FTB 07-19-2021: Heavy Rainfall Likely For Parts Of San Juan Mountains; Drier Elsewhere

Issue Date: Monday, July 19th, 2021
Issue Time: 9:50AM MDT

— A MODERATE flood threat has been posted for parts of the San Juan Mountains
— A LOW flood threat has been posted for parts of the Southwest Slope and Grand Valley

A plume of monsoonal moisture continues to stream unusually far north, as shown in the visible satellite image below. Meanwhile, substantial drying is occurring on the eastern side of the upper-level ridge guiding the monsoonal surge. This has created a strong gradient in moisture this morning. Grand Junction PW came in at 1.05 inches, while Denver’s is only 0.65 inches. Model initializations this morning suggest the southwest corner of Colorado is seeing PW in the 1.0-1.2 inch range, which is much above normal. Additionally, dewpoint temperatures are 3-5F higher this morning. For example, Cortez and Durango have dewpoints of 55F or higher, which is noteworthy given their elevation of ~6,000 feet or higher. While some drying will erode moisture a bit over the eastern San Juan Mountains, we expect plenty of moisture to hang around the far southwest part of the state to support at least one if not several rounds of heavy rainfall this afternoon and evening. Yesterday’s rainfall totals were not overly impressive, but a look at individual storm cores showed intensity up to 0.4 inches in 10 minutes. Instability today will be higher, up to 1,400 J/kg. This, coupled with veering southerly steering flow over time should limit cloud cover to the south of the region (due to lower terrain in northern New Mexico), and support a longer period of instability. Thus, a MODERATE flood threat is warranted for a climatologically preferred but narrow region on the west side of the San Juans where up to 2.0 inches of rainfall is possible through tomorrow morning. Mud flows and debris slides are possible given the steep terrain here. Further north, a LOW flood threat extends through the Uncompahgre Plateau, but drier air north of there is expected to limit rainfall efficiency.

Elsewhere, very limited rainfall is expected though isolated to widely scattered storms, possibly strong, will again be possible along the KS border. However, strong steering flow of 25mph or greater should limit point rainfall so flooding is not expected here.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

San Juan Mountains, Southwest Slope, Grand Valley and Central Mountains:

Scattered to numerous showers and thunderstorms especially over the higher terrain of the San Juan Mountains where max 30-min rainfall up to 0.9 inches and max 1-hour rainfall up to 1.4 inches is expected. Total rainfall up to 2.0 inches is possible through tomorrow morning. A MODERATE flood threat has been posted for parts of the region for isolated flash flooding, debris slides and mud flows with the stronger storms. Reminder that mud flows and debris slides can occur well after the rainfall ends. A LOW flood threat extends slightly to the north where max 1-hour rainfall up to 1.1 inches is possible. Elsewhere, widely scattered storms are expected with max 1-hour rainfall up to 0.7 inches.

Primetime: 1PM through 11PM

Northeast Plains and Southeast Plains:

Isolated to widely scattered showers and thunderstorms possible this afternoon and evening. Max 1-hour rainfall up 1.5 inches, which is just below flood threat thresholds. However, nuisance street/field ponding is possible under the strongest cells. Gusty winds and hail will be possible with the strongest storms especially in eastern areas along the KS and NE borders.

Primetime: 2PM through 9PM

Northwest Slope, Grand Valley, Northern Mountains, Front Range, Palmer Ridge and Urban Corridor:

Hot with an isolated shower or storm possible especially over the higher terrain and foothills. Max 1-hour rainfall up to 0.6 inches possible. Flooding is NOT expected today.

Primetime: 1PM through 9PM