864 FXUS65 KBOU 011021 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 421 AM MDT Sun Jun 1 2025 .KEY MESSAGES. - A few more showers and storms today, with warmer weather. Highs in the mid/upper 80s across the plains. - Still on track for a widespread rainfall event Monday afternoon into Tuesday, along with a return to below normal temperatures. - Snowmelt plus rainfall will lead to high flows on mountain streams. && .UPDATE... Issued at 406 AM MDT Sun Jun 1 2025 Today is expected to be the warmest day of the week, as our CWA sits near the axis of the upper level ridge. Temperatures should easily reach the mid 80s across the plains, with some near 90F readings possible depending on how quickly convective cloud cover develops this afternoon. Showers and storms will be isolated to scattered across mainly the higher elevations, with a few making the trek down into the I-25 corridor. Gusty winds would be the main threat with any storms today with DCAPE values closer to 1200-1400 J/kg this afternoon. Meanwhile, we continue to see a trend away from the more significant rainfall advertised by guidance Monday night into Tuesday. Most overnight model runs have continued to pull away from a clean interaction between the southern cutoff and the larger scale trough axis over the northern Rockies. As a result... the surface low ends up weaker and further southeast, and synoptic scale lift is quicker to exit the region. There still looks to be a favorable window for some meaningful rainfall Monday afternoon into Monday night... first from the advancing surface cold front as it encounters a fairly warm/unstable airmass, and then in the post-frontal/upslope regime where we still have some QG ascent. Some overnight CAMs were surprisingly dry across the CWA, but the large scale signal from ensemble and AIFS guidance would still favor a corridor of >0.50" of rainfall across much of our eastern plains. The more notable drying trend is further northwest across Middle/North Park and the surrounding mountains. Finally, it is worth nothing that there is at least a conditional severe weather threat Monday afternoon across most of lower elevations. With surface dew points remaining in the upper 40s to mid 50s, MLCAPE should increase to 1000-1500 J/kg. While deep layer shear is modest, at least a few multicell clusters or perhaps a splitting supercell or two (given relatively straight line hodographs) would be possible, especially closer to the Nebraska border where the better parameter space and forcing lie during the afternoon. && .DISCUSSION /Through Saturday/... Issued at 251 PM MDT Sat May 31 2025 Weak convection has begun over the mountains and should increase through the late afternoon. There`s still potential for a few cells to move southeast off the mountains, though the environment at lower elevations is less favorable than yesterday. The current forecast seems to have a decent handle on this, though we did increase the cloud cover overnight for lingering mid level clouds. Sunday`s environment isn`t much different, although a Denver cyclone boundary could help get a bit more storm activity going south of Denver in the mid to late afternoon. Temperatures will be 4 to 8 degrees warmer. Denver will likely be near 90, but probably just short of that mark. The model handling of the Monday/Tuesday system is not that bad, though the resultant QPF still varies. One model trend is to move the synoptic scale lift steadily eastward and bring an end to the significant precipitation earlier on Thursday. Some model runs had slowed the departure over the plains resulting in larger rain amounts. Now the consensus is that the rain in the mountains will mostly be Monday afternoon and Monday night, and on the plains 4 to 6 hours later. This reduces the forecast rain amounts a bit. Most models are still in the 0.5 to 1.5 inch range for the mountains, and 0.75 to 2 inches for the plains. The NBM is in the middle of these ranges overall and seems adequate. There are some runs with lighter amounts, and a few outliers that have twice that. It seems unrealistic to have that much over a large area, but there is potential for embedded thunderstorms or a mesoscale focus that lasts a few hours. The most likely chance for intense rain would be with convection Monday afternoon and evening. See below for hydro impacts. There`s pretty good agreement on keeping a cool moist environment with another shortwave late Wednesday. Amounts should be much lighter overall, but PoPs will still be high and there could be some localized heavy showers. There`s a drying and warming trend after that, with Thursday as the transition day. NBM guidance is on the warm edge of the envelope, but this is probably OK, with temperatures back around 80 in Denver Friday and Saturday. With drying aloft but probably less and the surface, this looks like a fairly typical June pattern and there could be some threat of all kinds of severe weather and burn area flooding by the end of the week, though there`s nothing that looks like a particular threat. Just a fairly normal early June pattern. && .AVIATION /06Z TAFS through 06Z Monday/... Issued at 1138 PM MDT Sat May 31 2025 VFR through the TAF period. Winds are gradually turning back to typical drainage flow and should continue overnight. A fairly typical late spring diurnal pattern is likely again for Sunday, with winds eventually turning towards some sort of light E/ENE component during the day. Some high-based convection is expected by tomorrow afternoon, with most guidance getting a few storms near the terminals by about 21-22z. Main impact would likely be a couple of hours of variable/gusty winds, with stronger storms producing a few gusts near 35kt. Any convective activity should quickly diminish by about 03z with drainage flow returning. && .HYDROLOGY... Issued at 251 PM MDT Sat May 31 2025 Forecast temperatures, snowmelt, and rainfall will be enough to cause rises on streams in the mountains, and minor flooding could result late Monday and Tuesday in areas that get heavier rainfall. In areas east of the mountains and burn areas, the threat is lower, but there could be enough rainfall to cause localized minor flooding. Larger rivers should be able to handle the most likely rain and snowmelt, though they will be running high. There is a small chance of rain that is heavier than expected on a scale of a county or two, that could produce flooding on one of the larger rivers. At this time, this appears to be a low probability (10 percent or less). && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Hiris DISCUSSION...Gimmestad AVIATION...Hiris HYDROLOGY...Gimmestad