Flash Flood Prediction Program

Time: 830 AM Wed June 21, 2023 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- STRONG TO SEVERE THUNDERSTORMS WITH HEAVY RAINFALL AND HAIL LIKELY FROM LATE AFTERNOON INTO THE EARLY OVERNIGHT PERIOD Today is the longest day of the year and officially the first day of summer. Conditions will feel like summer through early afternoon with highs reaching the mid to upper 80’s over the plains with 70’s in the Front Range foothills prior to thunderstorms developing late this afternoon and evening. Multiple ingredients will be coming together for thunderstorms to become strong to severe producing heavy rainfall, large hail and gusty winds. Storms will begin to initiate between 4-6pm and continue likely through midnight-2am. 1-2 rounds of strong/severe thunderstorms are expected. Areas further west into the foothills will have a lesser chance for strong/severe storms with areas along and E of I-25 in a more favorable environment as moisture is deeper, but stronger storms may back up to near the base of the foothills. Thunderstorms will be relatively slow moving and may produce extended periods of heavy rain, large hail and possibly accumulating hail. Message 1’s will be issued by early to mid afternoon and most likely be valid until midnight or 2am. After 2am Thursday, chances for thunderstorms diminish but do not completely go away and there could be additional showers and weaker thunderstorms into the early morning hours on Thursday. STORM RAINFALL POTENTIAL AND DURATION: Weak thunderstorms will produce 0.2-0.6" of rain. Moderate to strong thunderstorms will have the potential to produce rainfall rates of 0.6-1.8" in 10-30 minutes. WORST CASE SCENARIO: A nearly stationary strong thunderstorm may result in up to 3.0" of rain in 45-60 minutes. A LOOK AHEAD: The chance for severe weather will decrease on Thursday but with abundant moisture in place there will likely be more numerous showers and thunderstorms in the afternoon and evening with a higher likelihood of heavy rain that may result in excessive runoff and possibly isolated areas of flooding. The weather pattern will dry out on Friday and dry conditions will continue through the upcoming weekend. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 400 PM TO 200 AM THU 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Arapahoe 400 PM TO 200 AM THU 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Boulder 400 PM TO 200 AM THU 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Broomfield 400 PM TO 200 AM THU 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Denver 400 PM TO 200 AM THU 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Douglas 400 PM TO 200 AM THU 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Jefferson 400 PM TO 200 AM THU 0.2" (90%) to 0.6" (60%) to 1.8" (30%) HIGH Foothills above 6500ft       Boulder 300 PM TO 200 AM THU 0.2" (75%) to 0.6" (50%) to 1.8" (15%) HIGH Douglas 300 PM TO 200 AM THU 0.2" (75%) to 0.6" (50%) to 1.8" (15%) HIGH Jefferson 300 PM TO 200 AM THU 0.2" (75%) to 0.6" (50%) to 1.8" (15%) 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]