Flash Flood Prediction Program

Time: 1026 AM Sat July 20, 2024 Forecaster: Justin Brooks ------------------------- Heavy Precipitation Outlook Flash Flood Prediction Program ------------------------- Slightly cooler, multiple rounds of scattered showers and thunderstorms possible Flow aloft continues its northerly shift today as the southwestern US summertime ridge shifts slightly northwest, creating a setup for cooler Canadian air and Pacific moisture to make its way into the Rockies. The arrival of this cooler, relatively moist airmass (dewpoints around 50 and precipitable water (PWAT) values between 0.85-1") will spark convective initiation over the high country outside the district between Noon-1pm this afternoon, with an initial round of scattered storms possible for the District between 1-4pm. Storms today will move from N/NW to S/SE at 10-20mph, and be capable of producing up to 1.5" of rainfall in 60 minutes, hail, and gusty outflow winds. Forecast model guidance from the convection-allowing models suggest that another round of storms is possible this evening, with potentially higher rain impacts. At this time, a certain degree of uncertainty exists as to this second round's timing and impacts, as outflow boundary driven initiation appears likely in and around the District later today. STORM RAINFALL POTENTIAL AND DURATION: Typical showers thunderstorms today will produce 0.25"-0.50” in 10-30 minutes. Stronger/slower moving storms will produce up to 1.00" in 10-30 minutes. WORST CASE SCENARIO: A strong/stationary thunderstorm, or training of thunderstorms, may be capable of producing up to 2.00” or more in 30 minutes or less. A LOOK AHEAD: Low level moisture continues to increase tomorrow under the current upper-level setup, supporting another day with moderate/high potential for message issuance. Showers and thunderstorms are possible before noon, persisting through the afternoon and likely clearing around sunset. More scattered storms are possible Monday, though moisture decreases leading to lower heavy rainfall chances. LOCATION PRIME TIME 30-Minute Rainfall and % Probability Message Potential Plains       Adams 200 PM TO 1000 PM 0.25" (75%) to 0.5" (50%) to 1" (25%) MOD Arapahoe 200 PM TO 1000 PM 0.25" (75%) to 0.5" (50%) to 1" (25%) MOD Boulder 200 PM TO 1000 PM 0.25" (75%) to 0.5" (50%) to 1" (25%) MOD Broomfield 200 PM TO 1000 PM 0.25" (75%) to 0.5" (50%) to 1" (25%) MOD Denver 200 PM TO 1000 PM 0.25" (75%) to 0.5" (50%) to 1" (25%) MOD Douglas 200 PM TO 1000 PM 0.25" (75%) to 0.5" (50%) to 1" (25%) MOD Jefferson 200 PM TO 1000 PM 0.25" (75%) to 0.5" (50%) to 1" (25%) MOD Foothills above 6500ft       Boulder 100 PM TO 1000 PM 0.25" (80%) to 0.5" (60%) to 1" (40%) HIGH Douglas 100 PM TO 1000 PM 0.25" (80%) to 0.5" (60%) to 1" (40%) HIGH Jefferson 100 PM TO 1000 PM 0.25" (80%) to 0.5" (60%) to 1" (40%) HIGH MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS. MHFD Flood Prediction Center: 303-458-0789    F2P2 Website [http://udfcd.org/Flash+Flood+Prediction+Program]