FTB 09-29-2015: Another Day of Unsettled Weather

Issue Date: 9/29/2015
Issue Time: 9:00 AM

NO FLOOD THREAT IS FORECAST.

In the wake of yesterday’s cool front, plenty of low-level moisture has been transported into eastern Colorado with widespread dewpoints in the 40’s and 50’s from along the Front Range/Southeast Mountains and eastward. Meanwhile, in the mid-/upper-levels, good moisture is streaming across the state from the west. Combined, these moisture sources have led to climbing IPW values as noted in the chart below. This amount of moisture raises some concerns about heavy rain, but other ingredients tell the story as to why there is no flood threat being issued today.

IPW_09292015

First of all, the timing of the weak shortwave disturbance is not ideal, as the best lift is already over eastern Colorado (associated with the group of showers/thunderstorms ongoing across the Northeast Plains) and continuing to move east. Second, the amount of instability that will be present coincident with good moisture will be marginal, at best. The best corridor of instability will be along the Front Range/Southeast Mountains and foothills, where higher terrain offers some help. Extensive cloud cover is in place across the Front Range, and to the east, at this time, and will need to clear out in order for sunshine to do its job. This will be a tough sell. Further to the south along and near the Southeast Mountains, more sunshine will be present, making it more likely to see the instability come to fruition. Without good upper-level support, though, only quick pulses of strong-to-marginally severe storms will be possible, and not for a long enough timeframe to create flooding issues. Street/field ponding in poor drainage areas will be the most likely result from any bouts with moderate-to-heavy rainfall, so no flood threat is warranted.

To the west of the Front Range/Southeast Mountains, the higher terrain will see isolated-to-widely scattered showers/weak thunderstorms develop as sunshine/terrain circulations work on residual moisture. No heavy rain is expected. For more details, namely regarding timing and 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

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

Isolated-to-widely scattered showers/weak thunderstorms are expected across the higher terrain, with high temperatures still above normal for the date. Lower valleys will have a hard time seeing any rainfall as storms will likely stay anchored to the preferred terrain. Rain rates will be less than 0.25 inches/hour.

Timing: Noon – 8 PM. A couple of showers will linger into the nighttime hours.

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

Currently, a complex of showers/weak thunderstorms is working across portions of the Northeast Plains, Palmer Ridge, and northeastern extents of the Southeast Plains. This will move eastward and out of the state by later this morning.

For the rest of today, isolated-to-widely scattered showers and thunderstorms are expected, with the best coverage along and near the Front Range/Southeast Mountains where the corridor of instability will reside. Storms will spread to the south and east through the afternoon and evening, but will fade in intensity the further east they travel. Maximum rain rates will break down like this:

Front Range and Southeast Mountains: 0.6-0.8 inches/hour
Urban Corridor and Palmer Ridge: 0.6-1.0 inches/hour
Raton Ridge and Southeast Plains: 0.7-1.1 inches/hour
Northeast Plains: 0.4-0.7 inches/hour

Timing: 11 AM – 9 PM, with a few lingering showers/weak thunderstorms until midnight.