FXUS65 KBOU 181533 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 933 AM MDT Mon May 18 2026 .KEY MESSAGES... - Rain and mountain/foothill snow increases in coverage this morning with much colder temperatures arriving. - Accumulating snow for the mountains and the foothills. Only a few wet snowflakes possibly mixed in for the I-25 Corridor. - One last spring freeze possible on some of the plains Monday night. - Delay in the warming and drying trend for the week ahead, but still warmer and drier by Friday - next weekend. && .UPDATE... Issued at 933 AM MDT Mon May 18 2026 Snow was increasing over the mountains, with a heavy accumulating drizzle along the Front Range I-25 Corridor. Precipitation rates will increase as expected this morning through mid afternoon with the heart of QG forcing and band of heavier precipitation on the western slope moving in. It was thinning a bit as it moves this way, but still expect a 2-3 hour period of enhanced precipitation rates and even a possibility of a couple thunderstorms as this moves in. Given snow levels are lower already (down to about 6500-7000 feet in Larimer County), we added a Winter Weather Advisory for the northern Foothills as snow intensity should pick up to >1" per hour. Also there was even a narrow corridor of freezing drizzle in the higher foothills of Boulder County (7,500-8,500 feet) where deeper seeder/feeder action was not in play yet. That will change soon as deeper moisture arrives. We also added North Park to the Winter Weather Advisory considering heavier snow had begun falling there this morning and will continue through mid afternoon with rates 1" per hour or more at times. Finally, still can't rule out a few snowflakes mixing in with our rain in the I-25 Urban Corridor despite high surface wet bulb temps. Main opportunity would be with convective showers early to mid afternoon, and then into evening if precipitation lingers long enough under the influence of further cold advection aloft. && .DISCUSSION /Through Sunday/... Issued at 1222 AM MDT Mon May 18 2026 An upper level trough over the Great Basin will move across the area on Mon with favorable mid level ascent thru the aftn hours. At the sfc, upslope flow will be in place which will combine with the upper level trough to produce widespread precip across the area. Snow will occur in the mtns and higher foothills with several inches of accumulation. Have upgraded areas north of I-70 in the mtns to a warning as east facing slopes could see from 12 to 18 inches in some areas. Elsewhere will keep amounts in the advisory range. In the foothills, some of the higher areas may see 4 to 8 inches above 8000 ft with lessor amounts below 8000 ft. By Mon night precip should gradually decrease over the area during the evening hours. Overnight lows late Mon night into early Tue morning may drop down to freezing or slightly below across portions of the plains. For Tue, a disturbance will move across the area late in the aftn thru Tue night. As this feature moves across, there will be another round of precip over the higher terrain and across portions of the plains. Highs on Tue will remain cool as readings stay in the 50's across the plains. Looking ahead to Wed, an unsettled pattern will continue as decent lapse rates will combine with lingering moisture to produce a good chc of showers and thunderstorms over the higher terrain and portions of the plains. Most of the activity will occur in the late aftn and early evening hours. Highs will remain below normal as readings only reach the upper 50's to mid 60's across the plains. By Thu into Fri, latest data shows another upper level trough moving southeast towards the area. Not sure about the timing of this system, however, as it moves across should see another good chance of showers and thunderstorms. Temperatures both days will be in the mid 60's to lower 70's across the plains. For next weekend, will see a return to a drier and warmer pattern based on latest data. && .AVIATION /12Z TAFS through 12Z Tuesday/... Issued at 518 AM MDT Mon May 18 2026 Ceilings are currently bouncing between 300ft and 600ft at the TAF sites. These IFR and LIFR conditions are expected to continue through the rest of today and into tomorrow morning. Scattered light rain showers this morning will increase in coverage late this morning and early this afternoon, leading to multiple hours of rain showers and a few thunderstorms. Coverage should start to decrease near the TAF sites around 21Z to 23Z, leaving some scattered showers for the late afternoon and evening. Rain chances will decrease for the overnight period, but there still is a low chance (<30%) that a few showers could stick around. Ceilings will start to lift slightly for Tuesday morning, but they still look to remain below VFR thresholds. Winds today will generally be out of the north or east with periods of gusty winds late this morning at KDEN and KAPA. Variable gusty winds could also accompany any thunderstorms in the area. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 9 PM MDT this evening for COZ030- 031-034-035. Winter Storm Warning until 9 PM MDT this evening for COZ033. Freeze Watch from late tonight through Tuesday morning for COZ038>051. && $$ UPDATE...20 DISCUSSION...RPK AVIATION...AP