Time: 945 AM Sun May 9, 2021 Forecaster: Brad Simmons

Heavy Precipitation Outlook
Flash Flood Prediction Program

SLOW MOVING LOW PRESSURE SYSTEM TO IMPACT THE DISTRICT THROUGH TUESDAY PRODUCING COPIOUS AMOUNTS OF RAIN AND SNOW
-Slow moving upper level low pressure system will be impacting the District today through Tuesday with rain/thunderstorms and eventually some snow! Cloudy and cool today with a chance for a few light rain showers this morning giving way to a wetting rain and possibly isolated thunderstorm activity in the afternoon.
-Thunderstorms will depend on daytime heating and if temperatures remain too cool then precipitation will favor rain versus thunder and precipitation rates will remain on the light side. Chances for rain showers increase through noon with scattered to numerous rain showers and isolated weak thunderstorms possible through the afternoon. Best chances for wetting rain today and possibly some thunder looks to be from roughly noon to 8pm.
-After 8pm there is likely going to be a lull in the precipitation into the evening before a light soaking rain sets up during the overnight and continues into the day on Monday. The snow line may lower to around 6k briefly Monday morning before changing back to rain which may result in some wet snow in/near the foothills with minor snow accumulations above 7k.
STORM RAINFALL POTENTIAL AND DURATION: Light to moderate rain showers will produce 0.04-0.20" in 60 minutes. Heavier non-thunderstorm rain showers and weak thunderstorms will have the potential to produce 0.20-0.40" 10-30 minutes.

WORST CASE SCENARIO: A strong thunderstorm is able to develop with the potential to produce 0.30-0.80" in 10-30 minutes.

A LOOK AHEAD: Additional rain showers are likely all day Monday with temperatures expected to become cold enough for snow by 6pm down to 6-7k and between 10pm to midnight Monday down to 5k. Monday night into Tuesday morning a “wet” snow is expected with minor snow accumulations likely into Tuesday morning below 6k and significant snow likely for the foothills. Snow early Tuesday changes back to mix/rain showers by noon with showers continuing possible through the day. Tuesday evening and overnight shower activity is expected to decrease with dry conditions by daybreak Wednesday. If precipitation lingers long enough there could be some additional snow into Wednesday morning? Between 1-2” of moisture is expected over the District from today through Tuesday as rain or melted snow equivalent. Snow could result in a "branch breaker" type event if accumulations are significant along the I-25 corridor as many trees have begun to leaf out. Creeks and streams will also be running high due to non-thunderstorm rain and there could be some runoff issues as the ground is near saturation with all of the moisture as of late.

Location Prime Time 60-Minute Rainfall
and % Probability
Message
Potential
Plains      
Adams Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Arapahoe Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Boulder Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Broomfield Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Denver Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Douglas Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Jefferson Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Foothills above 6500ft      
Boulder Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Douglas Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
Jefferson Current TO 1000 AM SUN
Trace (100%) to 0.2" (75%) to 0.4" (40%) NONE
MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS.