FTB 08-26-2016: Locally Heavy Rainfall Remains a Threat

Issue Date: 8/26/2016
Issue Time: 9:25 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHEAST PLAINS, RATON RIDGE, AND NORTHEAST PLAINS.

A LOW FLOOD THREAT IS ALSO FORECAST FOR PORTIONS OF SOUTHEAST MOUNTAINS, FRONT RANGE, URBAN CORRIDOR, AND PALMER RIDGE.

A LOW FLOOD THREAT IS ALSO FORECAST FOR PORTIONS OF THE SOUTHWEST SLOPE AND SAN JUAN MOUNTAINS.

A somewhat tricky forecast is in the cards today/tonight, thanks to the potential for a few stronger thunderstorms to mix in with otherwise garden variety activity. Currently, a weak shortwave aloft is tracking east-northeastward across Colorado, producing extensive cloud cover across eastern and southern Colorado, along with a few scattered showers/thunderstorms (mainly across the Northeast Plains). Expect these showers/weak thunderstorms to wind down later this morning. As far as moisture is concerned, the IPW chart below shows mainly average moisture for the 3 usual reporting stations (Boulder, Grand Junction, and Schriever AFB); enough for a few locally heavy rainfall reports, but not enough for widespread heavy rain.

IPW_20160826

Today’s main event, so to speak, will begin early this afternoon as daytime heating and orographic effects help kick off scattered showers/storms across the High Country and Western Slope. These storms will generally move eastward with time, pushing over the lower elevations of eastern Colorado by early-to-mid afternoon. Wind shear and moisture will be sufficient for a few strong-to-marginally severe storms across eastern Colorado, but cloud cover will be a limiting factor. Where clouds can break earliest, allowing the most daytime heating, will be where the strongest storms can occur. Further west, the concern for locally heavy rainfall is mainly limited to the San Juan Mountains and Southwest Slope regions, thus the issuance of the low flood threat area. For more information on 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.
20160826_LFT

Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Northeast Plains, Palmer Ridge, Southeast Plains and Raton Ridge:

Scattered showers/thunderstorms are expected, with a couple strong-to-marginally severe storms mixed in with the garden variety majority. The main threats from stronger storms will be small hail, strong winds, lightning, and locally heavy rainfall. Maximum rain rates are as follows:

Urban Corridor and Palmer Ridge: 0.8-1.2 inches/hour
Northeast Plains and Southeast Plains: 2.0-2.5 inches/hour
Raton Ridge: 1.0-1.4 inches/hour

Timing: Noon – 10 PM, with a few showers/storms continuing until around midnight

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

Scattered showers/thunderstorms are expected during this period, mainly over the higher terrain. Storm motions will help carry a few showers/storms over lower valleys. Most activity will be produce gusty winds and light-to-moderate rainfall, but a bit better moisture/instability over the San Juan Mountains, Southwest Slope, Front Range, and Southeast Mountains will bring the potential for locally heavy rainfall to those regions. Maximum rain rates are as follows:

Southwest Slope, San Juan Mountains, Front Range, and Southeast Mountains: 0.8-1.2 inches/hour
Northwest Slope, Northern Mountains, Grand Valley, and Central Mountains: 0.4-0.8 inches/hour
San Luis Valley: 0.3-0.6 inches/hour

Timing: 11 AM – 9 PM, with a few lingering showers/weak thunderstorms over the higher terrain until midnight