FTB 06-30-2016: Widespread Showers/Thunderstorms Statewide

Issue Date: 6/30/2016
Issue Time: 10:15 AM

A MODERATE FLOOD THREAT IS ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, NORTHEAST PLAINS, PALMER RIDGE, AND SOUTHEAST PLAINS.

A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS, INCLUDING PORTIONS OF THE NORTHERN MOUNTAINS, NORTHWEST SLOPE, CENTRAL MOUNTAINS, GRAND VALLEY, SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, AND SOUTHEAST MOUNTAINS.

Instead of discussing the water vapor imagery, today is good opportunity to show one image from our model guidance that gets to the nitty-gritty of this forecast. The image shows the chance of precipitation across Colorado, and it’s fairly easy to see just how widespread the activity will be today. Monsoonal moisture, plus the presence of a cold frontal boundary/reinforcing outflow boundaries, will combine to leave a moist air mass in place across the state. Precipitable water (PWAT) values will be at (or above) 1 inch nearly everywhere along/east of the Front Range and Southeast Mountains, as well as far western Colorado. This signals just how moist the air mass is, and heightens the concern for flash flooding across the state.

POP_20160630

Scattered showers are ongoing over the high country and over the eastern plains. Activity will increase around 11 AM, first over the mountains, and then spreading eastward over the Urban Corridor and then across the plains. A couple storms across the plains will be strong/severe, with strong winds, hail (up to 1.5 inches), and heavy rain being the main threats. Scattered showers/thunderstorms will continue overnight and into tomorrow morning. For more details, including timing and maximum rain rates, please see the zone-specific 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.

Flood Threat Legend

Zone-Specific Forecasts

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

Widespread showers/thunderstorms are expected today and tonight, with isolated-to-scattered showers/thunderstorms continuing into the morning hours tomorrow. With precipitable water values at, or above, 1 inch, heavy rain is a heightened concern. Storm motions will help mitigate the flood threat somewhat, but that “help” is neutralized a bit with the past few days of rain saturating soils. Maximum rain rates are as follows:

Front Range: 1.0-1.5 inches/30-45 minutes
Urban Corridor: 1.2-2.0 inches/30-45 minutes
Palmer Ridge, Northeast Plains, and Southeast Plains: 1.8-2.5 inches/30-45 minutes

Timing: 11 AM – 4 AM

Southeast Mountains and Raton Ridge:

Scattered showers/thunderstorms expected to begin around lunchtime and continue through the evening, with a few isolated-to-scattered showers/storms lasting into the early morning hours. Although surface moisture will be a bit lower here, and it will be a bit removed from the best mid-level forcing, the environment will still support periods of locally heavy rainfall. Maximum rain rates will be 0.8-1.4 inches/hour, but most rain will fall at a rate of 0.4-0.8 inches/hour.

Timing: 11 AM – 3 AM

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

Widespread showers/thunderstorms are expected today and tonight, with scattered showers/thunderstorms continuing into tomorrow morning. Gusty winds, lightning, and locally heavy rainfall are the main threats. Maximum rain rates are as follows:

Northwest Slope and Grand Valley: 0.8-1.4 inches/hour
Northern Mountains and Central Mountains: 0.6-1.2 inches/hour
San Luis Valley: 0.6-1.4 inches/hour
Southwest Slope and San Juan Mountains: 1.2-1.6 inches/hour

Timing: 11 AM – 11 AM (tomorrow)

Note: Notice that some portions of central Colorado are not included in the low flood threat. This is due to the best instability/moisture existing east and west, so heavy rainfall causing flash flooding is not enough of a concern to warrant inclusion in the low flood threat.