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315 FXUS65 KBOU 182351 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 551 PM MDT Fri Sep 18 2026 .KEY MESSAGES... - Increasing coverage and intensity of afternoon/evening showers and t-storms for the lower elevations, peaking Saturday when locally heavy rainfall can be expected across the plains. - Warm temperatures through Saturday to give way to more seasonal conditions Sunday onwards. - Trending drier next week, although at least isolated to scattered showers will remain each day. && .DISCUSSION /Through Friday/... Issued at 210 PM MDT Fri Sep 18 2026 The day started off mostly cloudy, which slightly slowed heating, but skies cleared allowing temperatures to climb into the lower to mid 80s. ACARS soundings show 200-500 SBCAPE J/kg with little to no CIN. Precipitable water values are around an inch. We seem on track for scattered to numerous showers and thunderstorms later this afternoon and evening. Main threat will be brief heavy rain, small hail, and wind gusts up to 40 mph. Can`t rule out a stray severe thunderstorm or two across the eastern plains where better instability resides. The showers and thunderstorms will progress eastward and end by mid-evening over eastern Colorado. The upper level low over the Great Basin slowly moves eastward tonight and Saturday, increasing QG ascent over the region. Cross sections show skies should be mostly clear to partly cloudy tomorrow morning, allowing temperatures to reach their convective temperature (mid to upper 70s). Scattered to numerous showers and thunderstorms are expected again, but they should begin a couple of hours earlier. Storms increase in coverage and intensity as they move onto the plains during the late afternoon and early evening. MLCAPE reaches 500-1500 J/kg and 0-6km bulk shear is 30-40 knots. A few of the storms could become severe, mainly over the plains where the instability will be better. Models agree we will see a good amount of convection tomorrow, but they disagree by a few hours on the timing. Will it be an early to late afternoon event, or a mid afternoon to early evening one? The upper level low will move eastacross Wyoming on Sunday. At the surface east winds will bring cooler temperatures and keep low level moisture in place. We will likely see low clouds and if they are thick enough, there will be areas of drizzle Sunday morning. Highs are only expected to reach the mid 60s to lower 70s. Even with this, the cold air aloft and moist low levels yieldSBCAPE up to 1000J/kg. If temperatures can climb enough to break the cap, scattered thunderstorms are expected. Given the instability and good shear, a few severe storms will be possible. Historically, days like this favor the Palmer Divide where breaking the cap will be easier. Because of the cold air aloft, hail will be the main threat from the thunderstorms. On Monday, flow aloft returns to the southwest as the upper level low shifts east of the region. A shortwave ridge embedded in the southwest flow is expected to bring a mild and mostly dry day. However the 12Z ECMWF slowed the eastward progression of the low and shows showers lingering into Monday morning over eastern Colorado. For Tuesday through Friday, the models loosely agree, but detailsvary. It begins Tuesday when the closed low over southern California weakens and starts to lift northeast across the Great Basin. Models vary on the strengthen, timing, and track of this feature Tuesday and Wednesday, which sets up the pattern for the rest of the week. Ridging will take over the intermountain west and plains mid week. Does this upper level trough get trapped underneath the ridge (GFS solution) or does it quickly lift northeast into the Northern Plains (ECMWF solution)? We`ll see a chance for showers and thunderstorms Monday night and Tuesday as the upper-level feature embedded in the southwest flow lifts northeast across the region. If the slower solutions are correct, the chance for showers could linger into Wednesday. For Thursday and Friday, ridging is expected over the region. It could end up being a "dirty ridge" with subtropical moisture trapped underneath, continuing the chance for showers and thunderstorms. Temperatures from Tuesday onward are expected to be near to slightly above normal. && .AVIATION /00Z TAFS through 00Z Sunday/... Issued at 541 PM MDT Fri Sep 18 2026 Several things of note during this upcoming TAF period. First, convection has moved off to the north and east of DEN, and only a few weak showers are expected over the next hour or so. Winds are light and variable at DEN and APA, while BJC remains in fairly gusty westerly flow from decaying showers over the foothills. First concern is the potential for either FG or low stratus developing after midnight tonight. Most of the available 18z guidance is on board with a weak push of northerly flow (likely associated with this current cluster of convection) near 06z, with rich near surface moisture behind it. The last several cycles of the HRRR/RAP are in broad agreement, with support from both NAMs and a few other models. Given saturated/moist adiabatic profiles through a depth of ~1km, BUFKIT profiles would lean towards stratus over fog... which is also reflected in the TAF. Guidance burns most of this stratus off quickly during the morning hours Saturday, leading to sufficient destabilization before yet another wave of convection early Saturday afternoon. Fairly good agreement on the timing here, with TS moving off the foothills by 19-20z and pushing east of the metro by around 00z in the evening. Finally, guidance is a bit more bullish with FG/stratus potential Saturday night into Sunday... perhaps as early as sunset Saturday evening. Have at least included a mention for now at DEN, but a lot to get through before getting to worried about the specifics. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...BM AVIATION...Hiris