FTB 09-23-2017: Upper Level Dynamics and Moisture Combine for Widespread Showers and Thunderstorms

Issue Date: Saturday, September 23, 2017
Issue Time: 10:55 AM MDT

MODERATE flood threat for the Southeast Plains, Northeast Plains and Raton Ridge

LOW flood threat for portions of the Front Range, Urban Corridor, Southeast Mountains, Palmer Ridge, Raton Ridge, Southeast Plains, Northeast Plains

— Timing of flood threat is later than normal, and persists into the overnight hours

The visible satellite imagery below shows quite a bit of rainfall over the state as the trough moves in from the west. Beginning early this morning, the low pressure system over northwest Kansas began to pull in very moist air from Texas and Oklahoma into the eastern portion of the state. The dew points this morning over the Southeast Plains have already made it into the 60s, which is incredibly high for this time of year. Precipitable Water (PW) this morning at Denver was measured at 0.71 inches. Over Grand Junction, PW was 0.64 inches. Dodge City PW is probably a little high, but a better estimate for the SE Plains right now, which is at 1.26 inches. PW greater than 1 inch is marked in the map below. Aloft, expecting south and southwest winds, which will continue to pull moisture north and give storms a N/NE storm motion.

The upper low pressure system will slowly continue to slowly propagate eastwards with the majority of Colorado being under positive vorticity advection throughout the forecast period. Paired with high moisture, this lift will translate into the widespread showers over the state. Currently over the western portion of the state, the jet streak is creating a strong line of convection from north to south. Thunderstorms and widespread showers are likely through midnight until this feature begins to move to the northwest and western Colorado is under the influence of the dry slot.

As the upper low tracks to the northeast, showers and thunderstorms over the mountains are expected to increase. With colder temperatures behind the cold front, rain should transition into snow later this evening for elevations around 9,000 feet. With colder temperatures above 10,000 feet already, precipitation this afternoon will likely be all snow. That could mean a couple of inches by morning the higher elevations in the Northern and Central Mountains. East of the Continental Divide, the higher rainfall amounts will be over the southern portion of the high terrain due to the increase of moisture in this area. Southerly flow is expected to enhance precipitation along the Palmer Ridge/Teller County high elevation intersect. Over the Southeast Mountains, there is enough confidence that rain rates will reach 0.5 inches an hour near the Hayden Pass and Junkins burn scar, so storms that form in this area need to be monitored closely. A Low Flood threat has been issued for the higher terrains of the Palmer Ridge, Raton Ridge and Southeast Mountains. Threats include mud flows, debris slides and small stream and road flooding.

Over the eastern plains, enhanced thunderstorm and shower activity is likely this afternoon as the upper level energy, high low level moisture and lift along the slow moving cold front combine. With PW values over 1 inch, heavy rainfall is expected over the Southeast Plain beginning this afternoon. Rain is expected to continue over the far eastern plains into early morning, which warrants a Moderate Flood threat. Threats include stream and road flooding as well as field ponding.

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:

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

Cloudy this morning and increasing showers and thunderstorms throughout the day. Max 1-hr rain rates up to 0.7 inches possible (west) and 1.5 (east). Rain over the mountains will begin decreasing around midnight. Continuous rain is expected over the eastern plains through tomorrow making this an overnight threat. 24-hour totals up to 3.5 inches are possible over the far Southeast Plains. There is enough confidence 1-hour rain rates for storms near the Junkins and Hayden Pass burn scars will be over 0.5 inches. A Low Flood Threat has been issued. Threats include mud flows and debris slides (west), road and stream flooding (west/east) and field ponding (east). A Low/Moderate flood threat has been issued.

Primetime: 12PM to 7AM

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

Cloudy and rainy this morning with showers continuing to increase throughout the day. By late morning, more showers will form over the higher terrains. Southern high terrains could see 24-hour amounts up to 1.8 inches. Max 1-hour rain rates up to 0.5 inches are possible (south) and 0.3 inches (north).

Primetime: 11AM to 12AM