Flash Flood Prediction Program

Time: 1052 AM Tue April 27, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- AFTERNOON THUNDERSTORMS FOLLOWED BY A SOAKING RAIN THIS EVENING AND LIKELY SOME WET SNOW OVERNIGHT - A potent spring storm system is taking aim at the District today and tonight producing the first widespread thunderstorm activity of the season this afternoon. After the initial wave or two of showers and thunderstorms precipitation will transition to a steady "wetting" rain this evening and continue overnight with the snow level dropping to somewhere between 5,000-6,000ft by tomorrow morning. - Thunderstorms this afternoon between noon and 4pm may become strong to severe but with low freezing levels the stronger storms will be large hail or accumulating hail producers versus heavy rainfall producers. Weaker storms will be capable of moderate rainfall and small hail. Between 4-8pm the transition from convective storms to a more stratiform rain event is expected to unfold with a steady rain into Wednesday morning and the snow line lowering. - The snow line is expected to start out at or above 9,000' and lower to around 6,000' by around midnight with potentially a brief period of "wet" snow down to around 5,000' for the I-25 corridor after midnight into daybreak Wednesday. Just a degree or two will make the difference for rain or snow below 6,000'. Smaller creeks and streams will be filling with runoff as the event wears on. Cherry Creek through Denver may be most prone to running high or reaching bike path levels. STORM RAINFALL POTENTIAL AND DURATION: Strong thunderstorms are expected to produce more hail than rainfall. Weak to moderate thunderstorms will have the potential to produce 0.1-0.5" of rain in 10-30 minutes. Steady moderate to heavy rain may produce 0.3-1.2" in 45-75 minutes. A LOOK AHEAD: Any snow below 5,500' tomorrow morning will change back to rain as the snowline rises through the day. Precipitation likely remains snow at or above the 7-8,000' line. Widespread and persistent rain/snow showers tomorrow morning will begin to break up by noon slowly becoming dry through the afternoon. Additional precipitation on Wednesday will be light and with cooler temperatures in place no heavy rainfall is expected. LOCATION PRIME TIME 60-Minute Rainfall and % Probability Message Potential Plains       Adams 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) LOW Arapahoe 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) LOW Boulder 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) LOW Broomfield 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) LOW Denver 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) LOW Douglas 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) LOW Jefferson 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) LOW Foothills above 6500ft       Boulder 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) NONE Douglas 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) NONE Jefferson 1200 PM TO 1000 AM WED 0.1" (100%) to 0.5" (60%) to 1.0" (20%) 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]