Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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222 FXUS65 KBOU 220555 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1155 PM MDT Sat Sep 21 2024 .KEY MESSAGES... - Storm system will impact the area this evening through Sunday, with numerous showers and a couple storms. But expected rainfall amounts have trended toward the lower side of earlier guidance. - The first real mountain snow is still likely, with >70% probabilities of accumulating snow above 9,000ft. Minor travel impacts over high mountain passes this evening - Sunday morning. - Well below normal temperatures for Sunday, but likely a short- lived cooldown before a return to near or above normal temperatures next week. && .UPDATE... Issued at 826 PM MDT Sat Sep 21 2024 Only minor changes to the forecast at this hour. A leading band of showers has lifted north of the Denver area, with the more substantial area of rain just approaching now. We still expect this band to stall and pivot near or a little south of Denver overnight. So far the heaviest precipitation has been in the mountains of northern Park county which do really well in these southeast flow/deformation band situations. SNOTEL data suggests up to three-quarters of an inch of precipitation, with likely a rain/snow mix at 11,000 feet for a couple inches of snow accumulation. This focus will likely shift a bit further east, but the idea of substantial accumulations above 10 or 11 thousand feet in that stretch of mountains and perhaps as far north as Clear Creek and Gilpin counties still looks good. Lighter accumulations further north. We shifted the gradient in PoPs north and northeast a bit given the position of that leading band, but the general idea still looks good. && .SHORT TERM /Through Sunday/... Issued at 244 PM MDT Sat Sep 21 2024 Satellite shows the upper level low drifting northeast across northeast Arizona, while convection has blossomed ahead of it across central Colorado. From the looks of satellite imagery and latent heat release, the upper level low will reorganize into west central Colorado this evening before opening up and elongating eastward into eastern Colorado by Sunday morning. At the surface, upslope northeasterly winds dominated across the plains, with the upslope component on the wind profilers seen deepening over mountain top. Dewpoints were holding mostly in the mid 30s across the plains, and that remains one of the most concerning factors in this precipitation event. One other thing of note is that the forcing has been delayed with a slightly slower ejection of the upper level low, which means we`ll also be slowing down the arrival of precipitation into our forecast area this evening. As discussed earlier, the convection now in Central Colorado will be the initiating factor for our precipitation event, but given the slower motion of forcing we`ve delayed most of the precipitation development along the Denver metro area til this evening, and the rest of the plains through the night. Even then, there`s more uncertainty as to how this unfolds with the dry low levels, but would think eventually tonight we`ll see more widespread showers through a combination of upper level forcing as seen in the QG fields and the persistent upslope ahead of the ejecting upper level low. The limiting factor with regard to precipitation amounts will be the dry low levels. Latest model trends have caught onto this with lower QPF amounts, although there is still a rather large discrepancy in various model runs. Overall, we think most places will see amounts of 0.25-0.50 inch, although a few spots (perhaps one associated with mid level frontogenesis on the northern fringe of the upper low) could see 0.75 to 1 inch yet as a band rain could very well last through most of Sunday morning. There is still reasonable agreement in all the newest data that the northern border area will see little if any precipitation, as the upper low and forcing have trended slightly southward. With regard to the mountains, snow will be the main concern with colder temperatures arriving this evening through a combination of cold advection, latent heat of melting, and evaporative cooling/wet bulbing. QPF will be the trickiest aspect, as again the best precipitation will be generally south of Rocky Mountain National Park. This favors the higher mountains and passes of Park and Summit Counties, possibly into the Indian Peaks. We`ll keep the Winter Weather Advisory in effect, with the main impacts being slushy/snow covered roads over the higher passes (Vail Pass, Berthoud Pass, approaches to the Eisenhower Tunnel) with the heavier showers this evening into Sunday morning. Temperatures will be much cooler Sunday with low clouds and showers to start the day. But some sunshine is now expected to return in the afternoon, enough to support highs into the upper 50s and lower 60s over most of the plains. It will still be the coolest day in several months, quite appropriate for the first day of Autumn! && .LONG TERM /Sunday night through Saturday/... Issued at 244 PM MDT Sat Sep 21 2024 Any lingering showers are expected to ease by Sunday night as the upper-level low shifts east and out of Colorado. Overnight temperatures will feel cold for this first day of the official season change as widespread below normal temperatures are expected across the CWA. This is expected to bring our first frost of the season to portions of the plains overnight. Areas of fog may lower visibility in the early morning, mainly for Weld and Morgan Counties, but is expected to diminish as surface heating intensifies. Flow aloft will transition to a northwest regime as a building ridge develops over the western CONUS to begin the week. Within the flow aloft, a shortwave is expected to traverse the Rockies, bringing increasing mid-level moisture throughout the day. Forecast soundings indicate marginal instability, but there should be enough moisture available for scattered afternoon high-based showers to develop over the higher terrain, with little to no accumulations expected. After Sunday`s cool afternoon temperatures of 50s and 60s, Monday`s afternoon highs will rebound back up into the mid to upper 70s across the plains, 50s/60s for the mountains. A cold front looks likely to pass through northern Colorado Monday night into Tuesday morning bringing cloudy conditions and below normal temperatures for Tuesday. Models are persistent in diverging from this point, with the deterministic GFS closing off Monday`s passing shortwave once it enters the Mississippi River Valley region, where it sits through the remainder of the extended forecast period. The ECMWF pushes it straight south into Texas where it closes off and sits until ejecting to the northeast sometime over the weekend. Looking at the their ensemble partners, PWAT anomalies indicate continued dry conditions through the end of the forecast from Wednesday on with values ranging from 80 to 105% of normal across the CWA. As previous discussions have indicated, the NBM is a reasonable middle ground for now until models show some stronger agreement. Therefore, will let it ride for the latter half of the week. && .AVIATION /06Z TAFS through 06Z Monday/... Issued at 1155 PM MDT Sat Sep 21 2024 VFR conditions are possible until 09Z. Numerous rain showers continue for all sites which could lead to brief MVFR conditions 09Z-14Z for all terminals. Lower visibilities could continue through Sunday morning but there is low confidence in this outcome. Improving visibilities are possible by 18Z. Winds will remain northeast this morning then shifting southeast as the system moves out of northeast Colorado. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until noon MDT Sunday for COZ033-034. && $$ UPDATE...Gimmestad SHORT TERM...Barjenbruch LONG TERM...Bonner AVIATION...AD