Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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366 FXUS65 KBOU 262051 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 251 PM MDT Wed Jun 26 2024 .KEY MESSAGES... - Scattered to numerous thunderstorms later this afternoon and evening. Some storms will be strong to severe with heavy rain, large hail, and strong winds, mainly east of a Fort Morgan to Limon line. - Scattered-numerous thunderstorms return Thursday, some strong to possible severe, especially in the plains. The best chance of severe storms (30%) is across the far northeast corner. - Turning drier into the weekend, with some moderation in temperatures Saturday. - Potential for significant heat will rise again next week. && .SHORT TERM /Through Thursday/... Issued at 150 PM MDT Wed Jun 26 2024 A broken line of strong to severe storms could be moving across the eastern plains of Colorado this evening. The best instability remains east of the I-25 corridor, and and mostly east of a Fort Morgan to Limon line. The latest CAMS generate showers and thunderstorms from the higher terrain and move them east. While across the I-25 corridor instability is more limited, brief moderate rain and small hail can`t be ruled out. Overall coverage shouldn`t be that great which is the reason for just 50/50 chances of measurable rain. However, once the outflows from these storms make it east into the better moisture and instability, expect a broken line of strong to severe storms to develop. Main impact would be large hail, maybe up to golfball size based on the instability and vertical wind shear. The I-25 corridor should be storm/shower free by 9 PM, and by midnight the eastern plains should be clear of the showers and storms. Tonight, a really subtle 700 mb trough moves across the northern half of Colorado. CAMS want to break out light rain showers between 3-7 AM, probably related to this feature. No significant impacts are expected from these showers but a few hundredths of rain could fall. It will be partly to mostly cloudy much of the night across our area, resulting in a warm night (but thankfully not anywhere near "hot", the central and eastern US can keep their hot nights). Lows will be in the low to mid 60s across the plains, upper 60s for the urban heat islands and along the base of the foothills, and believe or not, close to 50 degrees in the mountain valleys like in Summit County, South Park, and North/Middle Parks. Tomorrow, the ridge shifts eastward and we have southwest flow aloft. There is a fair amount of subtropical moisture that gets advected from the southwest, resulting in PW values up to 1.2" across the plains below 7,000 ft, and 0.5-0.75" in the northern and central mountains/valleys and foothills. There is plenty of moisture around combined with the power of the late June sun to get storms going across the high country and Palmer Divide by early afternoon. South to southeast winds off the deck will bring in 60+ deg dewpoints across the far eastern plains, generally east of a Sterling to Burlington line. Initial storms should be capable of small hail and heavy rain. Coverage again looks to be spotty and not well organized given the weaker winds aloft. As the storms move east, a weak dry line should be in place across the far northeast corner. East of this boundary, any storm has a good chance of being severe with large hail and severe winds the most likely impacts. SPC has an enhanced risk out for this potential across the far northeast corner of Colorado. The main threats look to be after 6 PM for the far eastern plains. Before the storms get going we should have another day of 90+ deg heat across all of the plains below 6,000 ft elevation, but the heat impacts will be short lived with cloud cover and rain/cool outflows all afternoon, but especially after 3 PM for the I-25 corridor. The mountain valleys will also warm into the 70s before thunderstorms arrive after 12 PM. && .LONG TERM /Thursday night through Wednesday/... Issued at 150 PM MDT Wed Jun 26 2024 Thursday night into Friday a flat upper level ridge will stretch from the Desert Southwest into the Southeastern U.S. with an upper level shortwave moving across the Northern Rockies and Great Plains States. The combination of deep subtropical moisture, lift from the passing upper level shortwave/50KT upper level jet and adequate instability should result in continued scattered to numerous shower and storms through the evening hours. With anomalous PWAT`s of 0.35" above normal, some of the storms could produce heavy rainfall and localized flooding. Further east across the plains, one or two of the storms could be strong to severe during the early evening. Storm coverage and intensity are expected to decrease on Friday due to less available moisture and more stability. However, there still will be the potential for locally heavy rainfall across the forecast area along with one or two strong to severe storms across the far northeastern plains. On Saturday, a cooler and more stable airmass will be in place east of the mountains behind the passage of a cold front. Therefore, the Northeastern Plains will see some relief in the heat along with limited precipitation chances. Higher precipitation chances will be possible across the Front Range Mountains, foothills and Palmer Divide where a few storms could produce brief heavy rain and small hail. On Sunday, the upper ridge will be centered over the Texas/Oklahoma border with an upper trough over the Western States. This pattern may allow another surge of subtropical moisture to move into Colorado from the southwest. As a result, could see an increase in precipitation chances, mainly south of I-70, where locally heavy rainfall may be possible. Hot temperatures are also expected to return to the CWA on Sunday with highs climbing back into the 90s on the plains. Drier weather is expected along with continued hot temperatures early next week as the the upper flow becomes more zonal and the subtropical plume of moisture gets pushed south of our CWA. The main threat during this period will be gusty winds due to high based convection and some possible fire weather concerns across portions of plains and southern foothills. && .AVIATION /18Z TAFS through 18Z Thursday/... Issued at 1128 AM MDT Wed Jun 26 2024 VFR conditions expected through Thursday. Thunderstorms are expected to develop across the high country after 12 PM, and then move east. There is uncertainty about how well the storms can stay together across metro Denver and out to DIA. There are enough ingredients in place for severe storms east of DIA, but storms will be less intense if they move over any of the terminals this afternoon. Given the expected spottiness of the storms, TEMPO groups seem to be the best way to handle the potential of -TSRA. Showers and storms should clear the terminals by 7 PM. Gusty winds to 35 or maybe 40 kts are possible with the strongest storms this afternoon/evening. In the wake of the showers/storms, southeast winds may briefly gust over 25 kts, but by midnight drainage winds at the terminals should take over (SSW DEN and APA 8-12 kts, WNW at BJC 5-8 kts). Tomorrow winds should shift to the north by midday at 10 kts or less, and there will be another round of thunderstorms that either impact the terminals (40-50% chance) directly or generate gust fronts that result in winds shifts and gusty winds. && .HYDROLOGY... Issued at 150 PM MDT Wed Jun 26 2024 Enhanced levels of PW will remain in place through Thursday evening. Precipitation efficiency could even be slightly higher in the mountains given a more favorable low-level moisture profile and overall there are skinny CAPE profiles. Storm motion should ample to keep storms moving with 15-20 kts of steering layer flow out of the west-northwest. So while a few storms Thursday afternoon may be capable of heavy rain, they should be moving fast enough that the overall threat of flash flooding is very low. Over the burn scars, it`s only a tad higher (maybe a 10-20% chance of issuing a flash flood warning). PW`s are expected to decrease somewhat by Friday. However, there could still be localized areas of heavy rain with a limited flash flood threat across the burn scars. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...Schlatter LONG TERM...JK AVIATION...Schlatter HYDROLOGY...Schlatter