Time: 900 AM Sat September 21, 2024 Forecaster: Justin Brooks ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- Cool today with a good chance for afternoon/evening/overnight showers A cold front moved through the District early this morning which will effectively help limit high temperatures today. Currently temperatures are in the mid 50s and will increase to around 70 degrees this afternoon before clouds increase as a disturbance moves through the region. This disturbance will bring a good chance for showers this afternoon, picking up into the evening and lasting through the overnight and into Sunday. A slight chance a few thunderstorms develop this afternoon, likely turning over to just shower activity this evening. Thunderstorm activity will hinge on the amount of daytime heating today and will also play a big role in rainfall potential. If thunderstorms are able to develop this afternoon, the chances for a quick 0.50” in 10-20 minutes will increase significantly. This will result in a LOW chance for Messages to be issued this afternoon and into the evening hours. Shower activity becomes more widespread into the evening with some persistent moderate to brief heavy rainfall possible into the overnight. These persistent moderate to brief heavy showers will have the potential to stall over some locations resulting in a slight chance for up to 1.00” in under 60 minutes, which will also increase the threat for some isolated Low Impact Flooding this evening. Shower activity will likely persist overnight as temperatures drop into the low 40s on the plains with mid to upper 30s along the foothills. Showers will likely start to taper off by daybreak tomorrow with skies gradually clearing by midday. STORM RAINFALL POTENTIAL AND DURATION: Typical showers will produce 0.05”-0.30” total in 10-30 minutes. A moderate shower or thunderstorm will produce 0.30”-0.60” total in 10-30 minutes. WORST CASE SCENARIO: A stationary moderate shower or training of moderate to brief heavy showers will have the potential to produce up to 1.20” in under 60 minutes. A LOOK AHEAD: Tomorrow will be even cooler for the first day of Fall with high temperatures only reaching the mid to upper 50s. Skies will gradually clear throughout the day with dry conditions expected in the afternoon and evening. Temperatures bounce back Monday, into the mid to upper 70s under mostly sunny skies. Mild and dry conditions are expected throughout most of the day with a chance for showers during the evening hours. Off and on showers will be possible Tuesday with a chance for thunderstorms in the afternoon and early evening. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 200 PM TO 600 AM SUN 0.05" (90%) to 0.3" (60%) to 0.6" (30%) LOW Arapahoe 200 PM TO 600 AM SUN 0.05" (90%) to 0.3" (60%) to 0.6" (30%) LOW Boulder 200 PM TO 600 AM SUN 0.05" (90%) to 0.3" (60%) to 0.6" (30%) LOW Broomfield 200 PM TO 600 AM SUN 0.05" (90%) to 0.3" (60%) to 0.6" (30%) LOW Denver 200 PM TO 600 AM SUN 0.05" (90%) to 0.3" (60%) to 0.6" (30%) LOW Douglas 200 PM TO 600 AM SUN 0.05" (90%) to 0.3" (60%) to 0.6" (30%) LOW Jefferson 200 PM TO 600 AM SUN 0.05" (90%) to 0.3" (60%) to 0.6" (30%) LOW Foothills above 6500ft       Boulder 100 PM TO 500 AM SUN 0.05" (95%) to 0.3" (65%) to 0.6" (35%) LOW Douglas 100 PM TO 500 AM SUN 0.05" (95%) to 0.3" (65%) to 0.6" (35%) LOW Jefferson 100 PM TO 500 AM SUN 0.05" (95%) to 0.3" (65%) to 0.6" (35%) LOW MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS. MHFD Flood Prediction Center: 303-458-0789    F2P2 Website [http://udfcd.org/Flash+Flood+Prediction+Program]