Time: 922 AM Sun May 2, 2021 Forecaster: Brad Simmons

Heavy Precipitation Outlook
Flash Flood Prediction Program

STRONG TO SEVERE THUNDERSTORMS THIS AFTERNOON TRANSITIONING TO A SOAKING RAIN OVERNIGHT INTO MONDAY MORNING
-An approaching upper level low pressure system is already producing showers and thunderstorms over the mountains and its just a matter of time until the District will be impacted... Precipitation will be convective through the afternoon with thunderstorms having the potential to become strong to severe producing brief heavy rainfall, hail, gusty winds and frequent lightning.
-2-3 rounds of showers and thunderstorms is likely through the afternoon before precipitation becomes more stratiform and an "upslope" rain/snow event develops this evening and continues overnight into the first half of the day on Monday. The threat for heavy rainfall with the potential for excessive runoff will be limited to thunderstorm activity this afternoon and evening then rainfall rates will become lighter in the 0.05-0.20"/hr rate. Total rainfall amounts from noon today until noon Tuesday is expected to range from 0.5-1.5" with the higher end of the range likely in and adjacent to the foothills.
-As temperatures drop this evening the snow line will lower to 8,000-9,000ft between 10pm and midnight and between 7,000-8,000ft between midnight and 2am Tuesday. By daybreak Tuesday the snow line may be as low as 6,000' briefly before slowly rising as the morning progresses. Best chances for widespread rain/snow pending elevation will be through noon then become more isolated into the afternoon before drying out.
STORM RAINFALL POTENTIAL AND DURATION: Rain showers and weak thunderstorms will produce 0.1-0.3" in 10-30 minutes. Moderate to strong thunderstorms will have the potential to produce 0.3-0.9" in 10-30 minutes.

WORST CASE SCENARIO: Training of moderate/strong thunderstorm cells may result in up to 1.6" of rain in 60-90 minutes.

A LOOK AHEAD: A widespread light rain is expected Tuesday morning lower elevations with snow for the higher terrain continuing through about noon. In the afternoon shower activity will slowly diminish with dry conditions developing. Heavy rainfall is not expected.

Location Prime Time 30-Minute Rainfall
and % Probability
Message
Potential
Plains      
Adams 1100 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (20%) MOD
Arapahoe 1100 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (20%) MOD
Broomfield 1100 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (15%) MOD
Denver 1100 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (20%) MOD
Douglas 1100 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (20%) MOD
Jefferson 1030 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (15%) MOD
Boulder 1000 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (15%) MOD
Foothills above 6500ft      
Douglas 1100 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (15%) MOD
Jefferson 1030 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (15%) MOD
Boulder 1000 AM TO 800 PM
0.1" (100%) to 0.3" (75%) to 0.9" (15%) MOD
MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS.