Flash Flood Prediction Program

Time: 915 AM Fri August 13, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- AMPLE MOISTURE IS AVAILAIBLE TO FUEL THUNDERSTORMS... BUT WILL THEY DEVELOP? - Moisture began to increase over the District yesterday behind a cold front and continued to uptick overnight with ample surface moisture in place today to fuel thunderstorms... The main question is whether or not we will be able to tap into the moisture this afternoon and produce thunderstorm activity? As can be the case many days we seem to be missing a triggering mechanism as daytime heating alone may not be enough to generate thunderstorms and conditions could trend on the drier side. - Today is just one of those days where there is a low chance for thunderstorms to develop BUT if they do, they will be strong with heavy rainfall a good bet making for a tricky forecast. Potential triggers are more daytime heating than expected or the more likely culprit of outflow boundaries from thunderstorm activity outside the District pushing back into the District providing the needed lifting mechanism. - Thunderstorm activity will be slow to develop but by 3-4pm the first storms of the day become possible, favoring the foothills and Palmer Divide. There will then be a low chance for thunderstorms well into the evening hours. Thunderstorms that develop today will be slow moving and capable of producing heavy rainfall that may lead to excessive runoff and possibly flash flooding. STORM RAINFALL POTENTIAL AND DURATION: Rain showers and weak thunderstorms will produce a trace to 0.2". Moderate thunderstorms will produce 0.2-0.6" in 10-30 minutes with strong storms having the potential to produce heavy rainfall of 0.5-1.5" in 10-30 minutes. WORST CASE SCENARIO: A stationary strong thunderstorm may result in up to 2.5" of rain in 45-60 minutes. A LOOK AHEAD: Saturday will feature a chance for isolated afternoon thunderstorms favoring the higher terrain with a much lower threat for heavy rainfall. Thunderstorm activity is expected to increase on Sunday with strong storms capable of producing brief heavy rain. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 500 PM TO 1000 PM 0.1" (30%) to 0.5" (15%) to 1.5" (5%) LOW Arapahoe 500 PM TO 1000 PM 0.1" (30%) to 0.5" (15%) to 1.5" (5%) LOW Denver 500 PM TO 1000 PM 0.1" (30%) to 0.5" (15%) to 1.5" (5%) LOW Boulder 400 PM TO 1000 PM 0.1" (40%) to 0.5" (20%) to 1.5" (10%) MOD Broomfield 400 PM TO 1000 PM 0.1" (30%) to 0.5" (15%) to 1.5" (5%) LOW Douglas 400 PM TO 1000 PM 0.1" (40%) to 0.5" (20%) to 1.5" (10%) MOD Jefferson 400 PM TO 1000 PM 0.1" (40%) to 0.5" (20%) to 1.5" (10%) MOD Foothills above 6500ft       Boulder 300 PM TO 1000 PM 0.1" (65%) to 0.5" (40%) to 1.5" (15%) MOD Douglas 300 PM TO 1000 PM 0.1" (65%) to 0.5" (40%) to 1.5" (15%) MOD Jefferson 300 PM TO 1000 PM 0.1" (65%) to 0.5" (40%) to 1.5" (15%) MOD MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS. MHFD Flood Prediction Center: 303-458-0789    F2P2 Website [http://udfcd.org/Flash+Flood+Prediction+Program]