Time: 630 PM Thu August 12, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- THUNDERSTORM CHANCES ON THE INCREASE THIS EVENING/HEAVY RAINFALL POSSIBLE - Surface moisture has slowly increased through the afternoon with dew points currently in the low to mid 50's along and E of I-25. Outflow from thunderstorm activity to the SE is pushing back towards the District at this time and may trigger thunderstorm development this evening. There is a chance this boundary moves through and minimal if any development occurs but if a storm is able to develop heavy rainfall is a good bet. - With the increase in surface moisture potential rainfall amounts have also increased from this morning's HPO. Stronger thunderstorms, should they develop this evening will be capable of producing brief periods of heavy rain that may lead to excessive runoff and possibly flash flooding. - Best chances for thunderstorm activity will be over the plains where moisture is deeper with a lesser chance for strong storms over the foothills. Best chances for thunderstorms overall appears to be from roughly 10pm this evening until about 2am Friday morning. STORM RAINFALL POTENTIAL AND DURATION: Weak to moderate thunderstorms will produce rainfall rates of 0.2-0.5" in 10-30 minutes. Strong thunderstorms will have the potential to produce heavy rainfall of 0.5-1.5" in 10-30 minutes. WORST CASE SCENARIO: A nearly stationary strong thunderstorm may result in up to 2.5" of heavy rain in 45-60 minutes. A LOOK AHEAD: Thunderstorm chances increase area wide on Friday with widely scattered to scattered afternoon thunderstorm coverage in the afternoon/evening with slower moving strong storms capable of producing heavy rainfall that may lead to excessive runoff and possibly flash flooding. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams Current TO 200 AM FRI 0.2" (60%) to 0.5" (30%) to 1.5" (15%) MOD Arapahoe Current TO 200 AM FRI 0.2" (60%) to 0.5" (30%) to 1.5" (15%) MOD Boulder Current TO 200 AM FRI 0.2" (50%) to 0.5" (25%) to 1.5" (10%) LOW Broomfield Current TO 200 AM FRI 0.2" (60%) to 0.5" (30%) to 1.5" (15%) MOD Denver Current TO 200 AM FRI 0.2" (60%) to 0.5" (30%) to 1.5" (15%) MOD Douglas Current TO 200 AM FRI 0.2" (60%) to 0.5" (30%) to 1.5" (15%) MOD Jefferson Current TO 200 AM FRI 0.2" (50%) to 0.5" (25%) to 1.5" (10%) LOW Foothills above 6500ft       Boulder Current TO 200 AM FRI 0.2" (40%) to 0.5" (20%) to 1.5" (10%) LOW Douglas Current TO 200 AM FRI 0.2" (40%) to 0.5" (20%) to 1.5" (10%) LOW Jefferson Current TO 200 AM FRI 0.2" (40%) to 0.5" (20%) to 1.5" (10%) 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]