FTB 07-27-2017: Heavy Rainfall Continues Along the Eastern Mountains and Adjacent Plains

Issue Date: Thursday, July 27, 2017
Issue Time: 10:40AM MDT

—LOW/MODERATE flood threat for Front Range, Urban Corridor, Palmer Ridge, Central Mountains, Southeast Mountains, Raton Ridge, Southeast Plains

—LOW flood threat for San Juan Mountains, Northeast Plains

Afternoon Update (5PM): The low threat has been extended over the NE corner of the state along the NE, WY, KS border. Storms have been firing off the Cheyenne ridge this afternoon due to strong upslope flow and are moving S/SE over the state line. These storms will move into an area with PW values of 1.3 inches, high instability and 30-45 knots of shear. All of these ingredients combined are favorable for severe thunderstorm development. Threats include heavy local rain, large hail and small stream and street flooding. 1-hour rain rates will be as high as 1.9 inches and 3 hour totals up to 3.5 inches.

On the water vapor image below, the storms that persisted with the moist easterly flow overnight can be seen over the SE Plains. The High pressure has started to build back to the west and has elongated its center over NM, TX and OK. Today it will continue to build itself west and situate itself over the 4 corners region, which will cut off monsoon moisture into the region. The 500mb ridge is currently dipping into northern CO and has gained some strength since yesterday. This ridge will help produce westerly flow that will pull the drier air seen over Utah into western Colorado. Precipitable Water (PW) over the eastern portion of the state will also decrease today in the upper-levels with a more northwesterly/northerly flow. The gradient will decrease from north to south, but will still remain over 1 inch this afternoon and evening, which should be more than enough moisture for another round of heavy rainfall. Diurnal heating patterns will allow thunderstorms to fire over the higher terrain midday. Over the western slope, the limited moisture and westerly upper level flow should cause thunderstorms to be more isolated in nature and confine them to the higher terrains favoring the San Juan Mountains.

Along the Front Range and Southeast Mountains, storms will begin to organize just after noon. As they move off the mountains in the late afternoon and evening, expect a couple rounds of storms similar to yesterday. The steering winds aloft remain weak and storm motion will be S/SE at 15-20 mph. Cloud cover may limit instability over the far eastern plains, but plenty of instability from daytime heating will occur over the adjacent plains and Palmer Divide. So there is a potential for heavy, local rainfall once again today. Over the mountains, local 3 hour rainfall totals of 1.8 inches are possible. Once storms move off the mountains, stronger thunderstorms will favor the higher terrain of the Palmer Divide and could have 3 hour totals up to 3.5 inches. Outflow boundaries from storms will likely help provide low-level convergence for other storms, and an isolated storm may move into the northern portion of the state off the Cheyenne Ridge. Threats include mud flows, debris slides, gusty winds and urban and small stream flooding. Burn scars over the Southeast Mountains should be monitored closely for flash flooding, debris slides and mud flows. Fountain Creek in Colorado Springs ran into Action level yesterday evening, but should make a full recovery before the next round of storms this afternoon and evening.

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:

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

 Sunny this morning and drier air being entrained at the upper-levels from Utah. Limited moisture and westerly flow aloft should confine storms to the higher terrains this afternoon. Max 1-hour rain rates up to 1 inch are possible with local 24-hour totals up to 2.1 inches in the San Juan Mountains. Debris slide, mud flows and road flooding are the main threats over these saturated soils. A Low threat has been issued. Storm activity should start to subside after sundown.

Primetime: 12PM to 11PM

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

 Partly cloudy over the eastern plains this morning becoming more sunny later this morning. Front Range, Urban Corridor and Southeast Mountains should heat up rather quickly today building instability for multiple rounds of storms again today. Max 1-hour rain rates up to 1.3 inches over the higher terrain and 2.3 inches over the adjacent Plains. Burn scars will need to be monitored closely for mud flows, debris slides and flash flooding. There is a Moderate/Low flood threat issued for today.

Primetime: 12PM to 12AM