`ll be looking at several inches of high mountain snowfall (above 10,000 feet), but some accumulation down to ~8,500-9,000 feet if the heaviest/ coldest solution verifies. By early next week, there is decent agreement of northwesterly flow aloft developing as a ridge amplifies over the West Coast. This would allow additional shortwaves toward Monday and Tuesday with at least a small chance of showers, and temperatures remaining below normal on average. A warmup is expected toward the middle of next week with a return of upper level ridging. && .AVIATION /00Z TAFS through 00Z Friday/... Issued at 523 PM MDT Wed Sep 18 2024 VFR conditions tonight through Thu. NNE winds this evening will become light easterly by 06z. Drainage winds will develop by 10Z. By 15z winds will become more SE and continue thru Thu aftn. && .HYDROLOGY... Issued at 245 PM MDT Wed Sep 18 2024 We discussed the probabilities of precipitation amounts in the Long Term section above. Saturday afternoon and evening will have some threat of burn scar flooding if we see more convection develop ahead of the storm system. Following that, more stratiform type rainfall would be expected late Saturday night into Sunday, which typically doesn`t have a significant impact on burn scars. Nonetheless, if heavier scenarios unfold then some issues could still develop. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...Danielson LONG TERM...Barjenbruch AVIATION...RPK HYDROLOGY...Barjenbruch