Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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892 FXUS65 KBOU 211135 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 535 AM MDT Sat Sep 21 2024 .KEY MESSAGES... - Cooler today, with highs near or below normal. - Storm system will impact the area this evening through Sunday. - 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. - Well below normal temperatures for Sunday, but likely a short- lived cooldown before a return to near or above normal temperatures next week. && .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. && $$ SHORT TERM...AD LONG TERM...Hiris AVIATION...AD