FTB 06-28-2015: Uptick in Moisture Leads to an Uptick in Thunderstorm Coverage

Issue Date: 6/28/2015
Issue Time: 9:57 AM


The upper-level ridge centered over western Utah continues its dominance over the western US today. One main difference over yesterday is that a bit more subtropical moisture has rotated into Colorado under the ridge, pushing precipitable water values slightly above average. This will lead to a slight uptick in thunderstorm coverage today, but, due to mean storm motions from north to south, most will still remain confined to the High Country. An isolated thunderstorm or two will develop during the late afternoon/early evening over the Urban Corridor thanks to outflow boundaries from storms over the higher terrain.


A feature of note in today’s forecast is a weak shortwave that will move overhead during the early-to-mid afternoon hours. In addition to helping initiate storms over the higher terrain, this will also allow one or two isolated thunderstorms to develop over the Palmer Ridge and southern portions of the Urban Corridor, moving southward with time. With surface high pressure and upper-level high pressure in place, these storms will not last very long as they move away from the preferred terrain of the Palmer Ridge. Additionally, most of the precipitation that falls from these storms will likely not reach the ground due to high cloud bases. Maximum rain rates will be 0.2-0.5 inches/hour.

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 and Palmer Ridge:

Mostly sunny, with a couple of isolated thunderstorms possible this afternoon and evening. The main threat from storms will be gusty outflow winds, small hail, and lightning. With high cloud bases, though, much of the precipitation will evaporate before reaching the surface, resulting in maximum rain rates of 0.2-0.5 inches/hour. An outflow boundary moving off of the mountains and over the Urban Corridor will provide the best opportunity for a storm to produce locally heavy rain, though the chances are low. This is accounted for in the low flood threat area. If this situation occurs, maximum rain rates will push to 0.8-1.2 inches/hour.

Timing: 1 PM – 9 PM

Southeast Plains, Northeast Plains and Raton Ridge:

Mostly sunny and hot will be the main story today across these regions. Plains regions near the preferred terrain of the Palmer Ridge, and Raton Ridge near the Southeast Mountains, will hold a chance for an isolated thunderstorm to drift overhead, although chances are slim. Maximum rain rates will be 0.3-0.6 inches/hour.

Timing: 3 PM – 9 PM

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

Mostly sunny early, becoming partly sunny this afternoon as scattered thunderstorms begin to develop over the higher terrain. The main threats from thunderstorms will be gusty outflow winds, small hail, and lightning, but brief heavy rain cannot be ruled out. Most rain rates will be light (0.2-0.4 inches/hour), but a stronger storm or two could produce rain at 0.6-1.0 inches/hour. The San Juan Mountains will be the main region to watch today as the best moisture resides there. Maximum rain rates will be 1.0-1.4 inches/hour for the San Juan Mountains.

Timing: Noon – 8 PM, with a few showers/weak thunderstorms lingering until midnight over southern regions.