FTB 07-28-2017: Another Day, Another Round Of PM Heavy Rainfall

Issue Date: Friday, July 28th, 2017
Issue Time: 10:30AM MDT

MODERATE/LOW flood threat for Palmer Ridge, Urban Corridor, Northeast Plains, Southeast Plains, Southeast Mountains, Raton Ridge, San Juan Mountains and Southwest Slope

This morning’s water vapor image, below shows that ample amounts of low-level water vapor are found statewide. Precipitable water (PW) values range from about 0.7 inches over the North Platte basin to 1.1 inches in the southwest corner of the state to 1.3 inches along our far eastern border. PW is expected to remain steady or increase throughout the day as southwesterly flow aloft continues to usher in moist, monsoonal air. Some stronger jet stream level winds are noted along the CO/UT border and weak upper-level divergence is supporting a mid/high cloud deck seen in the southwest part of the state. This will slightly limit instability over the western slope, though very moist low-level air will partially offset this and another round of scattered to numerous storms are expected in the San Juans and surrounding regions.

East of the Continental Divide, a tricky forecast is in store with our high resolution ensemble guidance suggesting many different possibilities today. A weak and shallow wind shift axis is noted along the I-76 axis (also accompanied by localized areas of very dense fog), possibly from last night’s outflow boundaries. This should act as a demarcation between higher heavy rainfall chances to the east and lower chances to the west. Once again, lower precipitation chances are expected north of the Denver metro due to a relative minimum in moisture content and the strongest downsloping flow. Highest storm chances today will likely be in the southeast quadrant of Colorado once again. We expect two separate areas of action. First, diurnal upslope flow combined with plenty of moisture and adequate instability will trigger slow moving storms along the foothills from Denver south through the New Mexico border. The Palmer Ridge will once again likely serve as a hot spot. Second, another separate cluster of storms is expected to fire along the aforementioned wind shift. These storms will have the potential for very heavy rainfall due to their slow motion and possibility of back-building/training of individual cells. A Moderate threat is warranted for both of these areas with a broad Low flood threat surrounding the Moderate.

Once again, with adequate instability expected to persist well after sunset, so will the chance of heavy rainfall. Activity could persist through about 2AM.

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, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Partly cloudy early then scattered to numerous showers and thunderstorms developing by early afternoon. Highest coverage will be along Palmer Ridge and adjacent areas to the east. Max 1-hr rainfall up to 3.0 inches (east) and 1.9 inches (west) will support a Moderate flood threat with flash flooding and small stream flooding likely. Mud flows and debris slides will be a possibility in higher elevation locations. A Low threat has been posted for broad parts of eastern Colorado. The threat is expected to persist into the early overnight hours once again.

Primetime: 1PM to 2AM

Northwest Slope, Grand Valley, Southwest Slope, Northern Mountains, Central Mountains, San Juan Mountains, San Luis Valley:

Mostly cloudy with scattered showers turning to thunderstorms and increasing in intensity. High rainfall coverage will be in southwest areas, especially the western portion of the San Juans. A broad Low flood threat has been posted for max 1-hr rainfall up to 1.3 inches capable of producing isolated mud flows and debris slides. A Moderate flood threat has been posted for parts of the San Juans due to an expected longer duration of heavy rainfall there.

Primetime: 12:30PM to 10PM