Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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136 FXUS65 KBOU 252051 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 251 PM MDT Tue Jun 25 2024 .KEY MESSAGES... - Heat continues Wednesday and Thursday, though not as hot as earlier this week with lower 90s across the plains and urban corridor. - Increasing chances for thunderstorms Wednesday (30-60% high country; 20-40% Plains) and Thursday (50-80% high country; 30-60% plains). A few could be strong to severe. && .SHORT TERM /Through Wednesday/... Issued at 250 PM MDT Tue Jun 25 2024 A ridge of high pressure continues across Colorado. From this afternoon through Wednesday evening the ridge doesn`t move much, in fact slightly higher heights might even build in from the south. The center of the ridge is squarely over the U.S. Desert Southwest and northern Mexico. Flow at 500 mb and above is not surprisingly fairly weak with northwest flow 15-30 kts going west-southwest by Wednesday afternoon. However, closer to the surface things do change a little by Wednesday morning--a very weak trough at 700 mb moves across the northern Rockies on Wednesday with 20-30 kts of southerly flow ahead of it. Believe it or not, this air is a little bit cooler and quite a bit more moist than the air masses over the last few days. Here is what to expect at the surface: Weak convective showers should be over by midnight across our area, and any rainfall amounts will be very light. Skies will gradually clear with much less convective cloud debris aloft vs last night. That, combined with weaker winds near the surface will result in better nighttime cooling than the previous few nights. This should be true especially across the western suburbs where it was breezy all night last night with temps steady in the 70s. All areas, including the urban heat islands, should cool of nicely into the low to mid 60s tonight, same as the rest of the plains. The mountain valleys will also nicely cool into the upper 40s. On Wednesday moisture advection should begin in earnest ahead of the 700 mb trough, and combined with robust east winds just off the surface, should increase PW to near 1.5" across the eastern plains, and between 1-1.2" along the I-25 corridor, and 0.7" along the Divide and West Slope. HREF mean SBCAPE values are 700-1000 J/kg for most of the plains, including Larimer and Weld Counties. Across metro Denver the models don`t look bullish on brining in much low-level moisture. Lapse rates are pretty weak in the high country, so even though storms will get going by midday, they will be weak with just light rainfall. Given the better low-level moisture across the northern half of the I-25 corridor, PoPs are highest for Larimer and Weld Counties once convection moves off the mountains. When outflow boundaries/existing convection get far enough east into the richer moisture and instability, mostly along and east of a Fort Morgan to Limon line, better chances of thunderstorms (> 50%) are forecast into the evening hours. 0-6 km shear of about 40 kts should support strong storms, with a one or two possibly severe across the eastern Plains, where SPC has a marginal risk of severe storms. Even though there will be uptick in cloud cover and storm coverage, afternoon max temperatures will still be hot, with highs 90-95 across the plains, and 75-85 in the mountain valleys. && .LONG TERM /Wednesday night through Tuesday/... Issued at 250 PM MDT Tue Jun 25 2024 Wednesday night an upper level ridge of high pressure will be over the Rocky Mountain Region and Central Plains States with a weak to moderate westerly flow aloft over Colorado. Scattered showers and storms (25%-50%) will continue into the evening, most numerous across the plains east and northeast of Denver. With MLCAPES over 1000J/KG some of the storms from the Front Range Urban Corridor eastward across the plains could be strong to severe. In addition, with high PW`s ranging between 0.60" to 1.00" in the high country and 1.15 to 1.60" across Northeastern Colorado, heavy rainfall will also be a possibility. On Thursday, the upper ridge flattens in response to an upper level trough moving across the Northern Rockies. This will put Colorado under the influence of a moderate westerly flow aloft. With plenty of available moisture and instability, along with some lift from a 60KT upper level jet, we should see another round of afternoon and evening showers and storms move across the forecast area. Some of storms could produce heavy rain with the potential for a couple of strong to severe storms further east across the plains. On Friday, a somewhat cooler and drier airmass advects into the region as an upper level shortwave moves into the Northern Plains States and the upper level flow turns more northwesterly. As a result, shower and thunderstorm activity is expected to decrease somewhat. A cold front is expected to move across Northeastern Colorado Friday night resulting if cooler more stable day on Saturday with a chance (30%-50%) of afternoon and evening showers and storms mainly across the Front Range Mountains, foothills and Palmer Divide. Some of these storms could produce brief heavy rainfall. On Sunday, upper level high pressure builds over the Southern Plains States with an upper level trough of low pressure over the Western U.S. This pattern may allow some subtropical moisture to flow into Colorado from the south and southwest, which may bring an uptick in shower and thunderstorm activity to the high country. Despite the increase in moisture, models are showing light QPF in the mountains. Still think there is a remote chance that a storm in the high country could produce locally heavy rainfall, mainly south of I-70. Further east across the plains, thunderstorm activity may be limited due to lower dewpoints and more stability. However, can`t rule out a stronger storm due to local moisture convergence. Hot and mostly dry weather is expected early next week as a dry westerly flow aloft sets up over Colorado. && .AVIATION /18Z TAFS through 18Z Wednesday/... Issued at 1105 AM MDT Tue Jun 25 2024 VFR through Wednesday and with hot temperatures both this afternoon and Wednesday afternoon. There shouldn`t be any showers at the terminals this afternoon, but one or two wind shifts with gusty winds are expected. For now, the most likely wind shift looks to be west, northwest, or north given where some weak convection should be ongoing this afternoon. We will continue to handle the potential wind shifts with a TEMPO group for VRB gusts to 30 kts. Tonight will be a normal drainage wind night (SSW at APA and DEN 8-12 kts; WSW at BJC 5-9 kts). On Wednesday, winds should shift to the ESE and maybe gust over 20 kts from midday until storms gets going and move of the mountains. Thunderstorm coverage will be much more than that past couple of days. Those storms may directly impact the terminals, but it`s about a 40 percent chance of a storm going overhead. It`s almost a guarantee that several gust fronts will move across the terminals with a wind shift and gusty winds to 35 kts. && .HYDROLOGY... Issued at 250 PM MDT Tue Jun 25 2024 The potential for heavy rainfall will increase across North Central and Northeastern Colorado Wednesday and Thursday as subtropical moisture moves over Colorado from the south and southwest. Storm motions will be slightly faster Thursday, helping to taper the threat of flash flooding. Isolated to scattered afternoon thunderstorms will continue beyond Thursday, but moisture will generally be more limited, thereby reducing the potential for heavier rainfall. .BOU WATCHES/WARNINGS/ADVISORIES... Heat Advisory until 9 PM MDT this evening for COZ040. && $$ SHORT TERM...Schlatter LONG TERM...JK AVIATION...Schlatter