Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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559 FXUS65 KBOU 202119 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 319 PM MDT Fri Sep 20 2024 .KEY MESSAGES... - Dry and warm conditions replaced by cooler temperatures and late day showers Saturday. - Storm system will impact the area Saturday evening through Sunday. 80-90% chance of precipitation, with rainfall amounts the last remaining uncertainty. - The first real mountain snow is likely, with >70% probabilities of accumulating snow above 9,000ft. Minor travel impacts over high mountain passes Saturday evening - Sunday morning. - Turning much cooler by late Saturday through Sunday. Below normal temperatures early next week, then likely (60% chance) back to drier and warmer weather toward the end of next week. && .SHORT TERM /Through Saturday/... Issued at 245 PM MDT Fri Sep 20 2024 Satellite and surface analysis shows a pretty dry airmass over the region, with just a band of high level cirrus clouds. Those clouds will dissipate this evening with mostly clear skies and relatively cool temperatures overnight. A cold front arrives on the northern plains late tonight into the early morning hours Saturday. That cold front will be the start of a significant change in weather for us. There will still be a fair amount of sunshine to start the day Saturday, so despite the cold front and persistent cold advection all day, high temperatures should still be able to climb into the 65-70 degree range on the plains and I-25 Corridor. A touch of fall will definitely be in the air, but nothing compared to the cool expected for Sunday. With regard to precipitation, there was a trend toward slightly slower arrival of QG lift and precipitation, with most development of showers holding off until late afternoon. Instability will be limited, so only isolated thunderstorms possible. The showers will be spreading in from the south and southwest, so places like the northern tier of counties could still stay dry the entire day. The bulk of precipitation and mountain snowfall is still on target for Saturday night into Sunday morning. More on that in the Long Term Discussion below. && .LONG TERM /Saturday night through Friday/... Issued at 245 PM MDT Fri Sep 20 2024 Winter enthusiasts rejoice! Northern Colorado will get its first taste of the changing season as the first measurable snowfall is expected this weekend for our highest elevations. Fun Fact: This will also coincide with the autumnal equinox. By Saturday evening, precipitation will shift east and north across the forecast area. As temperatures decrease, snow-levels are expected to lower to roughly 8700` to 9000`. Uncertainty in QPF remains, however, the latest runs show yet another slight southward shift with a drier trend from what the midnight shift discussed in the previous AFD. Northeasterly upslope flow will help bring the most snowfall accumulations to the Front Range Mountains of Park and Clear Creek Counties where accumulations from 5-8" are expected. With the anticipated southward trend, totals to the north will be lighter, ranging from 2-5", with some greater uncertainty with regards to locations in northern Larimer County. The greatest expected impacts will likely be along I-70 from Vail Pass to Georgetown where snow showers may cause some slick roads and hazardous travel Saturday evening into Sunday morning above 10000`. Even though impacts are expected to be minimal, with this being the first snow accumulation of the season, we have issued a Winter Weather Advisory for the Front Range Mountains and Summit County for elevations above 9000`. This will be in effect from 6PM Saturday through noon Sunday. Flash Flood threats to the burn scars should be minimal, with the Quarry Fire scar having a limited threat. Across the plains, ensembles still show some discrepancies with amounts, but show agreement that the main stretch for the highest accumulations of rainfall look to be from Boulder to Sterling, along and south of I-76. The GFS ensemble shows a 40-50% chance of greater than .5" falling in this location, where the ECMWF ensemble shows 80-90% chance of this occurring. Precipitation is expected to shift north and east throughout the day, diminishing by the late afternoon. Temperatures will feel quite different than what we have felt over the past few months, with afternoon highs struggling to reach the 60s across the plains. Overnight lows will drop into the 20s and 30s for the mountains, and remain only slightly warmer across the plains, with widespread mid to upper 30s expected. Ridging is expected to build behind the exiting storm system to kick off the work week. A shortwave is expected to pass through bringing another chance for precipitation across the CWA on Tuesday. By mid-week, temperatures are expected to climb back into the 80s, but looks like the changing season is beginning to take hold, with no 90s expected in the extended forecast period. && .AVIATION /18Z TAFS through 18Z Saturday/... Issued at 1157 AM MDT Fri Sep 20 2024 VFR conditions continue through the TAF period. Winds are expected to hold from the ESE through the afternoon at KDEN and KBJC, with a shear zone/cyclone development this afternoon to the south. There is relatively high confidence this shear zone stays south (70-80%) through the afternoon. However, KAPA could be just far enough south for gusty south/southwest winds there (50-60% confidence) toward peak heating/mixing around 21Z. Winds get a little more tricky through this evening, with the shear zone/cyclone potentially lending toward more north/northwest flow 01Z-05Z, and then disrupted or lighter than normal drainage winds overnight. We`ve opted for a middle of the road approach with more westerly component overnight. A cold front arrives 12Z-14Z Saturday with switch to northerly winds. Low level moisture is rather scant behind the front, so the threat of any stratus or lower ceilings is still small (<20%). Showers will increase after 21Z Saturday with gradually lowering cloud decks then. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 6 PM Saturday to noon MDT Sunday for COZ033-034. && $$ SHORT TERM...Barjenbruch LONG TERM...Bonner AVIATION...Barjenbruch