Time: 932 AM Tue August 1, 2023 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- HEAVY RAINFALL PRODUCING THUNDERSTRMS TO IMPACT THE DISTRICT AGAIN THIS AFTERNOON AND EVENING/MESSAGE 2'S WILL BE ISSUED AROUND NOON Heavy rainfall producing thunderstorms impacted the District yesterday and a repeat performance is expected today. The high risk of heavy rainfall has resulted in the NWS issuing a Flood/Flash Flood Watch for the entire District from noon to midnight. Message 2’s will be issued by noon and will be valid until midnight but the best chance for thunderstorms looks to be through about 10:00 PM this evening. Thunderstorms will first develop over the mountains and foothills between 11:00 AM and 1:00 PM. Upper level steering winds from the SW will push storms onto the plains between 1:00-3:00 PM. Prime time for thunderstorms will be between 2:00-10:00 PM with multiple rounds of thunderstorms possible. After 10:00 PM the threat for stronger thunderstorms will diminish with additional weaker thunderstorms and light rain showers possible beyond midnight. The upper level steering winds are stronger than yesterday at 20-25mph and the storms should be faster moving but strong thunderstorms may move more slowly or against the grain and produce extended periods of heavy rain. Training of thunderstorm cells is another potential culprit for longer duration heavy rainfall that may lead to excessive runoff and flash flooding. Strong thunderstorms may also contain gusty winds and large hail. Temperatures will be a little cooler today with afternoon highs in the mid to possibly upper 80’s over the plains. The cooler temperatures may hinder storm strength slightly but widespread to numerous thunderstorms are expected regardless of temperature. Due to all of the rainfall yesterday it may not take as much rain today to produce excessive runoff and flooding. STORM RAINFALL POTENTIAL AND DURATION: Weak to moderate thunderstorms will produce rainfall rates of 0.2-0.8” in 10-30 minutes. Strong thunderstorms will have the potential to produce 0.8-2.0” of heavy rain in 10-30 minutes. WORST CASE SCENARIO: A slow moving large thunderstorm or training of strong thunderstorm cells may result in up to 3.5” of rain in 60-90 minutes. A LOOK AHEAD: Another high risk day for heavy rainfall producing thunderstorms on Wednesday but storms will likely be less numerous. The storms that do form will have attributes similar to the storms that form today. A decrease in thunderstorm coverage and intensity is expected Thursday and Friday but there will still be modest chances for afternoon thunderstorms and stronger storms will still have the potential to produce heavy rain. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 200 PM TO 1000 PM 0.5" (75%) to 1" (50%) to 2" (25%) HIGH Arapahoe 200 PM TO 1000 PM 0.5" (80%) to 1" (60%) to 2" (30%) HIGH Broomfield 200 PM TO 1000 PM 0.5" (75%) to 1" (50%) to 2" (25%) HIGH Denver 200 PM TO 1000 PM 0.5" (75%) to 1" (50%) to 2" (25%) HIGH Boulder 100 PM TO 1000 PM 0.5" (75%) to 1" (50%) to 2" (25%) HIGH Douglas 100 PM TO 1000 PM 0.5" (80%) to 1" (60%) to 2" (30%) HIGH Jefferson 100 PM TO 1000 PM 0.5" (75%) to 1" (50%) to 2" (25%) HIGH Foothills above 6500ft       Boulder 1200 PM TO 1000 PM 0.5" (80%) to 1" (60%) to 2" (25%) HIGH Douglas 1200 PM TO 1000 PM 0.5" (80%) to 1" (60%) to 2" (25%) HIGH Jefferson 1200 PM TO 1000 PM 0.5" (80%) to 1" (60%) to 2" (25%) 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]