Flash Flood Prediction Program

Much of the District missed out on the thunderstorm activity yesterday despite elevated moisture levels but that will not be the case today as widespread thunderstorms are expected as upper level steering winds become more southwesterly pushing foothill storms eastward. Moisture is also on the increase at the surface and aloft with dew points currently in the 60’s resulting in thunderstorms that develop being very capable of producing heavy rainfall that may lead to excessive runoff and possibly flash flooding. The high risk of heavy rainfall has resulted in the NWS issuing a Flood/Flash Flood Watch for the District which is valid from 4pm until midnight tonight and Message 2’s will be issued later this afternoon, possibly before 4pm.

Prime time for thunderstorms will be from 3 or 4pm until midnight but could linger beyond midnight into the early morning hours on Tuesday. Storms will generally move from WSW to ENE at around 15mph, but stronger storms may move more slowly or against the grain. Heavy rainfall will be the primary threats from thunderstorms today but large hail is also possible.

Temperatures will be near normal for this time of year with afternoon highs over the plains in the upper 80s to lower 90s. Normal high for Denver today is 90 degrees.

STORM RAINFALL POTENTIAL AND DURATION: Weak to moderate thunderstorms will produce rainfall rates of 0.2-0.8” in 10-30 minutes. Strong thunderstorms will have the potential to produce rainfall rates of 0.8-2.2” of rain in 10-30 minutes.

WORST CASE SCENARIO: A slow moving strong thunderstorm or a large thunderstorm complex may result in up to 4.0” of rain in 60-90 minutes.

A LOOK AHEAD: Thunderstorms will be widespread to numerous on Tuesday and Wednesday with highs only in the 80s. Thunderstorms will again be very capable of producing heavy rainfall that may lead to excessive runoff and possibly flash flooding and additional NWS Flood Watches/Message 2’s are likely. A decrease in thunderstorms is expected Thursday and Friday but there will still be a few storms with heavy rainfall a good bet.

Location	Prime Time	30-Minute Rainfall and % Probability	Message Potential
Plains
Adams	400 PM TO 1159 PM	0.5" (80%) to 1" (60%) to 2" (30%)	HIGH
Arapahoe	400 PM TO 1159 PM	0.5" (80%) to 1" (60%) to 2" (30%)	HIGH
Broomfield	400 PM TO 1159 PM	0.5" (75%) to 1" (50%) to 2" (25%)	HIGH
Denver	400 PM TO 1159 PM	0.5" (80%) to 1" (60%) to 2" (30%)	HIGH
Boulder	300 PM TO 1159 PM	0.5" (75%) to 1" (50%) to 2" (25%)	HIGH
Douglas	300 PM TO 1159 PM	0.5" (80%) to 1" (60%) to 2" (30%)	HIGH
Jefferson	300 PM TO 1159 PM	0.5" (75%) to 1" (50%) to 2" (25%)	HIGH
Foothills above 6500ft
Boulder	200 PM TO 1159 PM	0.5" (80%) to 1" (50%) to 2" (20%)	HIGH
Douglas	200 PM TO 1159 PM	0.5" (80%) to 1" (50%) to 2" (20%)	HIGH
Jefferson	200 PM TO 1159 PM	0.5" (80%) to 1" (50%) to 2" (20%)	HIGH

MONITOR NATIONAL WEATHER SERVICE FOR SEVERE WEATHER STATEMENTS.