Flash Flood Prediction Program

Time: 902 AM Sun September 12, 2021 Forecaster: Brad Simmons ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- SEASONAL TEMPERATURES WITH ISOLATED TO WIDELY SCATTERED AFTERNOON SHOWERS/THUNDERSTORMS - After record heat yesterday, temperatures will be more seasonal today in the low to mid 80's for highs over the plains with 70's in the Front Range foothills. Normal high for Denver today is 82 degrees. There is a very slight chance for a rain shower this morning, favoring northern areas of the District but most of the activity will hold off until this afternoon as a weak upper level disturbance passes through. - Showers and isolated to widely scattered thunderstorms will become possible over the foothills before noon and over the plains between noon-1pm. Best chances for shower and weak to briefly moderate thunderstorm activity will be between 1-7pm. After 7 or 8pm generally dry conditions are expected for the remainder of the evening and overnight period. - Showers and thunderstorms that develop today will be fast moving from SW to NE between 20-30mph. There is more moisture to work with than yesterday resulting in some storms potentially producing a brief period of moderate rainfall. Any heavy rainfall today is expected to remain well east of the District where moisture is deeper. STORM RAINFALL POTENTIAL AND DURATION: Rain showers will produce a trace to 0.1" of rain. Weak to moderate thunderstorms will have the potential to produce rain rates of 0.1-0.4" in 10-30 minutes. WORST CASE SCENARIO: A strong, slower moving thunderstorm is able to develop with the potential to produce brief heavy rainfall of 0.4-1.0" in 15-45 minutes. A LOOK AHEAD: A weak cool front is expected to move through tomorrow morning and shave 1-3 degrees off of highs from today's readings. There will continue to be a modest chance for isolated to widely scattered showers and thunderstorms but the timing will be pushed back with best chances between 3pm and midnight. A more robust upper level disturbance will move through on Tuesday and keep temperatures in the 70's for highs and result in a good chance for a much needed wetting rainfall over the District. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 100 PM TO 700 PM Trace (80%) to 0.2" (40%) to 0.4" (20%) NONE Arapahoe 100 PM TO 700 PM Trace (80%) to 0.2" (40%) to 0.4" (20%) NONE Broomfield 100 PM TO 700 PM Trace (80%) to 0.2" (40%) to 0.4" (20%) NONE Denver 100 PM TO 700 PM Trace (80%) to 0.2" (40%) to 0.4" (20%) NONE Boulder 1200 PM TO 700 PM Trace (85%) to 0.2" (50%) to 0.4" (20%) NONE Douglas 1200 PM TO 700 PM Trace (85%) to 0.2" (50%) to 0.4" (20%) NONE Jefferson 1200 PM TO 700 PM Trace (85%) to 0.2" (50%) to 0.4" (20%) NONE Foothills above 6500ft       Boulder 1100 AM TO 700 PM Trace (90%) to 0.2" (50%) to 0.4" (25%) NONE Douglas 1100 AM TO 700 PM Trace (90%) to 0.2" (50%) to 0.4" (25%) NONE Jefferson 1100 AM TO 700 PM Trace (90%) to 0.2" (50%) to 0.4" (25%) NONE MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS. MHFD Flood Prediction Center: 303-458-0789    F2P2 Website [http://udfcd.org/Flash+Flood+Prediction+Program]