479 FXUS65 KBOU 020621 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1221 AM MDT Mon Jun 2 2025 .KEY MESSAGES... - More widespread and locally heavy rainfall still expected late Monday - Monday night. Average precipitation amounts have decreased somewhat, however. - Snowmelt plus rainfall will lead to high flows on mountain streams. - Cooler Tuesday with a decrease in showers. Then gradual warming trend rest of the upcoming week, but a round of scattered showers/storms most days. && .DISCUSSION /Through Sunday/... Issued at 212 PM MDT Sun Jun 1 2025 Scattered showers and storms are getting going over the high country, and CIN is weakening sufficiently to allow some of these to drift onto the I-25 Corridor and adjacent plains. Despite greater CIN on the eastern plains, we do expect enough lift from gusty outflows to generate at least isolated convection if not a bit more on the eastern plains through this evening. Gusty outflow winds to 45+ mph will be the main threat with DCAPE (downdraft) of 1300-1500 J/kg, but MLCAPE (updraft) near 1000 J/kg so couldn`t totally rule out a brief severe/near severe storm with marginal hail/wind. The main concern for this forecast cycle shifts to potential for locally heavy rainfall late Monday afternoon and Monday night. As early discussed, there is not great linkage between the trough ejecting from the Desert Southwest and the trough arriving from the northwest. However, there is still a brief period very late Monday afternoon to late evening where things do line up, providing our highest threat of locally heavy rainfall then. Ingredients come together in this sense; deeper moisture associated with the southwest ejecting trough arrives 20Z-00Z, coincident with peak heating and instability. That should set off the first round of storms, spreading from the mountains onto the plains. A few of those storms could become severe on the northeast plains as MLCAPE increases to 1200-1800 J/kg as surface dewpoints hold in lower to mid 50s. The greatest risk of severe would be roughly east of line from Cheyenne to Fort Morgan to Akron. This is also when more uncertainty starts to enter the forecast. We`ll still be in favorable synoptic lift area and a cold front arrives 5 pm - 8 pm. Thus, storms could either regenerate behind the initial round of storms, OR the stronger storms could congeal into a more organized MCS and head into Nebraska - and then we just redevelop a more showery/isolated embedded storm event into the Monday evening and overnight hours. We think the latter is the most likely scenario at this time. That`s another reason both ensemble and deterministic QPF amounts have decreased over the last 24 hours. Nonetheless, we think most areas along/east of the Front Range have a high probability (>70% chance) of greater than 0.50" of rain, and a 40% chance of greater than 1". Those probabilities would be slightly lower/higher the farther north/south one progresses, respectively. Also worth noting, stronger storms will certainly be capable of producing locally heavy rain. There is a small risk of flash flooding from 1) an isolated strong storm moving over a burn scar Monday afternoon and 2) stronger storms merging or training on the northeast plains in the late afternoon and evening. More in the hydro discussion below. For Tuesday, we expect most precipitation to be shifting off to the south/east as the northern U.S. trough continues to push southeast, and rather rapid drying/stabilization occurs behind it. Forecast soundings show it will be difficult to regenerate any convection with daytime heating as it will be limited by the cooler boundary layer, so main chance of showers/isolated storms for the afternoon would be over the high country. Wednesday through the end of the week...The airmass will gradually warm and destabilize. Another trough is expected to eject out of the Desert Southwest late Wednesday or Wednesday night which should bring another round of at least scattered or greater coverage of showers and thunderstorms. This pattern is likely to continue through the latter part of next week as it appears modest mid level moisture will combine with additional shortwaves (these in the northern branch). MLCAPE is generally on the modest side through the week, but it`s early June so certainly any of these days could see a threat of a couple severe storms. && .AVIATION /06Z TAFS through 06Z Tuesday/... Issued at 1151 PM MDT Sun Jun 1 2025 VFR through Monday afternoon, with steadily deteriorating conditions Monday evening into Monday night. Light drainage winds tonight will again gradually turn back to some sort of east or northeast component by early tomorrow afternoon. There should be widely scattered thunderstorms across the area by about 21z. Not sure if there`s enough coverage of TS to justify a TEMPO, so dropped down to a longer PROB30 given some uncertainty in the timing of any convection near the terminals. A fairly strong cold front is expected by the evening, with cigs quickly lowering behind it. Widespread rain showers should develop closer to 03z-06z, with some potential for some heavier rain along with IFR ceilings by 06z. IFR conditions are likely to prevail most of Monday night. && .HYDROLOGY... Issued at 212 PM MDT Sun Jun 1 2025 Continued snowmelt and modest rainfall will be enough to cause additional rises on streams in the mountains. At this time, only the upper Colorado River is under enough threat of rises for minor flooding, and a Flood Watch remains in effect for a small section above Grand Lake Monday evening through Tuesday. The main threat of flooding along and east of the Front Range would be more localized and "flashier", given storms will still be capable of producing heavy rain, but the total amounts will likely be lower than earlier anticipated. The risk of flash flooding would be from 1) an isolated strong storm moving over a burn scar Monday afternoon or 2) stronger storms merging or training on the northeast plains in the late afternoon and evening. Warm cloud depth briefly increases to 6,000 feet on the plains, so storms would be efficient rainers. A stronger storm in the foothills could produce a quick inch of rain (less deep warm cloud depth) in 30 minutes, while a stronger storm on the eastern plains in a richer environment could produce 2 inches of rain in 45 minutes, so something to still watch considering potential for training or merging storms. Meanwhile, the I-25 Corridor seems to be caught in between for the most part, and we earlier collaborated with WPC to reduce the flash flood threat to Marginal from Slight Risk. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Barjenbruch AVIATION...Hiris HYDROLOGY...Barjenbruch