FTB 08-14-2018: Flood Threat Returns to Eastern Colorado

Issue Date: Tuesday, August 14th, 2018
Issue Time: 09:30AM MDT

— A MODERATE flood threat has been issued for Southeast Mountains and portions of the Front Range, Urban Corridor, Palmer Ridge and Raton Ridge

— A LOW flood threat has been issued for the Front Range, Southeast Mountains, Central Mountains, Urban Corridor, Palmer Ridge, Raton Ridge and portions of the Northeast and Southeast Plains

Quite a lot of fog over the eastern plains this morning with some very dense fog in spots. This means the low-level moisture has returned as the upper-level low traversed eastward into Kansas over the last 24-hours. The fog should begin to breakup throughout the morning, which should increase visibility. However, smoke from wildfires will limit the usual crystal blue skies again this afternoon. Expecting similar smoke conditions to yesterday. Today, the 500mb high begins to rebuild over the desert southwest, which will return weak, northwesterly flow aloft. A vorticity max over Utah is expect to rotate around the high, which should bring a little extra energy to the state for more widespread afternoon thunderstorm coverage. To our north, a passing trough will also push a shortwave near our norther border, which will help enhance thunderstorm activity over the area even more. Overall, looking at an active day with the threat of thunderstorms returning to the Front Range and Southeast Mountains.

As the shortwave passes over western Colorado early this afternoon, it should help kick off the thunderstorm action a couple hours sooner than usual over the higher terrains. Looking at widespread showers and thunderstorms similar to yesterday with Grand Junctions Precipitable Water (PW) around 0.75 inches. I wouldn’t be surprised if some of this moisture mixes out and rain rates drop off a bit from yesterday. This means thunderstorms that form today will again be capable of producing gusty winds and dangerous lightning. Increased cloud cover and integrated smoke will also help keep the high temperatures a bit cooler.

The shortwaves arrive to eastern Colorado around peak heating this afternoon, which means stronger thunderstorms and increased coverage along the Front Range and Southeast Mountains for initiation. PW values at Denver this morning were over 1 inch and with storm motion slowly to the southeast around 15 mph, the flood threat returns. Outflow boundaries from earlier thunderstorms will likely help kick off more thunderstorm action in the moist environment, so a couple rounds of storms over one area will be possible. Storms are expected to move into the adjacent plains later this afternoon, but the far eastern plains look to remain capped. Therefore, the flood threat should be confined west of Highway 71. There is also a possibility of a couple of storms moving off the Cheyenne Ridge into northern Weld County, so the Low threat has been extended into that area.

A Low flood threat has been issued for the eastern mountains and immediate adjacent plains. Burn areas will be need to be monitored closely as there is high confidence max 1-hour rain rates will exceed 0.5 inches/hour for all larger thunderstorms. Most of the problematic burn scars are included in the smaller area of the Moderate flood threat, which should suffice. Threats today include mud flows and debris slides over the steeper terrains and flash flooding of small streams. A quick 0.5 inches in 30 minutes is possible over urban areas, and with impervious surfaces, flooding of low-lying intersections and streets is possible. Storms will end a few hours after sundown, though some showers may linger in the Southeast Mountains overnight.

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.

Flood Threat Legend

 

Zone-Specific Forecasts:

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

Peak heating and the passage of a couple upper-level shortwaves coinciding will bring thunderstorms back to the eastern mountains and adjacent plains this afternoon. High low-level moisture has returned as well and with slower steering winds aloft, the flood threat returns. Instability and minimal shear will create an environment capable of producing some severe hail (up to 1.5 inches) and damaging winds under the strongest thunderstorms. Max 1-hour rain rates up to 1 inch/hour (north) and up to 1.25 inches/hour (south) are achievable. Further south, 2 to 3 hour totals up to 1.75 inches are possible. Threats today include mud flows and debris slides over the steeper terrains and flash flooding of small streams. This is especially true near recent burn scars, so these should be monitored closely this afternoon. A quick 0.5 inches in 30 minutes is possible over urban areas and with impervious surfaces, flooding of low-lying intersections and streets is likely.

Primetime: 1PM – 12AM

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

Similar setup to yesterday with rain rates dropping off a bit. Storm activity should be relatively widespread today and kick off a couple of hours sooner than usual with the passing shortwave arriving from Utah. Valleys may see some light rain again today, but gusty winds are the more likely scenario. Small hail is also possible under the stronger storms over the San Juan Mountains. Max 1-hour rain rates up to 0.1 inches (north) and 0.3 inches (south) are possible. Flooding is not expected.

Primetime: 12PM – 9PM