Time: 819 PM Wed May 17, 2023 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- CHANCES FOR SHOWERS AND THUNDERSTORMS INCREASING FOR THE OVERNIGHT PERIOD/HEAVY RAINFALL POSSIBLE THURSDAY Conditions have been relatively quiet over the District through the day today, but a cold front is on the approach and may generate additional rain showers and widely scattered thunderstorms late this evening into the early morning hours on Thursday. There are a few showers and weak thunderstorms in the area currently and a thunderstorm cannot be ruled out over the next couple hours, but better chances for showers and thunderstorms look to arrive between 10pm and midnight. There will then be a risk for overnight shower and thunderstorm activity through roughly 3 or 4am then any additional precipitation is expected to be light into mid-morning Thursday. Surface moisture will increase later this evening and overnight as the front moves through with stronger thunderstorms having the potential to produce moderate to briefly heavy rain resulting in a low Message potential for the overnight period. Best chances for precipitation over the next few hours will be south of I-70 then the better chances for storm activity will shift to the N of I-70 after 10pm. STORM RAINFALL POTENTIAL AND DURATION: Rain showers will produce a trace to 0.2” of rain in 15-45 minutes. Weak to moderate thunderstorms will produce rainfall rates of 0.2-0.5” in 10-30 minutes. WORST CASE SCENARIO: A strong thunderstorm will have the potential to produce rainfall rates of 0.3-0.7” in 10-30 minutes with a large strong thunderstorm or training of thunderstorm cells capable of producing up to 1.3" in 60 minutes or less. A LOOK AHEAD: Tomorrow morning behind the cold front we will wake up to cloudy skies with possibly a few light rain showers in the area. Rain showers and thunderstorms are anticipated to become more numerous Thursday afternoon and evening. The threat of thunderstorms and their intensity will hinge upon temperatures/cloud cover and other factors, but best chances will be in noon to 8pm time frame Thursday. Many creeks and streams are running high compared to “normal” due to recent rains or snow melt or a combination of both and should heavy rainfall develop Thursday excessive runoff may occur quickly and flash flooding is possible. Light rain showers continue likely at times overnight Thursday into Friday morning. A cloudy and cool day Friday with highs in the upper 50's to mid 60's with additional rain showers and possibly an isolated thunderstorm. The cool temperatures Friday may result in more showery conditions versus thunderstorms. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams Current TO 400 AM THU Trace (80%) to 0.2" (50%) to 0.5" (25%) LOW Arapahoe Current TO 400 AM THU Trace (90%) to 0.2" (50%) to 0.5" (20%) LOW Boulder Current TO 400 AM THU Trace (90%) to 0.2" (60%) to 0.5" (30%) MOD Broomfield Current TO 400 AM THU Trace (90%) to 0.2" (60%) to 0.5" (30%) MOD Denver Current TO 400 AM THU Trace (90%) to 0.2" (50%) to 0.5" (20%) LOW Douglas Current TO 400 AM THU Trace (90%) to 0.2" (50%) to 0.5" (20%) LOW Jefferson Current TO 400 AM THU Trace (90%) to 0.2" (50%) to 0.6" (25%) LOW Foothills above 6500ft       Boulder Current TO 400 AM THU Trace (95%) to 0.2" (60%) to 0.5" (30%) MOD Douglas Current TO 400 AM THU Trace (90%) to 0.2" (50%) to 0.5" (20%) LOW Jefferson Current TO 400 AM THU Trace (90%) to 0.2" (50%) to 0.5" (25%) 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]