FTB 08-29-2016: Upper-Level Low Will Lift Slowly Across Colorado

Issue Date: 8/29/2016
Issue Time: 10:09 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE URBAN CORRIDOR, FRONT RANGE, PALMER RIDGE, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, AND RATON RIDGE.

A LOW FLOOD THREAT IS FORECAST FOR SURROUNDING AREAS AND PORTIONS OF THE NORTHEAST PLAINS, CENTRAL MOUNTAINS, SAN JUAN MOUNTAINS, AND SOUTHWEST SLOPE.

The upper-level low (shown in the water vapor image below) that was discussed in yesterday’s forecast is not in a hurry to move, with its circulation center located across northwestern NM/southwestern CO. The low will lift slowly towards the northeast through this forecast period, providing favorable dynamics for the development of scattered showers/thunderstorms along/east of the Continental Divide. The best coverage of storms will be over/near the higher terrain south of I-70, and along the preferred terrain of the Palmer Ridge and Raton Ridge. Instability values are fairly low and wind shear is weak, but favorable dynamics aloft will overcome these limiting factors to produce a few stronger thunderstorms capable of heavy rainfall. Exacerbating any flooding concerns are saturated soils from previous days’ rainfall, and this is accounted for in the moderate flood threat area. Urban areas and burn scars, such as the Waldo Canyon and Hayden Pass fire burn scars, will need to be monitored closely.

WV_20160829

To the west, daytime heating and orographic effects will be relied upon to produce scattered activity, as the best dynamics will be situated to the east. Cloud cover will be a limiting factor on the amount and intensity of any showers/thunderstorms that develop, due to daytime heating being of primary importance. In general, expect isolated coverage for northern regions, with more scattered coverage across central and southern regions. For more information regarding timing and rain rates, please see the zone-specific forecast 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.
FTB_20160830

Flood Threat Legend

Zone-Specific Forecasts

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

Scattered-to-numerous showers and thunderstorms are expected along/near the higher terrain south of I-70, and along the preferred terrain of the Palmer Ridge and Raton Ridge. Elsewhere, more isolated-to-scattered coverage is expected. Heavy rainfall is expected within a few stronger thunderstorms, and flash flooding concerns are exacerbated due to previous days’ rainfall (especially last night’s heavy rain). Maximum rain rates are as follows:

Front Range and Southeast Mountains: 1.0-1.5 inches/hour
Urban Corridor: 1.2-1.6 inches/hour
Southeast Plains and Northeast Plains: 2.0-2.5 inches/hour
Palmer Ridge: 1.5-2.0 inches/hour

Timing: 11 AM – Midnight, with a few showers/thunderstorms lingering into the morning hours

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

Isolated-to-scattered showers/thunderstorms expected, with the best coverage across the Central Mountains, Southwest Slope, and San Juan Mountains. Instability will be fairly meager, as will wind shear, so storm organization/maintenance will be a limiting factor. Most activity will be garden variety, with only a couple isolated thunderstorms capable of brief heavy rainfall. Maximum rain rates are as follows:

Central Mountains, Southwest Slope, and San Juan Mountains: 0.8-1.1 inches/hour
Grand Valley: 0.4-0.6 inches/hour
San Luis Valley: 0.5-0.8 inches/hour
Northwest Slope and Northern Mountains: 0.2-0.5 inches/hour

Timing: 11 AM – Midnight