Flash Flood Prediction Program

Time: 913 AM Thu August 19, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- COOLER TODAY WITH FAST MOVING SHOWERS AND THUNDERSTORMS STARTING EARLY THIS AFTERNOON - Temperatures will be cooler today behind a cold front that moved through the District this morning with highs in the upper 70's to lower 80's along the I-25 corridor. A potent upper level low pressure system for this early in the season currently resides over central UT and will lift northeastward into WY during the day rotating a round or two of showers and thunderstorms through the District. - Shower and thunderstorm activity is currently just W of the Continental Divide and will begin to move into the Front Range foothills between 10am and noon. A broken line of showers and thunderstorms is expected to emerge onto the plains between noon-1pm. The initial line of showers and thunderstorms is likely the strongest and will be the dominant weather feature for the day but there could be a weaker secondary round later afternoon and evening. Best chances for thunderstorm activity will be from noon to 6pm. - Fast storm motions from SSW to NNE at 30-40mph will severely limit point rainfall amounts from a singe storm, likely keeping any heavy rainfall under 5 minutes in duration. Training of thunderstorm cells will produce the greatest risk for extended periods of heavy rainfall that may lead to excessive runoff. Strong thunderstorms may also become severe with gusty winds and hail being the primary threats. Stronger thunderstorm activity today is expected to favor I-70 northward and in particular areas just N of the District. STORM RAINFALL POTENTIAL AND DURATION: Fast moving rain showers and weak thunderstorms will produce a trace to 0.2". Moderate to strong thunderstorms will have the potential to produce 0.2-0.6" in 10-20 minutes. Note* - Due to fast storm motions we are using 20-minute rain totals today in the table for the first time this season (standard is 30-minute). WORST CASE SCENARIO: Training of moderate to strong thunderstorm cells may result in up to 1.5" of heavy rainfall in 45-60 minutes. A LOOK AHEAD: A slight chance for a few lingering rain showers or weak thunderstorms early Friday morning with dry conditions likely well ahead of noon. Down-sloping winds off the Continental Divide will dry us out for the afternoon with temperatures below seasonal averages in the 70's and 80's over the plains. LOCATION PRIME TIME 20-Minute Rainfall and % Probability Message Potential Plains       Adams 100 PM TO 600 PM 0.1" (80%) to 0.3" (50%) to 0.6" (25%) MOD Arapahoe 100 PM TO 600 PM 0.1" (65%) to 0.3" (40%) to 0.6" (15%) LOW Denver 100 PM TO 600 PM 0.1" (75%) to 0.3" (45%) to 0.6" (20%) LOW Douglas 100 PM TO 600 PM 0.1" (60%) to 0.3" (30%) to 0.6" (10%) LOW Broomfield 1200 PM TO 600 PM 0.1" (80%) to 0.3" (50%) to 0.6" (25%) MOD Jefferson 1200 PM TO 600 PM 0.1" (75%) to 0.3" (45%) to 0.6" (20%) LOW Boulder 1100 AM TO 600 PM 0.1" (85%) to 0.3" (60%) to 0.6" (30%) MOD Foothills above 6500ft       Boulder 1100 AM TO 600 PM 0.1" (95%) to 0.3" (75%) to 0.6" (50%) MOD Douglas 1100 AM TO 600 PM 0.1" (60%) to 0.3" (30%) to 0.6" (10%) LOW Jefferson 1100 AM TO 600 PM 0.1" (85%) to 0.3" (60%) to 0.6" (30%) 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]