Flash Flood Prediction Program

Time: 913 AM Wed July 21, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- MONSOON MOISTURE INCREASES HEAVY RAINFALL THREAT TODAY - Mid and upper level monsoon moisture will move overhead today and result in thunderstorm activity returning to the District. Thunderstorms that develop will be slow moving and have the potential to produce heavy rainfall that may lead to excessive runoff and possibly flash flooding. - Thunderstorms will develop over the mountains/foothills during the early afternoon. Thunderstorm chances increase over the plains between 2-3pm with prime time from 3-9pm. Additional showers and weaker thunderstorms will remain possible until midnight. 1-2 rounds of thunderstorms are expected with the initial round likely being the strongest. - Relatively slow storm motions from W to E at around 10mph will allow for extended periods of heavy rain under stronger storms. Storm motions may become erratic at times due to outflow boundaries/gust front interactions. STORM RAINFALL POTENTIAL AND DURATION: Weak to moderate thunderstorms will produce 0.2-0.6" in 10-30 minutes. Strong thunderstorms have the potential to produce 0.6-1.8" of heavy rain in 10-30 minutes. WORST CASE SCENARIO: A nearly stationary strong thunderstorm may result in up to 3.0" in 60 minutes or less. A LOOK AHEAD: Ridge of high pressure shifts back to the west on Thursday decreasing the chances for thunderstorms over the plains as the activity moves back over the mountainous areas of the state. There will still be a chance for isolated afternoon thunderstorm activity favoring the W and S areas of the District. Heavy rainfall threat lowers considerably. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 300 PM TO 900 PM 0.2" (75%) to 0.6" (50%) to 1.8" (25%) HIGH Arapahoe 300 PM TO 900 PM 0.2" (75%) to 0.6" (50%) to 1.8" (25%) HIGH Broomfield 300 PM TO 900 PM 0.2" (75%) to 0.6" (50%) to 1.8" (25%) HIGH Denver 300 PM TO 900 PM 0.2" (75%) to 0.6" (50%) to 1.8" (25%) HIGH Boulder 200 PM TO 900 PM 0.2" (80%) to 0.6" (50%) to 1.8" (25%) HIGH Douglas 200 PM TO 900 PM 0.2" (80%) to 0.6" (50%) to 1.8" (25%) HIGH Jefferson 200 PM TO 900 PM 0.2" (80%) to 0.6" (50%) to 1.8" (25%) HIGH Foothills above 6500ft       Boulder 100 PM TO 900 PM 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Douglas 100 PM TO 900 PM 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Jefferson 100 PM TO 900 PM 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS. MHFD Flood Prediction Center: 303-458-0789    F2P2 Website [http://udfcd.org/Flash+Flood+Prediction+Program]