STP 07-27-2017: Efficient Thunderstorms Drop Impressive Rainfall

Issue Date: Thursday, July 27, 2017
Issue Time: 10:50AM MDT


Yesterday was one of those July Colorado days where you could just feel the moisture when you walked out the door. Dew points were near 60F by noon across the Urban Corridor and Eastern Plains with the mid-levels saturated as well. The High pressure over Oklahoma allowed moisture to pour into the eastern plains. There was some drying throughout the day over the western portion of the state, which meant more isolated thunderstorms when compared to the previous couple days. Storms begin to fire off the high terrain a little later than normal with the upslope flow. Storm motion was more easterly, so the thunderstorms over the western portion of the state were confined to the mountains. In this area, local storm centers were estimated at 1.5 inches of rain. A CoCoRaHS station happened to be positioned over SW Gunnison County that captured that storm center at 3.11 inches. This gage should be read with a grain of salt as we were not able to confirm this 24-hour amount this morning.

The storms that formed over the higher terrain of the Front Range and Southeast Mountains moved off the mountains in the late afternoon and evening. The rainfall amounts from these widespread storms were nothing less than impressive. With the high moisture, multiple rounds of storms were able to fire. The extensive precipitation gage network throughout the Urban Corridor gave us an idea pretty early in the afternoon about how efficient these storms were going to be. Over Douglas County near Russellville Gulch, 2.68 inches fell in 1-hour with 1.14” falling in 10 minute. Another storm was recorded dropping 2.48 inches in 45 minutes. While these were some of the highest rainfall amounts, other storms had well over 1 inch in 30-min. The list of the most impressive storm reports are below. CDOT reported rain washing mud and rocks onto most roads west of Denver, Golden and Boulder including US 285, CO 74, US 40, US 6,  CO 72 and CO 119. In Cripple Creek, CR 11 at US 67 was flooded. Ponding in yards and fields was also reported over the Franktown area. Radar estimates the 3-6 hour totals were just over 3.5 inches with CoCoRaHS stations recording 3.18 inches. A USGS gage recorded 3.05 inches at Fort Carson and 3.78 inches at Franktown. Fountain Creek was running high last night which can be seen at the gage in Colorado Springs reaching action level last evening.

Over the Eastern Plains, the high moisture was paired with larger instability and weak shear. This allowed storms in the SE corner to persist into the night. The 1-hour max precipitation rates were as high as 2 inches. The 3-6 hour totals were up to 4 inches with 24 hour total just over 4 inches in Otero County. Unfortunately, this area isn’t as well gaged, but a CoCoRaHS station recorded 3.5 inches of rain over Otero County for the 24-hour period.

Impressive reported rain totals:

10-min: 1.14” at Russellville Gulch near Franktown

10-min: 0.67 at UDFCD gage in Littleton

15-min: 0.96” at Cherry Hills ASOS

15-min: 0.79” at the Rosedale gage just west of Evergreen

15-min: 0.59” at 4mi Creek south of Deckers

15-min: 0.5” in Park County

18-min: 1.25” at Berthoud

20-min: 0.75” in Falcon near Colorado Springs

30-min: 1.5” at Marston Lake in Littleton

30-min: 0.98” in For Collins; 3 other gages reported over 0.9” in 30-min

30-min: 0.94” Choke Cherry Reservoir near Genesee

30-min: 0.91” at the Rosedale gage just west of Evergreen

30-min: 0.88” at Cripple Creek in Teller County

30-min: 0.5” at Porphyry gage near Jamestown

1-hr: 2.68” at Russellville Gulch near Franktown

1-hr: 1.13” at Fort Carson

To see how much rain fell over your area the last 24-hours, check out our new Precipitation Map below.