FTB 09-10-2019: Deepening Trough will Bring Widespread Rainfall to Western Colorado

Issue Date: Tuesday, September 10, 2019
Issue Time: 10:30AM MDT

–A LOW flood threat has been issued for the 416 burn area

A strong shortwave trough is expected to move over western Colorado today bringing widespread thunderstorms and gusty winds to the area. Moderate levels of mid-level moisture can be seen in the blue and white colors over Utah in the mid-level water vapor imagery below. Embedded within the jet stream (orange line below) are a series of shortwaves that will move over western Colorado throughout the afternoon and well into tomorrow morning. These features and enhanced lift from the jet stream will bring widespread showers and thunderstorms over the west. PW was measured at 0.53 inches this morning over Grand Junction and is forecast to increase up to 0.75 inches this afternoon. Most of the moisture is concentrated in the mid-levels, which will keep storms high based this afternoon, so gusty winds in the storm vicinity area likely.

With the jet overhead steering flows are expected to be greater than 30 mph out of the southwest, so max 1-hour rainfall rates will be limited to 0.5 inches with most areas receiving rates closer to 0.25 inches. These fast flows will also create sustained wind gusts up to 35 mph over and to the west of the Continental Divide with wind gusts up to 55 mph possible in the vicinity of the stronger thunderstorms. The heaviest rainfall is expected over the Southwest Slope and San Juan Mountains where PW is expected to increase up to 1.1 inches briefly this afternoon, so a Low flood threat has been issued for the 416 burn area. Storms tracking over the burn area will be capable of reaching max 1-hour rainfall rates up to 0.4 inches with 2-3 hour totals capable of reaching 0.6 inches. Threats include debris flows, mud slides and flooding of small rivers and streams.

Low pressure remains over the Southeast Plains this morning with downsloping flow off of the Southeast mountains creating a dryline (red dashed line below) over the area. This feature will provide lift for early afternoon thunderstorms which will be capable of producing moderate rainfall rates, large hail and gusty winds. Steering flows are expected to be slow (8 mph – 13 mph) out of the south/southwest. This will cause storms to sit over the area with max 2-3 hour rainfall rates up to 1.4 inches possible. Storms are also expected to move off of the Front Range and over the northern Urban Corridor and western Northeast Plains this afternoon and tonight, but rainfall rates are forecast to be below flood threat criteria. Flooding is not forecast over these areas today.

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:

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

Widespread storms are expected to begin around noon over the southwest corner and along the CO/UT border. Storm coverage will then extend to the entire forecast region early this afternoon. Fast winds within the jet stream will keep storms moving quickly with max 1-hour rainfall rates of 0.5 inches possible over the Southwest Slope and San Juan Mountains. Multiple storms that track over the area will be capable of producing 2-3 hour totals up to 0.75 inches. Storms that track over the 416 burn area will be capable of causing mud slides, debris flows and flooding of small rivers and streams. The threat for the burn area will extend until sundown, but a few isolated showers are expected overnight and into tomorrow morning.  These showers are not forecast to cause flooding problems.

Max 1-hour rainfall rates of 0.25 inches are possible over the remaining forecast areas. These rates will decrease to 0.1 inches overnight as instability decreases. Sustained wind gusts up to 35 mph are possible with maximum wind gusts up to 55 mph within the vicinity of stronger storms this afternoon and early this evening. Flooding is not forecast outside of the 416 burn area.

Primetime: Noon to 9am

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

Storms coming in from the west are expected to reach the Front Range by 3 pm. Moisture will be low by the time they reach the area, so max 1-hour rainfall rates of 0.2 inches are forecast over the northern Front Range. As the storms move off of the mountains they will pick up moisture from the Northeast Plains and Urban Corridor, which will increase 1-hour max rainfall rates over the northern Urban Corridor and western Northeast Plains up to 0.75 inches. Storms will begin around 3 pm over the Front Range and exit the northern Colorado border by 10 pm. To the south a few isolated storms over the Southeast Mountains are possible this afternoon with max 1-hour rainfall rates of 0.1 inches.

The heaviest rainfall for the eastern regions today will be to the east of the dryline forming over the Southeast Plains this morning. Slow steering flows will allow storms to produce up to 1.4 inches over a 2-3 hour period. Large hail and wind gusts up to 55 mph are possible within the storm vicinity. Storms are expected to begin around noon and end by 8 pm tonight. Flooding is not forecast over the area.

Primetime: Noon to 10pm