FTB 09-22-2017: Strong Disturbance Enters Colorado

Issue Date: Friday, September 22, 2017
Issue Time: 8:45AM MDT

LOW flood threat for Hayden Pass burn scar
LOW flood threat for Southeast Plains and Raton Ridge (along with chance of severe weather)
— Timing of flood threat is later than normal, and persists into the overnight hours

It has been the center of attention for several days now, and is finally here. As shown in the water vapor image, below, a strong upper-level trough has begun to enter Colorado from the west. It is being supported by a large-scale jet stream with several embedded jet streaks exceeding 120 mph. Its sensible impact is already being felt in the form of mid-level clouds and scattered showers and even thunderstorms west of Continental Divide. One key variable is lacking right now, however. Precipitable water (PW) is only 0.5 inches at Grand Junction and less than that at Denver. However, much moister air resides just across the border into OK and KS with PW exceeding 1 inch there. At the surface, a cool front has entered northeast Colorado and is positioned right along the South Platte River valley as of 8AM MDT. With plenty of upper air support, expect this low pressure to intensify this afternoon, drawing in some of this moister air into its warm sector over the Southeast Plains.

In all, we expect isolated to scattered showers and weak thunderstorms to increase in coverage and intensity. While coverage will be almost the entire state, the only areas with potential for heavy rainfall will be in the Southeast Plains. Expect a round of thunderstorm associated with peak heating this afternoon, followed by another round of showers and thunderstorms this evening and into the overnight hours as numerous upper-level impulse combined with a strong moisture advection. There are two areas of concern today. First, in the far southeast part of the state, thunderstorms will be capable of producing heavy rainfall and isolated flash flooding. Severe weather will also be possible. Second, precipitation coverage is expected to increase during the overnight hours across central Colorado. Adequate moisture warrants a Low flood threat for the Hayden Pass fire scar for both short-term (< 1 hour) and medium term (3 hour) rainfall rates.

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:

Southeast Mountains, Raton Ridge, Southeast Plains:

Partly cloudy early, then increasing clouds with scattered showers and thunderstorms developing this afternoon and continuing into the evening/overnight hours. Max 1-hr rainfall up to 1.6 inches (far southeast) and 1.2 inches (Raton Ridge) with up to 0.7 inches elsewhere. A Low flood threat has been posted. In addition, severe weather will be possible with hail up to 2.25 inches during the afternoon storms and up to 1.5 inches with the overnight activity. Gusty winds up to 75 mph are also possible. A separate Low flood threat has been posted for the Hayden Pass fire scar where max 1-hour rainfall up to 0.5 inches and max 3-hour rainfall up to 0.8 inches is possible, enough to trigger mud flows and debris slides over the sensitive soil.

Primetime: 3PM to 6AM

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains:

Partly cloudy early, then increasing clouds with scattered showers and weak thunderstorms developing this afternoon and continuing into the evening/overnight hours. Max 1-hour rainfall up to 0.7 inches. Hail up to 0.75 inches and gusty winds up to 60mph could accompany the strongest cells on the Palmer Ridge and Northeast Plains. Snow showers will be possible at the highest elevations. Flooding is not expected today.

Primetime: 3PM to 6AM

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

Mostly cloudy with scattered showers and weak thunderstorms increasing in coverage and continuing into the late evening and overnight hours. Max 1-hour rainfall up to 0.5 inches with max 24-hour totals up to 1.6 inches in favored southwest facing slopes. Snow will be possible at the highest elevations. Flooding is not expected today.

Primetime: 11AM to 11AM