Flash Flood Prediction Program

Time: 919 AM Fri September 3, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- SEASONAL TEMPERATURES WITH A ROUND OR TWO OF THUNDERSTORMS THIS AFTERNOON/EVENING - Temperatures will be at or slightly below seasonal averages today over the District with highs reaching the upper 70's to lower 80's over the plains. Normal high for Denver today is 84 degrees. An upper level disturbance is on the approach and may result in an isolated rain shower this morning, favoring the higher terrain. - As the upper level disturbance moves through the District this afternoon it is expected to generate a round or two of thunderstorms between 1-7pm. Upper level steering winds from SW to NE at 15-20mph will keep storms moving along at a brisk pace but stronger storms will be capable of producing brief heavy rainfall that may lead to excessive runoff roadways and smaller creeks/streams. Favored areas for stronger storms at this time looks to be E of I-25 and S of I-70. In addition to brief heavy rainfall strong thunderstorms may also contain gusty winds and large hail. - After 7pm the main round or two of storms is expected to have pushed eastward with lingering more isolated light rain showers remaining possible until around 10pm. Beyond 10pm dry conditions are expected for the remainder of the evening/overnight period. STORM RAINFALL POTENTIAL AND DURATION: Weak thunderstorms will produce 0.1-0.4" of rainfall in 10-30 minutes. Moderate to strong thunderstorms will have the potential to produce heavy rainfall of 0.4-1.2" in 10-30 minutes. WORST CASE SCENARIO: Training of strong thunderstorm cells or a slower moving large thunderstorm may result in up to 2.4" of rainfall in 60-90 minutes. A LOOK AHEAD: Behind the disturbance that moves through later today temperatures will be similar on Saturday in the 70's and lower 80's for highs over the plains. A more stable atmosphere will be in place over the plains with possibly an isolated late afternoon thunderstorm, otherwise thunderstorm activity will favor the foothills and Palmer Divide on Saturday. Sunday and Monday are trending dry with warming temperatures. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 100 PM TO 700 PM 0.1" (80%) to 0.4" (60%) to 1.2" (20%) HIGH Arapahoe 100 PM TO 700 PM 0.1" (80%) to 0.4" (60%) to 1.2" (20%) HIGH Broomfield 100 PM TO 700 PM 0.1" (80%) to 0.4" (60%) to 1.2" (20%) HIGH Denver 100 PM TO 700 PM 0.1" (80%) to 0.4" (60%) to 1.2" (20%) HIGH Boulder 1200 PM TO 700 PM 0.1" (80%) to 0.4" (50%) to 1.2" (15%) MOD Douglas 1200 PM TO 700 PM 0.1" (80%) to 0.4" (60%) to 1.2" (20%) HIGH Jefferson 1200 PM TO 700 PM 0.1" (80%) to 0.4" (50%) to 1.2" (15%) MOD Foothills above 6500ft       Boulder 1100 AM TO 600 PM 0.1" (90%) to 0.4" (50%) to 1.2" (10%) LOW Douglas 1100 AM TO 600 PM 0.1" (90%) to 0.4" (50%) to 1.2" (10%) LOW Jefferson 1100 AM TO 600 PM 0.1" (90%) to 0.4" (50%) to 1.2" (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]