SPM 07-29-2020: Heavy Rain East of Continental Divide, More Flooding from Runoff Near Spring Creek Burn Area as Monsoon Moisture Shifts East

Issue Date: Wednesday, July 29th, 2020
Issue Time: 11:00AM MDT

Summary:

Yesterday was the final day of a prolonged period of monsoon moisture generating large precipitation totals across the state (6 day event). The rainfall on Tuesday was focused on the northern and eastern parts of Colorado. The northern mountains mainly saw isolated thunderstorms bring localized heavy downpours and gusty winds. The strongest of these storms occurred over Garfield County, which dropped over 0.5 inches of rain according to the QPE map. The highest rain gauge near the area picked up 0.64 inch (CoCoRaHS). This storm went on to generate a 48 mph wind gust at the Aspen airport.

A larger cluster of thunderstorms developed just east of the Continental Divide and rolled through the Denver metro before continuing eastward over the plains of eastern Colorado. A few small storm cells prompted Flood Advisories from NWS Boulder over Boulder and Weld counties, where over 0.5 inches of rain were estimated. A CoCoRaHS observer in Lyons, CO reported 0.69 inches of rain over 30 minutes in northern Boulder County within one of the Flood Advisory areas. An ALERT gauge just south of Lyons reported 0.71 inches of rain. Western portions of Fort Collins received up to 0.67 inches of rain according to rain gauges in the area, and the heaviest rain fell over Elbert County (1.8 inches) as storms moved just east of the Urban Corridor. A few other gauges over the Palmer Ridge picked up over 1 inch as well. Flooding was not reported over these areas.

Later evening storms also fired over the Southeast Mountains before tracking southeast over the Raton Ridge and Southeast Plains. These storms brought another flash flood warning for the Spring Creek burn area, and a flash flood was reported on the southwest side of the burn area in northeast Costilla County. This is the same area that had issues on Monday. A rain gauge near the flash flood report only reported 0.38 inches of rain, and with relatively quick motion, guessing this was over a short period of time. This also indicates that flash flooding is occurring from the lower rain totals due to saturated soils from the heavy rain over the last several days. Again, this lines up with the flash flood report from Monday.

The last 6 days have been quite the rainfall eventful for Colorado. The image below shows the AHPS observed precipitation estimate over the last 7 days, which covers this rainy period. Below is a summary of some observations and rain totals from this 6-day event:

• San Juan Mountains: 6-day rain totals = 3 – 4 inches (southwest & southeast), 2-3 inches (east & north)
• Southwest Slope: 6-day rain totals = 0.25 inches (southwest) to 3 inches (southeast)
• San Luis Valley: 6-day rain totals = 0.5-1.5 (interior), 2-3 inches (western & eastern edges)
• Southeast Mountains: 6-day rain totals = 2-4 inches (highest totals along crests of Sangre De Cristo Mountains)
• Numerous debris slides across San Juan Mountains, resulting in hikers being stranded and rescued near Silverton & Telluride
• Local rivers and streams have swelled over 90th percentile flows over Southwest Slope and San Juan Mountains

For rainfall estimates in your area over the last 24 to 72 hours, check out our State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The 24-hour, 48-hour and 72-hour total precipitation contain bias corrections that are not disaggregated into the hourly estimates, so there will likely be some differences. The accumulation ending time is 7AM of the date shown in the bottom right corner. Also shown optionally are vulnerable fire burn areas (post 2012), which are updated throughout the season to include new, vulnerable burn areas. The home button in the top left corner resets the map to the original zoom.