Time: 905 AM Mon May 10, 2021 Forecaster: Brad Simmons

Heavy Precipitation Outlook
Flash Flood Prediction Program

SHOWERS ON THE INCREASE TODAY WITH LATE SEASON SNOW EXPECTED OVERNIGHT AS SLOW MOVING LOW PRESSURE SYSTEM IMPACTS THE DISTRICT
-Cloudy, cool and showery this morning with a more widespread, soaking rain over the majority of the District expected to pick up between 10am and noon. Highs today will only reach the 40's over the plains with 30's in the foothills resulting in snow falling above 7.5k and a mix of rain and snow between 6.5-7.5k and generally rain below 6.5k through the afternoon. Then the snow line will lower to 5k or below tonight.
-With the cool temperatures precipitation is expected to favor light to moderate non-thunderstorm rain. If a thunderstorm were to develop it would likely be early on today and with low freezing levels small hail or graupel/snow pellets would be the primary culprit. Rainfall rates this afternoon expected to be in the 0.1-0.3"/hr range, with 0.5"/hr peak intensities possible.
-As the snow line lowers later this afternoon and evening there is great uncertainty as to exactly when the "changeover" from rain to snow will occur over the I-25 corridor. Elevation and intensity of precipitation will play a major role in when rain changes over to snow and total snow accumulations. Mainly snow is expected above 8k today and tonight with a changeover from rain to snow between 2-5pm at around 7k and 5-9pm down to 6k and 9pm to midnight or later down to 5k but could be earlier/later.
STORM RAINFALL POTENTIAL AND DURATION: Rain showers will produce precipitation rates of 0.1-0.3" per hour. Moderate to heavy non-thunderstorm rain may result in up to 0.5" per hour rainfall rate.

WORST CASE SCENARIO: Thunderstorms are able to develop and may result in 0.2-0.6" of rain in 10-30 minutes.

A LOOK AHEAD: Snow showers or a mix of rain and snow to start the day on Tuesday will slowly transition to rain over lower elevations of the District as the snow level rises through midday. Rain/snow pending elevation continues likely through the afternoon. Tuesday evening and overnight shower activity is expected to decrease with dry conditions by daybreak Wednesday. If precipitation lingers long enough Wednesday night there could be some additional snow accumulations into Wednesday morning over the plains. Over the District 0.6-1.3 inches of moisture as rain or melted snow equivalent is expected from today through Tuesday with isolated higher amounts possible. Smaller creeks and streams will be running high over the next few days.

Location Prime Time 60-Minute Rainfall
and % Probability
Message
Potential
Plains      
Adams Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Arapahoe Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Boulder Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Broomfield Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Denver Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Douglas Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Jefferson Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Foothills above 6500ft      
Boulder Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Douglas Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
Jefferson Current TO 900 AM TUE
0.1" (100%) to 0.3" (75%) to 0.5" (25%) NONE
MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS.