FTB 07-15-2018: Plume of Deep Moisture Has Overspread Much of Colorado

Issue Date: 7/15/2018
Issue Time: 8:30 AM

A HIGH FLOOD THREAT HAS BEEN ISSUED FOR THE SPRING, WESTON PASS, AND CHATEAU FIRE BURN AREAS.
A MODERATE FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE NORTHERN MOUNTAINS, FRONT RANGE, SOUTHEAST MOUNTAINS, CENTRAL MOUNTAINS, URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, SOUTHEAST PLAINS, AND RATON RIDGE.
A MODERATE FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF SOUTHWEST SLOPE AND SAN JUAN MOUNTAINS, NAMELY THE 416 FIRE, BURRO FIRE, AND WEST FORK COMPLEX BURN AREAS.
A LOW FLOOD THREAT SURROUNDS THE AREAS MENTIONED ABOVE, AND INCLUDES PORTIONS OF THE NORTHWEST SLOPE AND GRAND VALLEY.

The stage is set for scattered-to-widespread thunderstorms to impact Colorado today, with plenty of moisture available for heavy rainfall. An upper-level disturbance (denoted by the red line in the water vapor image below) has shifted yesterday’s high pressure to the east, and centered a plume of deep moisture (green arrow) over Colorado. This plume of moisture will shift east a bit through the day, placing many of the state’s burn scars under the gun for heavy rain, as well as the Urban Corridor. At the surface, a cold front is advancing through eastern Colorado, with easterly flow behind the front expected to push dewpoints in the upper-50s and low-60s back into the foothills. Altogether, these moist conditions set the stage for showers/thunderstorms capable of heavy rainfall.

The bulk of shower/thunderstorm activity will occur over the area outlined by the low flood threat, with drier air limiting storm coverage over the far Northwest Slope and far Southeast Plains. Between those two drier regions, showers/thunderstorms will begin to ramp up between 11 AM and Noon, with coverage increasing through the afternoon and into the evening hours. Much of the activity will diminish after sunset, with a few showers/thunderstorms lingering into the early morning hours across the Urban Corridor, Palmer Ridge, Raton Ridge, and eastern plains.

Burn scars and urban locations are the main areas to watch today for potential flash flooding issues, due to impervious soil/man-made surfaces exacerbating runoff. Recent/ongoing fires, such as the Spring, Weston Pass, Chateau, Lake Christine, Abode, 416, and Burro will need to be watched closely for mud flows, debris slides, and flash flooding issues. The Urban Corridor will likely see flooding issues underneath strong storms, especially in poorly drained locations, such as low-lying intersections, neighborhood streets, and low points in highways bordered by concrete barriers.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge:

Scattered showers/thunderstorms are expected today/tonight, with plenty of moisture available for efficient precipitation processes. Storms will get their start around lunchtime, increasing in coverage through the afternoon/early evening, decreasing in coverage after sunset as temperatures cool. A few thunderstorms are expected to continue into the nighttime/early morning hours. Maximum rain rates are as follows:

Urban Corridor and Northeast Plains: 1.0-1.5 inches/30 minutes
Palmer Ridge and Southeast Plains: 0.8-1.3 inches/30 minutes
Raton Ridge: 0.5-1.0 inches/hour

Timing: Noon – 11 PM, with a few showers/thunderstorms continuing into the overnight/early morning hours.

Northern Mountains, Northwest Slope, Front Range, Central Mountains, Grand Valley, San Juan Mountains, Southwest Slope, Southeast Mountains, and San Luis Valley:

Scattered showers/thunderstorms are expected today/tonight, capable of heavy rainfall. Burn scars deserve careful attention, especially the Spring Fire, Chateau Fire, and Weston Pass fire; high threats have been issued for those scars. Flash flooding in steep terrain, burn scar flash flooding, mud flows, and debris slides are the potential impacts. Maximum rain rates are as follows:

Front Range, Southeast Mountains, and Central Mountains: 1.2-1.6 inches/hour
Northern Mountains, San Juan Mountains, and Southwest Slope: 0.8-1.2 inches/hour
Grand Valley: 0.6-1.0 inches/hour
San Luis Valley: 0.5-0.9 inches/hour
Northwest Slope: 0.2-0.5 inches/hour

Timing: 11 AM – Midnight for the Front Range and Southeast Mountains, 11 AM – 10 PM for all other regions. A few showers/thunderstorms will linger into the early morning hours over the Front Range, Southeast Mountains, San Juan Mountains, and Southwest Slope.