Time: 934 AM Tue June 29, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- WARM LAYER OF AIR ALOFT MAY PUT A LID ON THUNDERTSORM DEVELOPMENT OVER THE PLAINS? - A warm layer of air aloft at around 20,000ft is expected to suppress thunderstorm activity over the plains today with any shower or thunderstorm activity generally limited to the foothills and Palmer Divide. Late in the day moisture will increase from the S and a stray storm cannot be completely ruled out over the plains but they will be far and few between. Temperatures today will nudge upwards a few degrees into the mid 70's to around 80 over the I-25 corridor. - Shower and weak to moderate thunderstorm activity will develop over the higher terrain W of the District during the early afternoon. Increasing moisture from the S or an outflow from foothill storms may be able to generate isolated showers and weak thunderstorms over the plains by 3 or 4pm. With ample surface moisture in place if a thunderstorm is able to develop it will have the potential for moderate rainfall. - The current impression is that the warm layer of air aloft will hold and keep thunderstorm activity limited to the higher terrain W and S of the District with any activity within the District being weak and short lived. If the warm layer of air aloft erodes the HPO may need to be updated to increase the chances for thunderstorms and heavy rainfall. STORM RAINFALL POTENTIAL AND DURATION: Rain showers and weak thunderstorms will produce a trace to 0.2" in 10-30 minutes. Moderate thunderstorms will produce 0.2-0.5" in 10-30 minutes. WORST CASE SCENARIO: The warm layer of air aloft erodes allowing for more numerous moderate and strong thunderstorms with the potential to produce heavy rainfall rates of 0.4-1.0" in 10-30 minutes. A LOOK AHEAD: We will tack on a few more degrees to high temperatures on Wednesday and add a little more moisture. With more heat and moisture to work with there will be a better chance for afternoon thunderstorms, some of which may contain heavy rainfall. Thursday and Friday will feature a high likelihood of afternoon and evening thunderstorms with the potential to produce heavy rainfall that may result in excessive runoff and flash flooding. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 400 PM TO 800 PM Trace (40%) to 0.2" (20%) to 0.5" (5%) NONE Arapahoe 400 PM TO 800 PM Trace (40%) to 0.2" (20%) to 0.5" (5%) NONE Broomfield 400 PM TO 800 PM Trace (40%) to 0.2" (20%) to 0.5" (5%) NONE Denver 400 PM TO 800 PM Trace (40%) to 0.2" (20%) to 0.5" (5%) NONE Boulder 300 PM TO 800 PM Trace (60%) to 0.2" (25%) to 0.5" (5%) NONE Douglas 300 PM TO 800 PM Trace (60%) to 0.2" (25%) to 0.5" (5%) NONE Jefferson 300 PM TO 800 PM Trace (60%) to 0.2" (25%) to 0.5" (5%) NONE Foothills above 6500ft       Boulder 200 PM TO 800 PM Trace (80%) to 0.2" (50%) to 0.5" (15%) LOW Douglas 200 PM TO 800 PM Trace (80%) to 0.2" (50%) to 0.5" (15%) LOW Jefferson 200 PM TO 800 PM Trace (80%) to 0.2" (50%) to 0.5" (15%) 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]