Issue Date: 8/25/2018
Issue Time: 9:05 AM
A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, SOUTHEAST MOUNTAINS, RATON RIDGE, AND SOUTHEAST PLAINS.
The approach of an upper-level trough from the west (and associate disturbances denoted by red dashed lines) has forced the upper-level ridge eastward, placing Colorado squarely underneath southwest flow aloft for today/tonight. This has allowed precipitable water to tick upwards to near, or slightly above, 0.75 inches across the state. This moisture, although confined mainly in the mid-levels, will provide the fuel for showers and thunderstorms today and tonight across the state, with the best coverage south of I-70 coinciding with the best low-level moisture, relatively speaking.
With all of that said, moisture below cloud bases still lacks, so the threat of heavy rainfall is fairly marginal today. Plus, storm motions will be moving at a decent rate to the east-northeast, so it will be difficult for storms to produce rainfall above flash flood thresholds. Despite these factors, a low flood threat has been issued, mainly for the sensitive burn scars in southern Colorado. Multiple rounds of marginally heavy rainfall may be enough to set off these areas, so the low threat is warranted. For more details on timing and rain rates, please see the zone-specific forecast 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.
Zone-Specific Forecasts
Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge:
Isolated-to-scattered showers and thunderstorms expected, with the best coverage south of I-70. Dry air below cloud bases will keep rain rates fairly low, especially along/near the higher terrain. Further east, near the eastern CO border, a bit more low-level moisture will be present, allowing a couple thunderstorms to become strong-to-severe, producing brief periods of heavy rain, strong winds, and plenty of lightning. Maximum rain rates are as follows:
Urban Corridor and Palmer Ridge: 0.1-0.3 inches/hour
Raton Ridge: 0.2-0.35 inches/hour
Southeast Plans and Northeast Plains: 0.6-1.1 inches/hour
Timing: 1 PM – 11 PM, with a couple isolated showers/storms lingering into the early morning hours over the Southeast Plains and Raton Ridge
Northern Mountains, Northwest Slope, Front Range, Central Mountains, Grand Valley, Southeast Mountains, San Juan Mountains, Southwest Slope, and San Luis Valley:
Scattered showers and thunderstorms are expected today/tonight, with the first storms of the day popping over the San Juan Mountains and Southwest Slope by 11 AM. Storm motions and dry air below cloud bases will keep most rain rates light, especially for areas north of I-70. The best chance at heavy rainfall will be over the San Juan Mountains, Southwest Slope, and Southeast Mountains, where low-level moisture is best, relatively speaking. The low flood threat is warranted due to the possibility of multiple rounds of thunderstorms in these areas. Maximum rain rates are as follows:
Northwest Slope and Grand Valley: 0.05-0.2 inches/hour
Northern Mountains, Front Range, and Central Mountains: 0.1-0.35 inches/hour
Southwest Slope, San Juan Mountains, and San Luis Valley: 0.4-0.6 inches/hour
San Luis Valley and Southeast Mountains: 0.25-0.5 inches/hour
Timing: Noon – 10 PM for locations north of I-70; 11 AM – Midnight for locations south of I-70, with a couple isolated showers/weak thunderstorms lingering over the San Juan Mountains and Southeast Mountains into the early morning hours.
