FTB 09-11-2019: Strong Trough Passes Overhead Bringing with it Increase Chances for Rainfall and Gusty Winds

Issue Date: Wednesday, September 11th, 2019
Issue Time: 9AM MDT

The infrared imagery below shows the strong trough to our west with a little dry air (break in rainfall) between this system and the one from yesterday. Large ascent out in front of the trough is marked with the orange “X”. There is still some ongoing rainfall this morning associated with the disturbance over eastern Colorado. Snow is also being reported over the highest elevations of the eastern San Juans with just fog over the Northeast Plains. Expecting this disturbance to continue to move eastward and weaken throughout the morning. As the strong trough and jet move overhead this afternoon, gusty winds are forecast with the cold front, tightening surface gradient and thunderstorms that develop. Wind gusts up to 45mph are forecast for western Colorado/high terrains with gusts up to 40 mph along the front over the eastern plains tonight. Not expecting the cold front to drop though until later this afternoon (west) and tonight (east), but some very cold temperatures are forecast overnight with a little bit of snow for the highest elevations. Largest accumulations (a couple inches above 10,000 feet) will be possible over the northern high terrains by tomorrow morning with some lighter accumulations as you move south. A cool start to Thursday morning, statewide, will finally allow it to feel more like fall.

As far as rainfall today, storms look to favor the northern half of the state with a couple isolated storms possible over the far Southeast Plains. Storm motion is forecast to be quick to the northeast with the jet overhead, but trailing storms should help produce greater accumulations with less instability able to develop over western Colorado and the mountains. Over eastern Colorado, a surface low develops over the far Northeast Plains. This should dry out the low levels to the south of its location, but keep higher moisture on its north and east side. This should limit storm coverage over the Southeast Mountains as the trough passes through.

PW was measured at 0.73 inches at Denver this morning, which should increase as the low develops. Thus, stronger, more widespread storms will be possible over the northern portion of the Urban Corridor, Front Range and Northeast Plains. A couple of these storms may become severe over the Northeast Plains with large hail and damaging winds as the main threats as daytime heating and dynamics will be better over the area. A boundary is forecast to set up somewhere over the eastern plains, so a couple weak thunderstorms will be possible again this afternoon over the Southeast Plains where the cap can break. With better moisture over Kansas, flooding is not anticipated. Additional weak, overnight storms will be possible over the Northeast Plains with the cold front dropping south and upper level support from the trough. So forecasting some fog tomorrow morning as well over the northeast quadrant of the state. Flooding is not forecast today due to the speed of the storms with the jet overhead.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Front Range, Central Mountains, Northern Mountains, Northwest Slope, Grand Valley, Urban Corridor, Northeast Plains:

Best chance for rainfall and highest totals will be over these regions. Trailing storms will allow for some decent accumulations over the Northwest Slope and Northern Mountains. The former very much needs the wetting rain, but both areas are the in either the D0 or D1 drought categories. Isolated storm totals up to 1 inch will be possible by tomorrow morning with some snow over the Northern Mountains (above 10K feet). Over the northern Front Range, Central Mountains and Grand Valley, max 1-hour rain rates up to 0.25 inches will be possible.

As storms move into the northern Urban Corridor and develop over Northeast Plains, totals should increase with higher moisture on the north side of the surface low. Max 1-hour rain rates up to 0.75 inches (west) and 1.5 inches (east) will be possible. A couple storms may become severe near the Nebraska border with large hail (up to 1.75 inches) and damaging winds also being a threat. The main severe threat will stay north, but a couple of these stronger storms will be possible near the Nebraska border. Linger showers and weak thunderstorms will be possible overnight with the cold front sliding south and upper level support from the passing trough. Flooding is not forecast due to the quick storm motion to the northeast.

Primetime: 9AM to 2AM

Southwest Slope, San Juan Mountains, San Luis Valley, Southeast Mountains, Raton Ridge, Palmer Ridge, Southeast Plains:

Better dynamics and moisture will be to the north for western Colorado, but rainfall is still forecast for the San Juan Mountains, Southwest Slope and Southeast Mountains with PW at Grand Junction at 0.67 inches and rising. Storm motion should favor the southwest facing slopes for accumulation with light rainfall anticipated for the San Luis Valley as well. Max 1-hour rain rates up to 0.75 inches (west) and 0.25 inches (east) are possible with totals under 0.1 inches for the San Luis Valley. Storm rain rates should be little lower near the 416 burn area and with only gradual rainfall forecast over the longer duration, so flooding is not anticipated at this time.

Dry air to the west should keep the Raton and Palmer Ridge dry this afternoon with only increasing cloud cover forecast along a convergence boundary. Temperatures will be able to reach the 90°Fs again, so I’m sure they’re looking forward to the cold front dropping through tonight. A couple of weak thunderstorms will also be possible over the Southeast Plains along the convergence boundary created by the surface low. Max 1-hour rain rates up to 0.75 inches will be possible with storm rainfall total slightly higher.

Primetime: 1PM to 9PM