Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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900 FXUS65 KBOU 102141 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 341 PM MDT Sat Aug 10 2024 .KEY MESSAGES... - Scattered to numerous showers and thunderstorms into this evening. Threats include flash flooding on the recent burn areas as stronger storms will produce locally heavy rainfall. A severe storm still possible over the Palmer Divide area. - Sunday`s storm threat should decrease a bit over the mountains, but a a couple more severe storms likely over the plains. - Slightly cooler Monday with better precipitation chances. The main threat will be heavy rainfall with a couple strong to severe storms possible. - Gradual drying trend expected from Wednesday through the end of the work week with near to slightly above normal temperatures. && .UPDATE... Issued at 338 PM MDT Sat Aug 10 2024 So far, the storms have not been able to tap into the richer moisture and instability on the northeast plains. We still can`t rule out a couple stronger storms near the Palmer Divide area late this afternoon and evening where CIN is less significant, but overall threat is decreasing per observations and convective trends over the northeast plains. && .SHORT TERM /Through Sunday/... Issued at 143 PM MDT Sat Aug 10 2024 Scattered strong to severe thunderstorms were developing across the northern mountains and near the Wyoming border early this afternoon. They will continue to increase in coverage and spread east/southeast across the plains late this afternoon and evening. While instability so far is rather marginal (MLCAPE 500-1000 J/kg), there is abundant deep layer shear (>50 kts). Considerable CIN still resides on the plains due to this morning`s stratus, but MLCAPE is growing and somewhat organized convection could be enough to keep this generating above the low level stability. And, a couple of these could be severe with high wind the primary threat. There is also some hail threat if sufficient instability can be tapped. For the overnight, at least weak QG lift occurs, and with the jet max to our north we`ll keep a chance of showers/storms going in most locations into the early morning hours. The greatest chances over the northeast plains and in the mountains. A combination of shower outflow and partial clearing could lead to patchy fog once again (30-40% chance but relatively low coverage). On Sunday, there is reasonable agreement that some drying will enter the picture from the north. This should cut down on the intensity and coverage of showers and storms in the northern mountains, but overall another round of scattered afternoon thunderstorms is expected. The storms will be higher based and produce gusty outflow winds in/near the Front Range, but a ribbon of higher instability will likely spawn a couple more severe storms over the northeast plains. At this time, the greatest risk of that would be roughly east of Greeley to Byers (east of Denver) and Kiowa. The far northeast corner could once again be on the cooler side of guidance, while temperatures in the I-25 Corridor are expected to heat into the lower 90s with contribution of downslope flow and westerly breezes in the afternoon. && .LONG TERM /Sunday night through Saturday/... Issued at 143 PM MDT Sat Aug 10 2024 Sunday night, scattered showers and storms may continue across the CWA into the evening, mainly south of I-70. Locally heavy rain and small hail will be possible mainly over the eastern sections of the Palmer Divide and portions of the plains. Monday through Tuesday night, upper level high pressure will amplify across the Rocky Mountain Region with a weak to moderate westerly flow aloft over Colorado. On the western periphery of the high, monsoonal moisture is progged to flow north across Arizona and then westward over Colorado. This moisture combined with daytime heating and occasional upper level disturbances should result in scattered to numerous afternoon and evening showers and thunderstorms across the forecast area. With PW anomaly`s 0.35" above normal, some of the storms could produce heavy rainfall with the potential for some localized flash flooding, especially in and near the recent burn areas. Latest model runs have trended lower with the amount of instability on Monday which suggests a lower threat for severe weather; however, can`t rule out a marginal severe storm somewhere across the Northeastern Plains. Models are now showing Tuesday having the greater potential for severe weather on the plains with CAPE`s over 1500 J/KG. However, the 0-6KM AGL Bulk Shear is marginal with values less than 35kts. Due to run to run inconstancy`s, there is quite a bit of uncertainty in Tuesday`s severe potential. Therefore, we will need to keep an eye on any changes in upcoming model runs. For the Wednesday and Thursday period, there is quite a bit of uncertainty in the forecast as models have diverged considerably. The GFS is showing warmer and drier conditions as upper level high pressure builds over Colorado from the southeast. On the other hand, many of the other deterministic solutions from the ECMWF, CFS, UKMET and ICON models are showing much cooler temperatures across the CWA due to some upper upper level troughiness over Colorado. For Friday and Saturday, most of the models are suggesting hot and mostly dry weather across the forecast area as a strong upper ridge builds over the Central and Southern Rockies. On the other hand, the GFS places the high further east over the Southern Plains States with some monsoonal moisture providing a better chance for afternoon and evening thunderstorms during the afternoon and evening hours. For now will go with the isolated to scattered (10% - 50%) pops given by the NBM. && .AVIATION /18Z TAFS through 18Z Sunday/... Issued at 1159 AM MDT Sat Aug 10 2024 Next forecast challenge will be thunderstorms this afternoon and evening. Appears the greatest threat will exist from about 21/22Z-03Z tonight. There`s enough risk (>50%) to mention TEMPO TSRA with VRB gusts. Possible scenario would be for one early round 21Z-23Z, with potentially another closer to 02Z-03Z, but given CAMs are already behind with convective development so timing confidence is still low. For later tonight, there is still potential for lingering showers, but the main concern would be for shower/storm outflow and another surge of richer low level moisture. That would lend itself toward a fog threat 10Z-15Z. For now, confidence is low, but enough to mention VCFG in the KDEN TAF. A stronger west/northwest wind with gusts around 20 kts is expected to develop Sunday after 18Z. && .HYDROLOGY... Issued at 143 PM MDT Sat Aug 10 2024 Intensity of storms could offset the faster storm motion this afternoon, with the Alexander Mountain burn area being the most susceptible given the recent burn. Otherwise, enough storm motion (25-30 mph) to keep a limited threat for the most part. Still, an intense storm could put down a quick one half to 1 inch of rain in 20 minutes. Sunday`s threat should decrease a little with less intense convection expected over the burn scars, especially in northern Colorado. The combination of monsoonal flow over Colorado and increased low level moisture east of the mountains Monday will lead to the threat of heavy rain across the forecast area. Consequently, some of the storms could produce localized flash flooding, especially in and near the recent burn areas. The Alexander Mountain burn area will see the highest threat (elevated) with limited risks for the older burn scars. Similar conditions are expected on Tuesday with slightly lower chances for heavy rain and flash flooding due to somewhat lower moisture values. Drier and warmer conditons are expected by Wednesday. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Barjenbruch SHORT TERM...Barjenbruch LONG TERM...JK AVIATION...Barjenbruch HYDROLOGY...Barjenbruch/JK