Time: 308 PM Tue June 29, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- FOOTHILL THUNDERSTORMS MAY SUPPLY TRIGGER FOR STORMS ON THE PLAINS LATER THIS AFTERNOON/EVENING - Slow moving to nearly stationary thunderstorms over the foothills of Jefferson County may push a rain cooled outflow boundary eastward onto the plains shortly. This rain cooled outflow boundary/gust front may be able to overcome a warm layer of air aloft and generate new storm development over the plains in the coming hours. - Thunderstorms if they develop will be slow moving and capable of producing moderate to heavy rainfall for extended periods of time. There is no indication that storms are imminent but may develop over the coming hours if conditions come together just right. - Best chances for thunderstorm activity continues to be over the foothills through about 8pm this evening. STORM RAINFALL POTENTIAL AND DURATION: Weak to moderate thunderstorms will produce 0.3-0.9" in 10-30 minutes. Strong thunderstorms will have the potential to produce 0.5-1.5" in 10-30 minutes. WORST CASE SCENARIO: A stationary moderate to strong thunderstorm may result in up to 2.5" of rain in 60-90 minutes. A LOOK AHEAD: We will tack on a few more degrees to high temperatures on Wednesday and add a little more moisture. With more heat and moisture to work with there will be a better chance for afternoon thunderstorms, some of which may contain heavy rainfall. Thursday and Friday will feature a high likelihood of afternoon and evening thunderstorms with the potential to produce heavy rainfall that may result in excessive runoff and flash flooding. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 400 PM TO 800 PM 0.1" (60%) to 0.3" (30%) to 0.9" (15%) MOD Arapahoe 400 PM TO 800 PM 0.1" (60%) to 0.3" (30%) to 0.9" (15%) MOD Boulder 400 PM TO 800 PM 0.1" (65%) to 0.3" (40%) to 0.9" (20%) MOD Broomfield 400 PM TO 800 PM 0.1" (60%) to 0.3" (30%) to 0.9" (15%) MOD Denver 400 PM TO 800 PM 0.1" (60%) to 0.3" (30%) to 0.9" (15%) MOD Douglas 400 PM TO 800 PM 0.1" (65%) to 0.3" (40%) to 0.9" (20%) MOD Jefferson 400 PM TO 800 PM 0.1" (65%) to 0.3" (40%) to 0.9" (20%) MOD Foothills above 6500ft       Boulder Current TO 800 PM TUE 0.1" (95%) to 0.3" (70%) to 0.9" (30%) MOD Douglas Current TO 800 PM TUE 0.1" (95%) to 0.3" (70%) to 0.9" (30%) MOD Jefferson Current TO 800 PM TUE 0.1" (95%) to 0.3" (70%) to 0.9" (30%) MOD MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS. MHFD Flood Prediction Center: 303-458-0789    F2P2 Website [http://udfcd.org/Flash+Flood+Prediction+Program]