Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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734 FXUS65 KBOU 211540 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 940 AM MDT Sat Sep 21 2024 .KEY MESSAGES... - Cooler today, with below normal temperatures. - Storm system will impact the area this evening through Sunday, with numerous showers and a couple storms. But expected rainfall amounts trending toward the lower side of earlier guidance. - The first real mountain snow is 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 940 AM MDT Sat Sep 21 2024 Cold front moved across the plains and back through the foothills early this morning. Temperatures cooled with gusty north/northeast winds across the plains. Those winds will slowly diminish through the day and turn more northeasterly. Unfortunately the low levels are quite dry behind the front, with dewpoints holding mostly in the mid 30s. That`s one strike in what could otherwise be a good QPF event for us. Forcing has also been delayed, which means we`ll also be slowing down the arrival of precipitation into our forecast area. There was one surge and precipitation shield in northeast New Mexico, but for the most part that will slide east of our forecast area, only clipping Lincoln County. In between that there is a large dry slot noted on water vapor and satellite imagery, before the main synoptically forced precipitation area in northeast AZ and southeast UT. We should see that gap fill in later this afternoon and evening as enough moisture and instability is present in the low/mid levels for convective development in southern/central CO. This should end up being the start of our precipitation event, but given the slower motion of forcing we`ve delayed most of the precipitation development along the Denver metro area and adjacent plains til this evening. 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 as well as persistent upslope ahead of the ejecting upper level low. However, due to the dry low levels the effective intensity of precipitation will be limited, so we`re trending toward less QPF in this forecast. In fact, the latest model averages are really dropping QPF numbers - mostly under 0.5 inch for Denver, and much less than that farther north toward Longmont and Fort Collins. The more southern trend continues as well, and this is noted in the latest QG forcing and upper low track being farther south, and a little weaker too. This puts into question the Winter Weather Advisory for the mountains north of I-70 with less QPF to work with. We`ll continue to watch that trend as well. && .SHORT TERM /Through Tonight/... Issued at 344 AM MDT Sat Sep 21 2024 A few changes to precipitation chances and temperatures the short- term forecast. Overall, model guidance has a good handle on the location of the upper level low compared to current GOES-18 water vapor satellite imagery. As this system continues to slowly push into western Colorado, there will be a delay in precipitation as majority of forcing will arrive late this afternoon. Before the arrival of our storm system, a cold front should drop temperatures along the forecast area. Afternoon temperatures will peak between the upper 60s to low 70s for the plains. Additionally, the Front Range mountains and valleys should drop between the mid 40s to low 60s. As the trough approaches, mid level to surface winds increase shifting to northeast this afternoon. Lower elevations could experience wind gusts up to 40 mph before any precipitation develops. Any heating and instability remains limited, thus the flash flood threat over the burn areas has significantly decreased given the isolated thunderstorm threat. There is a low chance of early showers developing this afternoon but will likely fight drier air near surface. Stratiform rainfall should begin this evening across the lower elevations. Although majority of ensemble members for both the GFS and ECMWF have decreased, QPF fields between 0.30-0.90 inches seem reasonable for areas below I-76 corridor. There is some uncertainty for areas such as Fort Collins and northern Weld county due to this upper level low becoming weaker. The only advantage these areas have are periods of upslope flow possibly tonight. It is evident majority of the forecast areas will receive much need precipitation especially for areas in severe drought. Snowfall amounts remain unchanged in the forecast with areas above 9,000 feet receiving 3-8 inches therefore the Winter Weather Advisory continues starting this evening. Tonight, low temperatures drop below normal for the entire forecast areas and rainfall continues overnight. && .LONG TERM /Sunday through Friday/... Issued at 344 AM MDT Sat Sep 21 2024 The 500mb vort max should be pretty close to the Denver metro by Sunday morning, and there should be a narrow band of showers arcing around this vort max at the start of the forecast period. Guidance still isn`t in great agreement on where this sets up, but the multi-model mean would place this somewhere near Denver and off to the east-northeast. A couple slower solutions keep some rain around through around noon, but most of the area should by dry as we get into the afternoon hours. Gradually clearing skies are expected through the day, but this will do little to warm up temperatures during the day. 700mb temperatures do manage to climb back to around +2-4C by the afternoon, and high temperatures should reach the upper 50s to perhaps the low 60s across the plains. Northwesterly flow aloft is expected on Monday, with temperatures warming back up into the 70s. Moisture aloft gradually increases by the late afternoon hours ahead of an approaching shortwave and it may be enough for a few rain or snow showers across the higher mountains by the evening hours. Guidance begins to quickly diverge by Monday night/Tuesday. A shortwave tracking out of the northern Rockies should dive southeastward towards the Central Great Plains, with an accompanying cold front pushing towards the region late Monday or early Tuesday. The GFS/UKMET keep this feature much closer to the parent trough over the upper Midwest, while most other models have the s/w detached from the longwave trough (and eventually cuts it off into a closed low), which ushers in cooler weather into our area again. There isn`t any particularly clear reason to go with one solution over the other, and for now the NBM is a reasonable middle ground. Forecast uncertainty grows as we get into the latter half of the week, with the GFS developing a strong ridge over the region, while other models still have lingering effects from the weakening upper low over the Central/Southern Great Plains. At this point the GFS and its ensemble are outliers compared to the rest of the long range guidance, and our forecast is closer to the non GFS/GEFS mean. Despite the uncertainty in temperatures, there is rather high confidence that this period will be dry with no meaningful PoPs. && .AVIATION /12Z TAFS through 12Z Sunday/... Issued at 521 AM MDT Sat Sep 21 2024 VFR conditions are possible through Saturday afternoon. Rain showers and lower ceilings will lead to MVFR conditions as early as this evening through early Sunday. There is uncertainty in showers occurring before the storm system arrives. The greatest time period falls between 22Z-24Z. By 00Z, widespread rainfall should arrive to all terminals. It is possible MVFR conditions could occur as early as 02Z. These conditions could continue through Sunday morning. Northerly winds should shift northeast this afternoon increasing in speed. By this evening easterly winds are likely. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 6 PM this evening to noon MDT Sunday for COZ033-034. && $$ UPDATE...Barjenbruch SHORT TERM...AD LONG TERM...Hiris AVIATION...AD