Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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330 FXUS65 KBOU 161745 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1145 AM MDT Mon Sep 16 2024 .KEY MESSAGES... - Scattered showers and isolated high based thunderstorms with strong, gusty outflow winds this afternoon into early evening. Highest coverage in/near the mountains. - Windy conditions become widespread on Tuesday. - Scattered thunderstorms Tuesday afternoon, some may be severe over the plains with high winds. - Cooler temperatures Friday and next weekend with good chances (30-60%) for rain and high mountain snow. && .UPDATE... Issued at 1022 AM MDT Mon Sep 16 2024 Mid and high level clouds were beginning to increase from the southwest as a weak shortwave is moving up from the Four Corners area. Instability will be limited as the bulk of this moisture, barring the far northeast corner, is confined to the mid/upper levels and the low levels are quite dry. Thus, with DCAPE near 1000-1200 J/kg, the main threat will be gusty outflow winds to around 40-45 mph. We are seeing diminishing CIN over the plains with some sunshine this morning, so we may have to spread the low PoPs (mostly virga though!) a little farther east onto the plains late this afternoon/early evening. The highest (scattered coverage) is still anticipated to be over the mountains. We`ll have to bump the forecast high temperatures a degree or so over the northeast plains where there`ll be more sunshine and good mixing. && .SHORT TERM /Through Tonight/... Issued at 350 AM MDT Mon Sep 16 2024 There a still a few showers over the eastern border at this time. They were moving east-northeastward out of the CWA. Normal drainage wind patterns were in place for much of the CWA. Models show a pretty strong upper low over northern California to move into the western Great Basin by 00Z this afternoon, then to be centered over northeastern Nevada by 12Z Tuesday morning. The winds aloft for the CWA will be 45 to 80 knots from the southwest both today and tonight. The synoptic scale energy for the CWA on the QG Omega fields is somewhat noisy today and tonight and pretty weak downward in nature anyway. The flow aloft seems to translate somewhat down to the low levels this afternoon and evening, with gusty southwesterlies surface winds progged fro a bit this afternoon. For moisture, the precipitable water fields show 0.50 to 1.10 inches over the forecast area today into this evening, with a tad of drying over the plains tonight. Instability wise, the CAPE is meager today into this evening, with the highest values over the far northeast corner this afternoon only. There is nothing over the CWA for the tonight period. The model QPF fields have limited measurable rainfall over the mountains, foothills and Palmer Ridge this afternoon and early evening only. Will keep the highest pops going over the mountains and foothills this afternoon. The plains can expect 0-10% pops for the first two periods. There may be some outflows around, but not as strong as Monday`s were. For temperatures, today`s highs are 2-5 C cooler than yesterday`s highs. The 90s F look to be over. && .LONG TERM /Tuesday through Sunday/... Issued at 300 AM MDT Mon Sep 16 2024 A strong negatively tilted upper level trough will lift northeast across Colorado Tuesday. Center of the trough at 12Z Tuesday will be over eastern Nevada with the axis extending into northwest Arizona. By 18Z, the trough axis will be over southwest Colorado and by 00Z it will be over northeast Colorado...very fast moving. Ahead of the trough axis showers and thunderstorms will form Tuesday morning over the Four Corners and northward into western Colorado. Best lift over north central and northeast Colorado will be Tuesday afternoon, lining up with peak heating. Pretty much all the high resolution models show a line or broken line of storms tracking across the area Tuesday afternoon. SBCAPE reaches 500-1000 J/kg, so the storms should have decent updrafts. Strong lower and mid level winds, inverted-V sounding, and DCAPE of 1000-1500 J/kg all add up to strong to severe winds from the thunderstorms. This looks to be a higher threat than most days with the NAMNest and HRRR models showing some gusts to 70 mph. The storms race east of the area by early to mid evening. For Wednesday and Thursday, an upper level low drops south along the west coast Wednesday and then slowly progresses eastward Thursday. By late Thursday the upper level low will be centered over California and Nevada. Ahead of this low, ridging in the southwest flow aloft will prevail over Colorado. The airmass dries out with mostly clear skies expected. Temperatures will be a little cooler, but still above normal with highs in the lower to mid 80s over northeast Colorado. Even though it will be dry (low relative humidities), winds will generally be weak, easing fire weather concerns. On Friday, the upper level low slowly moves east across the Great Basin and over the Central Rockies Saturday. Models still somewhat struggling to find run to run consistency. The latest 00Z runs of the GFS and ECMWF are slower than previous runs keeping the area precipitation free until Saturday. Ensemble members are not entirely on board with this solution. This low as it tracks across the Central Rockies could stay north of Colorado resulting in mainly dry conditions. If it takes a more southern and stronger route, much of the area will see a good shot of rainfall. Because of the uncertainty, will continue with the broad brush approach with 20-50 PoPs for Friday and Saturday. Temperatures will run at or below normal with highs expected to be in the 70s over northeast Colorado. Though if we happen to see a cloudy/rainy day, highs will likely be in the 60s. This storm system should be east of the area Sunday. With the uncertainty in the weather pattern, will keep low PoPs (10- 20%) in the forecast for Sunday. && .AVIATION /18Z TAFS through 18Z Tuesday/... Issued at 1145 AM MDT Mon Sep 16 2024 Main concern for relatively early in this TAF period is VRB and gusty outflow winds from passing showers/isolated storm. Main threat would be 21Z-24Z, but can`t rule out something as early as 20Z considering radar trends to our south. Also, the coverage and propagation of showers to our south would suggest the main threat of stronger outflows would be from the south/southwest, but can`t rule out a lighter east/northeast flow behind any passing showers given the dry sub-cloud layer. The chance of any TS at the airports is quite low - about 10-20%. By 00Z-01Z, most of the convection should have passed with a return to slightly enhanced south/southwest winds expected overnight. We have higher confidence of storms and stronger outflows affecting the airports on Tuesday. The most likely timeframe for that would be after 21Z. A linear feature of convection is expected, which means more organization of strong, gusty outflow winds to 40-50 kts with the storms that move across. Fortunately, if the storms are linear, that should be mainly just a brief 30-60 minute blast of wind and high airport impacts before winds settle down again. However, there`s a 20-30% chance a second batch would be possible with gusty/variable winds - this time not as strong. Before the storms arrive Tuesday afternoon, we`ll see gusty southerly winds increase 16Z-21Z with daytime heating and increased mixing/flow aloft driving the strengthening winds. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Barjenbruch SHORT TERM.....RJK LONG TERM......Meier AVIATION...Barjenbruch