Time: 853 AM Wed June 30, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- SLOW STORM MOTIONS AND AMPLE MOISTURE = INGREDIENTS FOR HEAVY RAINFALL PRODUCING THUNDERSTORMS - Moisture is on the increase today, the only question is whether or not there will be any significant storm coverage within District boundaries... Light upper level winds from the SW at 10mph or less and ample surface moisture with dew points in the upper 40's to lower 50's will result in thunderstorms that develop today having the potential to produce extended periods of moderate and heavy rainfall. - Thunderstorms will first develop over the foothills and Palmer Divide by around noon or shortly after. With light steering winds and a lack of a trigger over the plains the storms are expected to favor the higher terrain. Rain cooled outflow from foothill storms or the development of a surface wind convergence line over the plains will likely be needed to initiate convection within the District itself. Between 2-4pm a gust front from foothill storms or the set-up of a surface wind convergence line may develop or a combination of the two and thunderstorms will become more likely over the plains. - The foothills and Palmer Divide stand the best chance for stronger storms today with a lesser chance over the plains, however the plains storms will be just as capable of producing heavy rain, the overall threat is just lower. Prime time for thunderstorms today is from 2-8pm, beyond 8pm any thunderstorm activity is expected to be weak with conditions trending dry for the overnight period. STORM RAINFALL POTENTIAL AND DURATION: Rain showers and weak thunderstorms will produce 0.1-0.4" in 15-45 minutes. Moderate to strong thunderstorms will have the potential to produce 0.4-1.6" in 15-45 minutes. WORST CASE SCENARIO: A stationary strong thunderstorm may result in up to 2.5" of rain in 60-90 minutes. A LOOK AHEAD: Thunderstorm chances increase all areas of the District on Thursday with scattered to numerous thunderstorms expected during the afternoon and evening. There is a high likelihood of excessive runoff and possibly flash flooding as moisture levels continue to rise and storm motions remain very slow. Not much change on Friday with scattered to numerous thunderstorms with the potential for heavy rainfall. There will likely be a flooding event of some type over the next few days but the severity and location cannot be pinpointed until the events begin to unfold. LOCATION PRIME TIME 45-Minute Rainfall and % Probability Message Potential Plains       Adams 200 PM TO 800 PM 0.1" (65%) to 0.4" (30%) to 1.0" (15%) MOD Arapahoe 200 PM TO 800 PM 0.1" (65%) to 0.4" (30%) to 1.0" (15%) MOD Broomfield 200 PM TO 800 PM 0.1" (65%) to 0.4" (30%) to 1.0" (15%) MOD Denver 200 PM TO 800 PM 0.1" (65%) to 0.4" (30%) to 1.0" (15%) MOD Boulder 100 PM TO 800 PM 0.1" (75%) to 0.4" (50%) to 1.0" (25%) MOD Douglas 100 PM TO 800 PM 0.1" (80%) to 0.4" (60%) to 1.0" (30%) MOD Jefferson 100 PM TO 800 PM 0.1" (75%) to 0.4" (50%) to 1.0" (25%) MOD Foothills above 6500ft       Boulder 1200 PM TO 800 PM 0.1" (95%) to 0.4" (75%) to 1.0" (35%) HIGH Douglas 1200 PM TO 800 PM 0.1" (95%) to 0.4" (75%) to 1.0" (35%) HIGH Jefferson 1200 PM TO 800 PM 0.1" (95%) to 0.4" (75%) to 1.0" (35%) 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]