405 FXUS65 KBOU 241826 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1226 PM MDT Wed May 24 2023 .UPDATE... Issued at 1220 PM MDT Wed May 24 2023 First storms over the east slopes, mainly in Larimer County, were enough to create brief heavy rain but convection has been very pulsy with new development outflow driven. The Cameron Peak burn area may be pretty well worked over soon which may reduce the flood threat in a less unstable cloudy air mass. Hopefully we'll wind up with a low level convergence area that's east and south of the current activity and new and potentially stronger convection will be focused there. Not much to change with the rest of the forecast at this point. The clouds will reduce instability, but with very slow storm motions we could see productive pulse storms. Probably not much of a threat of flooding except if we get a second pulse that drifts back over the same area, which could happen somewhere. After the next hour or so, the main threat of problems is probably in our urban areas mid to late afternoon. && .SHORT TERM...(Today and tonight) Issued at 325 AM MDT Wed May 24 2023 Key messages: 1) Active weather pattern starts today 2) Locally heavy rain and a couple severe storms possible 3) Smoke departs Today will mark the beginning of an extended period of unsettled and stormy weather. The upper level high over Nebraska is shifting ever so slowly east today and will eventually center itself into Iowa. That will allow the mid level dry pocket over the northeast plains to retreat, with deeper moisture and a less capped environment taking hold across the forecast area. At the same time, low level moisture has continued to increase, with dewpoints into the mid 50s as far back as Fort Collins. With the increase in low level moisture, MLCAPE are advertised by most short range models to reach 1000-1500 J/kg across the plains. Meanwhile, deep layer shear is quite weak and under 20-25 knots which means the severe threat will come from pulse storms. Large hail is the main threat. But, of course, it's Colorado so you can never rule out a landspout should the right combination of developing updrafts/boundaries line up just right. Heavy rain will also be a threat given the very weak shear and mean wind for storm motion only between 6 and 10 knots. More details on the heavy rain and burn area flash flood threat in the Hydrology section below. Storm coverage, despite the lack of much forcing, is expected to be considerably higher than yesterday given the absence of a cap. In fact, most convective allowing models show convection developing over the Front Range noon - 3pm, and then organizing/moving slowly east across the plains 3-10 pm, before weakening toward the northeast corner of the state 10 pm - midnight. We've gone along with that scenario given the good agreement and conceptual model for today's event. A couple things could throw that off a bit; 1) It appears some stratus/patchy fog development will occur this morning, and that could impact destabilization 2) Mid level clouds are starting to thicken across the mountains and into portions of south central Colorado, so there could be a few showers as early as this morning already. We do think most areas will see rain and hear thunder this afternoon and evening. Finally, for those looking for relief from the smoke and poor air quality - it's coming! As the flow aloft eventually turns southwest and we reach convective temperatures, we'll start to see some improvement this afternoon, and then a big improvement as the expected area of showers/storms moves across with further smoke dispersion and a cleaner airmass following the storms. .LONG TERM...(Thursday through Tuesday) Issued at 325 AM MDT Wed May 24 2023 The synoptic pattern will change very little over the next several days, leading to a 7 day forecast that almost feels copied and pasted from day to day. Daily afternoon thunderstorms... with a heavy rain, hail, and wind threat... will be the main headline for the upcoming week. A broad trough axis will remain in place to the west of the region, with ridging across the Central CONUS. Weak south- southwesterly flow aloft should prevail across Colorado, though the overall airmass will be a bit drier. Overall storm coverage should be diminished somewhat compared to Wednesday, but isolated to scattered thunderstorms are still expected by the afternoon across the Front Range and plains, where modest instability is anticipated. Light upper level winds will limit deep layer shear to around 20-25kt, but a few marginally strong/severe storms are still possible. The instability axis should very slowly shift eastward Friday into the weekend, with scattered storms likely across the plains each afternoon. A few strong to severe storms will be possible given the marginally favorable environment. Guidance does eventually get slightly faster mid-level winds into the region by the weekend, and there may be a better chance for more organized severe weather sometime this weekend... but confidence in that is fairly low. Models begin to diverge by early next week, though the general pattern would still support afternoon convection. Temperatures through the period should be a few degrees above normal values, with highs across the plains reaching the mid to upper 70s. && .AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon) Issued at 1220 PM MDT Wed May 24 2023 Thunderstorms are expected in the Denver area later this afternoon, mainly between 21z and 01z. Some of the storms will produce wind gusts to 40 knots and IFR conditions in heavy rain. VFR conditions are expected overnight, with a chance of IFR in fog between 10z and 14z. && .HYDROLOGY... Issued at 325 AM MDT Wed May 24 2023 Increased moisture will lead to stronger storms with heavier rainfall this afternoon and evening. Storms will also be slow moving, and thus we'll see an elevated threat of flash flooding in some of the burn areas. It appears the largest influx of moisture will be on the east side of the Front Range mountains, putting the Cameron Peak burn area as the most vulnerable burn area today for flash flooding. Little has changed farther west for the East Troublesome and Williams Fork, so the threat remains more limited there. The main time frame for burn areas would be from Noon - 5pm. We'll also see locally heavy rain develop from storms across the I-25 Corridor and adjacent plains. These storms should be fairly efficient rain producers given warm cloud depths near 3000-3500 feet and relatively low LCLs (near 5000-6000 ft AGL). Storm intensity will also be there given MLCAPE of 1000-1500 J/kg so stronger storms will be capable of producing 1 to 1.5 inches of rain in 30 minutes, before weakening (most of these will be pulse storms given weak shear environment). That would be enough rain to produce considerable street flooding and even some lowland flooding considering the recent wetness. At least a limited threat of burn area flash flooding will continue through most of the extended forecast period. Near to above normal moisture, weak flow aloft, and modest instability will support slow moving thunderstorms each afternoon. There may be a day or two where an elevated risk is justified, but at this point no particular day stands out. Daily thunderstorms should continue into early next week. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Gimmestad SHORT TERM...Barjenbruch LONG TERM...Hiris AVIATION...Gimmestad HYDROLOGY...Barjenbruch/Hiris