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591 FXUS65 KBOU 171938 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 138 PM MDT Thu Sep 17 2026 .KEY MESSAGES... - Active weather pattern through Saturday. Locally heavy rainfall will be the greatest storm threat, although a couple severe storms possible Saturday. - Near or just above normal temperatures settling below normal for Sunday and Monday. - Trend toward drier weather by early next week. && .DISCUSSION /Through Thursday/... Issued at 138 PM MDT Thu Sep 17 2026 A very moist south-southwest flow aloft will continue through Friday between an upper level high over Texas and an upper level low to the northwest. Latest ACARS soundings show precipitable water valuesof 1.2 inches, about 220 percent of normal. The soundings also show a long skinny CAPE profile of about 500 J/kg, which favors heavy rainfall. Storms willmove somewhat quickly, which will limit the heavy rain and flooding threat. Best chance for thunderstorms through this evening will be east of I-25 where it`s been mostly sunny the past few hours and temperatures have climbed into the upper 70s to lower 80s. To the west of I-25, it`s been cloudy most of the day with on and off light rain. Temperatures have been slow to warm with 1:00 PM readings in the upper 60s to lower 70s. Light rain showers are expected to linger into the evening, with the radar showing additional showers lined up to the south-southwest. The upper level low will slowlytravel eastward across southern Idaho and Wyoming this weekend. Moisture decreases, but remains well above normal (140-180 percent of normal) through Sunday. Mid level temperatures will cool as the upper levellow nears the region. MLCAPE reaches 600-1200 J/kg both days. Increasing westerly flow aloft around the upper level low will help produce good shear Saturday with 25-40 knots of bulk shear (06 km) and 35-50 knots of bulk shear (06 km) Sunday. High temperatures are expected to be mainly in the 70s with Sunday being a few degrees cooler, which could end up capping convection. The passage of a shortwave trough is expected to result in better shower/thunderstorm coverage on Saturday. Models hint at heavier rainfall over the northeast plains after 00Z Sunday. On Monday, the upper level low will weaken as it exits the region. A new upper level low/trough moves over California and the Great Basin. In the meantime, an upper level ridge will be centered over Texas. For Colorado, weak ridging is expected to prevail, resulting in dry conditions. For Tuesday through Thursday, an upper-level ridge remains over the south central and southeast part of the country, while troughiness will be over the southwest. This pattern will allow shortwave troughs to lift northward out of Mexico/the eastern Pacific and move across the region. There isn`t much agreement yet on the location and timing of the shortwave troughs, but there should be a day or two with at least scattered showers and thunderstorms. Temperatures during this time frame will be near normal with highs in the 70s across northeast Colorado. && .AVIATION /18Z TAFS through 18Z Friday/... Issued at 1221 PM MDT Thu Sep 17 2026 Showers have started popping up over the foothills this morning, with several skirting over KBJC. Throughout the afternoon and evening, we expect the greatest threat for showers/heavy rain to be over the foothills and northeastern plains, though we can`t rule out some scattered showers or storms over the metro area. While there`s not much instability, plenty of moisture has been brought in from the south, and so, should any showers or storms move over the TAF sites, locally heavy rainfall would be the main concern. Still, confidence regarding storms over the TAF sites is lower than it was yesterday, so we`ve dropped the previous TEMPO group down to a PROB30 for this afternoon and evening. Most models keep precipitation in the area through at least 02Z, with rainfall lingering a little longer in the vicinity of KBJC. As showers leave the area, winds should be light and variable before settling into drainage overnight. The late afternoon/evening tomorrow will be our next best chance for storms/showers. As such, we`ve introduced a TEMPO starting at 22Z and lasting through the end of the TAF period; that being said, model guidance currently suggests the bulk of convection to be later (23Z-03Z), so we`ll likely be extending the TEMPO group at the 00Z TAF update this evening. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...BM AVIATION...AGS