Time: 845 AM Sun June 20, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- ISOLATED AFTERNOON THUNDERSTORMS WITH A STRONG COLD FRONT MOVING THROUGH THIS EVENING PRODUCING RAIN SHOWERS OVERNIGHT - Behind the upper level disturbance that moved through the District yesterday conditions will be relatively quiet through the day today until a cold front arrives this evening. A weak, preliminary cold front has moved through this morning which will help to keep temperatures in the 80's for highs over the plains. Daytime heating may generate an isolated thunderstorm or two this afternoon, favoring the higher terrain producing light to briefly moderate rain. - The main focus is on the approaching strong cold front that will move through the District this evening, sometime between 6-9pm. Along and just behind the front there will be a chance for thunderstorms but as temperatures cool any thunderstorm activity will quickly transition to general rain showers. Best chances for thunderstorm activity outside isolated storms this afternoon will be just ahead of and behind the front in the 6-10pm time frame. After 10pm any thunderstorm activity will diminish with intermittent light rain showers continuing overnight into early Monday morning. Heavy rainfall is not expected today but cannot be ruled out if strong thunderstorms develop this evening. - Lingering rain showers into Monday morning have likely ended by around daybreak or shortly after. Clouds will decrease and eventually become sunny/mostly sunny in the afternoon. STORM RAINFALL POTENTIAL AND DURATION: Rain showers and weak thunderstorms will produce a trace to 0.2" of rain. Moderate thunderstorms will have the potential to produce 0.2-0.4" in 10-30 minutes. Intermittent rain showers overnight will generally produce rainfall rates of 0.1-0.2"/hr. WORST CASE SCENARIO: A strong slower moving thunderstorm is able to develop this evening along the front this evening with the potential to produce 0.3-0.8" of rain in 10-30 minutes. A LOOK AHEAD: Any lingering rain shower activity Monday morning will end by around 10am. Skies will become sunny to mostly sunny with highs only reaching the 70's for highs over the plains with 60's in the Front Range foothills. Daytime heating may generate a shower or thunderstorm Monday afternoon over the higher terrain with the plains looking to remain on the dry side. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Arapahoe 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Boulder 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Broomfield 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Denver 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Douglas 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Jefferson 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Foothills above 6500ft       Boulder 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Douglas 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE Jefferson 600 PM TO 800 AM MON Trace (100%) to 0.2" (65%) to 0.4" (15%) NONE MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS. MHFD Flood Prediction Center: 303-458-0789    F2P2 Website [http://udfcd.org/Flash+Flood+Prediction+Program]