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218 FXUS65 KBOU 181801 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1201 PM 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 /18Z TAFS through 18Z Tuesday/... Issued at 1159 AM MDT Mon May 18 2026 Model cross sections indicate that the ceiling should be at or below BKN-OVC015 at DIA the rest of the today and all of tonight. Will leave a PROB30 in for TSRA this afternoon for 3 or 4 hours based on the instability available. Visiblities should range from 1SM-5SM, lower with the better rain showers through around 00Z this afternoon. Will get visibilities to P6SM after that. There could be patchy fog developing after 06Z tonight, but there is enough uncertainty at this time so I will it out for now. && .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. Frost Advisory from midnight tonight to 8 AM MDT Tuesday for COZ038-042>051. Freeze Warning from midnight tonight to 8 AM MDT Tuesday for COZ041. && $$ UPDATE...20 DISCUSSION...RPK AVIATION...66