Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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982 FXUS65 KBOU 171647 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1047 AM MDT Tue Sep 17 2024 .KEY MESSAGES... - A line of thunderstorms with high and potentially damaging winds are likely this afternoon. Peak gusts from severe storms up to 60 mph likely in the mountains, 60-70 mph along the I-25 Corridor, and a few gusts of 70-80 mph possible over the eastern plains. - Cooler temperatures this weekend with good chances (30-60%) for rain and high mountain snow. && .UPDATE... Issued at 1046 AM MDT Tue Sep 17 2024 Forecast for a high impact weather day is coming together pretty much as anticipated. Satellite shows a fair amount of sunshine with warming temperatures across the Front Range and plains, while scattered thunderstorms had already developed on the West Slope of Colorado. There is excellent agreement in the short range convective allowing models (CAMs) showing storms organizing linearly over the mountains and then pushing east onto the I-25 Corridor 2-4 pm, and then moving quickly east across the plains 4-7 pm, and should be pretty much over by 8 pm. The one thing there`s a little uncertainty with is the storms potentially having too much outflow along the Front Range in a low MLCAPE/high DCAPE environment. That could lead to new storm initiation east of original convection, and then organizing again more effectively east of the I-25 Corridor where damaging winds would be even more likely. Ingredients are all there for high winds with the storms, including DCAPE >1200 J/kg, strong mid level environmental flow (35-40 kts at 700 mb), and linear (QLCS-like) organization although MLCAPE a bit limited as discussed above. Thus, we think peak gusts from severe storms would be near 60 mph in the mountains, 60-70 mph along the I-25 Corridor, and potentially 70-80 mph over the plains east of I-25. Those would be the strongest, but almost all areas should see gusts of at least 40-55 mph. Winds will reach a given location well ahead of the showers and storms, so we`re messaging safety for water recreationists. Also, scattered power outages can be expected in areas that see the strongest winds. Fire danger is also a threat with strong southwesterly winds and warm temperatures on the plains this afternoon, and the gust fronts along and ahead of storms. We are expected wetting rains with most storms in the mountains - hopeful we see that over the new Pearl fire or significant fire behavior possible there with the gusty winds associated with storms and/or post-frontal downslope winds. && .SHORT TERM /Through Tonight/... Issued at 352 AM MDT Tue Sep 17 2024 Water vapor satellite imagery showing a broad upper level low spinning over Nevada. Ahead of the low, lift is producing isolated showers over western Colorado already. The center of the low lifts into Wyoming by this evening, but as the trough axis races northeast across Colorado it will provide plenty of lift. By mid morning expect scattered thunderstorms across western Colorado. Daytime heating will destabilize the airmass ahead of the trough over central and eastern Colorado. Still good model agreement that a line of storms form by late morning over western/central Colorado. Mid level flow increases in the afternoon with 700 mb winds increasing to 35-40 kts by 21Z-00Z. Storm movement looks to be 35-40 knots as well and as mentioned in the last discussion a 20 knot forward outflow out of a storm moving 35 knots yields 55 knot winds. So it won`t take much to produce strong winds today. Greatest instability will be over the eastern plains with SBCAPE of 500-1000 J/kg. The greater instability should produce more organized storm segments, leading to winds of 70 mph or locally stronger. The enhanced severe risk over the northeast plains looks on track. No big changes to the forecast for today. Will continue to highlight the strong wind threat from the thunderstorms this afternoon. For this evening and tonight, storms continue to race northeastward and exit the state by early to mid evening. Subsidence quickly moves in and brings mostly clear skies overnight. This may keep windy conditions going across the higher foothills and Front Range Mountains this evening. Expect a chilly night compared to recent nights due to the clear skies and dry airmass. Lows are expected to fall into the 40s overnight across northeast Colorado. The mountain valleys, likely fall below freezing with lows in the mid 20s to lower 30s. A little moisture hangs on overnight over the northeast plains where a little fog will be possible towards sunrise Wednesday morning. && .LONG TERM /Wednesday through Monday/... Issued at 352 AM MDT Tue Sep 17 2024 A few small changes were made this update to long-term forecast package. Strong subsidence values are displayed by QG fields along northeastern Colorado Wednesday. Strong southwesterly flow continues under high pressure creating mostly sunny skies and dry conditions. 700mb temperatures between 11-13C will lead to above normal temperatures Wednesday and Thursday. Additionally, low relative humidities are possible. Although wind gusts remain below 25 mph for the lower elevations, there is an increasing chance southwesterly winds could increase mid- afternoon given the upper level stationed in southeastern Wyoming especially for the foothills areas. This may lead to elevated fire weather conditions for the foothills briefly but mainly for Jackson and Larimer counties Wednesday afternoon. Additionally, Thursday similar conditions could occur with slightly stronger winds for the lower elevations leading to near critical conditions for the plains, Palmer Divide, and Lincoln county. There is good agreement for a weak shortwave increasing mid-level moisture starting Friday late afternoon. While MLCAPE values are low near 100 J/kg, isolated high based showers and storms with little rainfall are possible through late evening for both the mountains and plains. DCAPE values approach 1000-1300 J/kg therefore expect gusty winds associated with storms and showers. This forecast package included a decrease to PoPs Friday. Our next upper level trough pack a punch which could arrive this weekend. Majority of ensemble members are favoring precipitation arriving late Saturday morning first for the higher elevations. QPF fields range between 0.5-2 inches for the region. There is some uncertainty if this system hovers in the northeastern corner through Sunday afternoon. This resulted in NBM isolated PoPs for northeast Colorado through Sunday evening. ECMWF also keeps showers and storms lingering through Sunday evening. Either way, there is high confidence in cooler afternoon temperatures this weekend through early next week especially Saturday and Sunday. Saturday compared to Friday, temperatures are almost 10-15 degrees cooler for both the mountains and plains. Maybe even a chance for sweater weather? && .AVIATION /12Z TAFS through 12Z Wednesday/... Issued at 533 AM MDT Tue Sep 17 2024 Strong southwest flow aloft will bring gusty south to southwest winds through this afternoon. South winds increase to 30 knots around 18Z. Forecast guidance is in good agreement with a line of thunderstorms moving across the mountains and into the Denver area between 20-23Z. This is expected to bring a brief period of strong outflow winds (40-50kts) as the storms move through. Winds decrease after 00Z and may turn northerly or become variable as weak cold front moves through in the 02-04Z window. Drier air moves in after 00Z Wednesday with mostly clear skies tonight and Wednesday. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Barjenbruch SHORT TERM...Meier LONG TERM...AD AVIATION...Meier