Flash Flood Prediction Program

Time: 1046 AM Sun May 23, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- THUNDERSTORM CHANCES INCREASING AS MOISTURE IS SLOW TO SCOUR OUT - A "dry line" is approaching from the west but ahead of it ample moisture remains in place along the I-25 corridor to fuel storms as temperatures are now warming. Ahead of the dry line there will be a modest chance for thunderstorms, some of which may become strong to severe and could produce brief heavy rainfall. - As the dry air arrives from the west thunderstorm chances will diminish significantly. The drier air is currently just W of the Continental Divide and the progression of the drier air over the next few hours will be a determining factor on how the day will play out. Thunderstorms will be fast moving from S to N reducing the threat for heavy rainfall from a single storm. Training of thunderstorm cells may result in extended periods of heavy rainfall. - If thunderstorms initiate over the foothills instead of E of the I-25 corridor, Message 1's may need to be issued as brief heavy rainfall will accompany the stronger thunderstorm cells. STORM RAINFALL POTENTIAL AND DURATION: Typical rain showers/isolated thunderstorms will have the potential to produce a TR-0.3" in 10-30 minutes. A strong thunderstorm will have the potential to produce 0.3-0.8" in 10-30 minutes. WORST CASE SCENARIO: Training of thunderstorm cells may result in up to 1.5" of rain in 45-60 minutes. A LOOK AHEAD: High pressure starts to build over the region tomorrow leading to dry conditions starting the week with temperatures in the upper 70's to low 80's through Wednesday. A slight chance Wednesday afternoon for widely scattered high-based thunderstorms with no heavy rainfall threat at this time. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 1130 AM TO 400 PM Trace (90%) to 0.3" (60%) to 0.8" (15%) MOD Arapahoe 1130 AM TO 400 PM Trace (90%) to 0.3" (60%) to 0.8" (15%) MOD Broomfield 1130 AM TO 400 PM Trace (90%) to 0.3" (60%) to 0.8" (15%) MOD Denver 1130 AM TO 400 PM Trace (90%) to 0.3" (60%) to 0.8" (15%) MOD Boulder 1100 AM TO 400 PM Trace (90%) to 0.3" (60%) to 0.8" (15%) MOD Douglas 1100 AM TO 400 PM Trace (90%) to 0.3" (60%) to 0.8" (15%) MOD Jefferson 1100 AM TO 400 PM Trace (90%) to 0.3" (60%) to 0.8" (15%) MOD Foothills above 6500ft       Boulder Current TO 400 PM SUN Trace (80%) to 0.3" (50%) to 0.8" (10%) LOW Douglas Current TO 400 PM SUN Trace (80%) to 0.3" (50%) to 0.8" (10%) LOW Jefferson Current TO 400 PM SUN Trace (80%) to 0.3" (50%) to 0.8" (10%) 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]