FTB 07-12-2018: Moisture Increase Brings Threat of Heavy Rainfall

Issue Date: 7/12/2018
Issue Time: 8:37 AM

A HIGH FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHWEST SLOPE AND SAN JUAN MOUNTAINS TODAY, NAMELY THE 416 FIRE AND BURRO FIRE.
A HIGH FLOOD THREAT IS FORECAST FOR THE SPRING FIRE BURN AREA.
A MODERATE FLOOD THREAT IS FORECAST FOR OTHER AREA BURN SCARS, INCLUDING THE WEST FORK COMPLEX, CHATEAU FIRE, WESTON PASS, JUNKINS, HAYDEN PASS, LAKE CHRISTINE, AND WALDO CANYON.
A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF ALL 14 REGIONS.

Moisture has increased across the state, setting the stage for scattered showers and thunderstorms capable of heavy rainfall. As you can see in the surface plot below, dewpoints are generally in the 50s across lower elevations, with upper-30s/40s readings across higher elevations. These numbers are indicative of a moist atmosphere. Additionally, there is a surface low pressure centered just south of the Palmer Ridge. This center will shift slightly throughout the day in response to upper-level forcing, but the result will remain the same – it will pull moisture from the plains into eastern Colorado, with the deepest moisture near the CO/KS border.

Due to this increased moisture statewide, a few disturbances embedded in the mid-level flow, and plenty of daytime heating, scattered showers and thunderstorms are expected across all regions. Precipitable water values are in the 90th percentile for the date in this morning’s 12Z soundings from Denver and Grand Junction, highlighting just how primed the atmosphere is for thunderstorms capable of heavy rainfall. Recent/ongoing burn scars across the Southwest Slope, San Juan Mountains, Southeast Mountains, Central Mountains, and Southern Front Range are of particular concern today, with moderate-to-high flood threats being issued. Flash flooding, mud flows, and debris slides will all be threats for those areas. More information on rain rates and timing can be found in the zone-specific discussions below.

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 coverage peaking between 3 PM and 7 PM (in concert with peak daytime heat). The best moisture, relatively speaking, will be situated across two regions: near the CO/KS border where Great Plains moisture transport will be best, and south of Highway 50, where monsoonal moisture will be deepest. Westerly flow along the Urban Corridor will keep the lower-levels somewhat drier, reducing the risk of heavy rainfall in those areas somewhat. Every region will have an area of Low Flood Threat, due to the maximum rainfall rates possible:

Urban Corridor and Palmer Ridge: 0.8-1.2 inches/hour
Northeast Plains: 1.5-2.0 inches/hour
Southeast Plains and Raton Ridge: 1.2-1.6 inches/hour

Timing: 1 PM – Midnight, with a few lingering showers/thunderstorms across the eastern plains into the 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-to-widespread showers and thunderstorms are expected beginning late this morning, peaking during the late afternoon/early evening hours, and diminishing after sunset, with a few lingering until around Midnight over the higher terrain of the Southwest Slope and San Juan Mountains. Due to the depth of moisture available, precipitation will reach the surface more efficiently than previous days, increasing the risk of heavy rainfall. Thus, the issuance of multiple high and moderate flood threats for area burn scars, and a widespread low flood threat. Maximum rain rates are as follows:

Northwest Slope: 0.4-0.8 inches/hour
Northern Mountains, Grand Valley, and Central Mountains: 0.8-1.2 inches/hour
Front Range: 0.6-1.2 inches/hour
Southeast Mountains, Southwest Slope, and San Juan Mountains: 1.0-1.5 inches/hour
San Luis Valley: 0.5-1.0 inches/hour

Timing: 11 AM – 9 PM, with a few lingering showers/thunderstorms over the higher terrain of the San Juan Mountains and Southwest Slope until around Midnight.