Time: 925 AM Mon May 17, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- CONTINUED CLOUDY AND COOL WITH AFTERNOON/EVENING RAIN/THUNDERSTORMS WITH THE POTENTIAL FOR HEAVY RAINFALL - Continued cloudy and cool through the first half of the day today with temperatures warming into the 60's over the plains with 50's in the foothills. All the ingredients are in place for showers and thunderstorms to develop today the only question is will temperatures warm enough for thunderstorms to become moderate to strong or will temperatures remain too cool and result in only weak thunderstorms and a bias towards non-thunderstorm rain? Just a few degrees could be the difference... - There is a chance for an isolated rain shower or sprinkle this morning with conditions trending dry into early afternoon. The cloud cover and cooler temperatures will help to push back the initiation of shower and thunderstorm activity with the first storms likely developing between 2-4pm over the foothills then spread out onto the plains between 4-6pm. Scattered to widespread shower and thunderstorm coverage is then expected to around 10pm and could linger until midnight. After midnight conditions trend drier with isolated light rain showers possible overnight into Tuesday morning. - Thunderstorms that develop today will be capable of producing heavy rainfall that may lead to excessive runoff and possibly flash flooding. Storm motions will be slow, at 10mph or less and thunderstorms may have a tendency to anchor along the foothill/plains interface potentially producing moderate to heavy rainfall for an extended period of time. STORM RAINFALL POTENTIAL AND DURATION: Rain showers and weak thunderstorms will produce 0.1-0.4" in 10-30 minutes. Moderate to strong thunderstorms. Moderate to strong thunderstorms will have the potential to produce 0.4-1.0" in 10-30 minutes. WORST CASE SCENARIO: A strong thunderstorm becomes stationary producing an extended period of heavy rainfall with up to 2.0" in 60 minutes or less. A LOOK AHEAD: A chance for a few isolated light rain showers persist into daybreak Tuesday then an increase in non-thunderstorm rain showers increase through the morning with a chance for generally weak isolated afternoon thunderstorms. Temperatures will remain below seasonal averages in the 60's for highs over the plains. Heavy rainfall is not expected at this time. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 400 PM TO 1159 PM 0.1" (90%) to 0.4" (50%) to 1.0" (15%) MOD Arapahoe 400 PM TO 1159 PM 0.1" (90%) to 0.4" (50%) to 1.0" (15%) MOD Denver 400 PM TO 1159 PM 0.1" (90%) to 0.4" (50%) to 1.0" (15%) MOD Boulder 300 PM TO 1159 PM 0.1" (95%) to 0.4" (60%) to 1.0" (20%) HIGH Broomfield 300 PM TO 1159 PM 0.1" (90%) to 0.4" (50%) to 1.0" (15%) MOD Douglas 300 PM TO 1159 PM 0.1" (95%) to 0.4" (60%) to 1.0" (20%) HIGH Jefferson 300 PM TO 1159 PM 0.1" (95%) to 0.4" (60%) to 1.0" (20%) HIGH Foothills above 6500ft       Boulder 200 PM TO 1159 PM 0.1" (100%) to 0.4" (75%) to 1.0" (25%) HIGH Douglas 200 PM TO 1159 PM 0.1" (100%) to 0.4" (75%) to 1.0" (25%) HIGH Jefferson 200 PM TO 1159 PM 0.1" (100%) to 0.4" (75%) to 1.0" (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]