Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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231 FXUS65 KBOU 162101 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 301 PM MDT Mon Sep 16 2024 .KEY MESSAGES... - Severe storms with high and potentially damaging winds will be likely Tuesday afternoon. - Scattered showers and isolated high based thunderstorms with gusty outflow winds this afternoon into early evening. Highest coverage in/near the mountains. - Cooler temperatures Friday and next weekend with good chances (30-60%) for rain and high mountain snow. && .SHORT TERM /Through Tuesday/... Issued at 300 PM MDT Mon Sep 16 2024 Isolated to scattered weak convection is attempting to develop across the forecast area this afternoon. MLCAPE is negligible in all but the far northeast corner, where the airmass is more strongly capped. Thus, we think the main threat over the next few hours will be gusty outflow winds to around 40 mph, with only light rain. Most of this activity is expected to decrease rather quickly this evening with stabilization and passage of the weak shortwave moving northeast across the area right now. Clearing skies are expected overnight, with low temperatures near normal and lighter winds. Tuesday is still shaping up to be an atypical active September day with severe/damaging winds possible from fast moving showers and storms in the afternoon. Many ingredients are coming together for this threat. A fast moving shortwave kicks out of the Great Basin and lifts northeast across Colorado in the afternoon - coinciding with peak heating and destabilization. Mid level flow increases in the afternoon with 700 mb winds increasing to 35-40 kts by 21Z-00Z. DCAPE increases to 1000-1200 J/kg (not great for high winds, but will definitely lead to acceleration of the environmental flow). Thus, just 20 kts of acceleration added onto 35 kt mean flow in a well mixed environment would suggest shower and storm outflows easily reaching 55 kts (~65 mph). In addition, these storms may align linearly, suggesting more widespread high winds possible than our typical thunderstorm days and similar to a QLCS type structure. That said, our instability is lacking which will be the one limiting factor in our severe weather/high wind threat for tomorrow. Still, it was interesting to see some high resolution model output showing a couple pockets of 70-80+ mph storm outflows tomorrow afternoon. Whatever the case, tomorrow is shaping up to be a rare day with a legitimate severe storm/high wind threat. Boaters, paddleboarders, and water enthusiasts beware of the weather tomorrow! It is best to be off the water before noon. Sudden onset of gusty winds are likely on open waters, which could leave you in peril with little advanced warning. && .LONG TERM /Tuesday night through Monday/... Issued at 300 PM MDT Mon Sep 16 2024 A potent upper level shortwave will be move across the Central and Northern Rockies Tuesday night with a moderate southwesterly flow aloft over Colorado. There may still be a line of convection across the far eastern zones during the early evening which should quickly move out of our forecast area. Some of these storms could be severe with strong wind gusts being the main threat. In addition, the models are hinting at a mountain wave setting up Wednesday evening with wind gust to 65 mph possible across the higher foothills and Front Range Mountains. For Wednesday through Friday, Colorado will be in between storm systems under the influence of a moderate southwesterly flow aloft and weak upper level ridging. With limited moisture, dry weather is expected across all of the CWA with breezy conditions across the high country. The combination of low relative humidity and gusty southwesterly winds may lead to elevated to near critical fire weather conditions from South Park eastward across the Southern Foothills, Palmer Divide and east across Lincoln County. There should be a uptick in shower and thunderstorm activity on Friday as mid level moisture and QG ascent increases ahead of a large closed upper low centered over the Mohave Basin. Despite the increase in moisture, the airmass is expected to remain quite dry. Consequently, the showers and storms are expected to be high based with gusty winds and mainly light rain. Cooler unsettled weather is expected Friday night into the weekend as the next storm system and associated cold front moves across the Central and Southern Rockies. At this time, there is quite a bit of uncertainty on how the pattern will unfold due to run to run inconsistencies and model discrepancies. For example, the latest GFS run has trended further south and has a weaker upper low tracking across the Colorado/New Mexico border. This solution only produces light QPF across the CWA. On the other hand, the ECMWF, suggest a heavy swath of precipitation stretching from the Central and Northern Mountains across the plains north of I-76. This is in response to a stronger upper low tracking from the Four Corners Region into Northeastern Colorado. If the ECMWF solution verifies, we could even see some severe weather across the far Northeastern Plains as well as several inches of snow across the high mountains. Due to all the uncertainty, have decided to stay with the pops given by the model blend. Mostly dry and warmer weather is expected early next week as the storm system moves east of the region and a dry west to northwesterly flow aloft sets up over Colorado. && .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. && .FIRE WEATHER... Issued at 300 PM MDT Mon Sep 16 2024 Good humidity recovery can be expected in most areas tonight, with winds also decreasing after showers/isolated storms end this evening. Tuesday is shaping up to have elevated fire weather conditions ahead of the approaching cold front, with southwest winds increasing in most areas in a still warm/dry environment. Showers and storms will provide for scattered wetting rains, but also produce very strong outflow winds. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...Barjenbruch LONG TERM...JK AVIATION...Barjenbruch FIRE WEATHER...Barjenbruch