FTB 08-26-2015: Monsoon Disturbance On the Way

Issue Date: 8/26/2015
Issue Time: 9:10 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, GRAND VALLEY, NORTHWEST SLOPE, AND NORTHERN MOUNTAINS.

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, GRAND VALLEY, CENTRAL MOUNTAINS, FRONT RANGE, NORTHERN MOUNTAINS, AND NORTHWEST SLOPE.

An increase in shower/thunderstorm coverage today is expected over the mountains as a monsoon disturbance rotates across Colorado from the west/southwest. Ahead of this disturbance, moisture has increased fairly significantly over western Colorado, with IPW readings greater than 1 inch at Grand Junction. Widespread showers and thunderstorms are expected over the High Country and adjacent low valleys, with areas east of the mountains looking to remain mainly dry and hot today.

With the increase in moisture and support from the monsoonal disturbance, showers and thunderstorms will produce rainfall fairly efficiently today, resulting in locally moderate-to-heavy rainfall. Instability will not be great due to extensive cloud cover and storm motions will be decent; both of which will help to limit the flood threat somewhat.

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Along the Urban Corridor, as well as western portions of the Northeast Plains, Southeast Plains, Raton Ridge, and Palmer Ridge, gusty outflow winds from mountain convection will be the main impact today, but a very low chance of isolated thunderstorms will exist during the afternoon and evening hours. Further east, mostly sunny skies and hot temperatures will be the name of the game.

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.

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Flood Threat Legend

Zone-Specific Forecasts

Front Range, and Southeast Mountains:

Isolated-to-scattered showers and thunderstorms expected, though moisture will be slightly less than the deep moisture found over points to the west. Brief, localized, moderate rainfall is possible, with rain rates breaking down like this:

Front Range: 0.5-1.0 inches/hour
Southeast Mountains: 0.3-0.6 inches/hour

Timing: Noon – 10 PM, with a few showers/weak thunderstorms lingering into the early morning hours tomorrow

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

Aside from low chances of an isolated storm or two across the Urban Corridor, and western portions of the other regions, these zones will stay mostly sunny and hot today. The mixing out of moisture will keep rain rates low, with maximum rates of 0.25-0.6 inches/hour from any thunderstorms.

Timing: 2 PM – 8 PM, with a shower/weak thunderstorm or two during the early morning hours as the disturbance rotates across eastern Colorado.

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

Widespread showers and thunderstorms expected today and tonight, with locally heavy rainfall possible. Widespread cloud cover will keep temperatures much cooler than yesterday. Maximum rain rates will be 1.0-1.5 inches/hour, 1.8-2.2 inches/3 hours. The best chances of realizing these rates will be over the San Juan Mountains, Southwest Slope, Grand Valley, Northwest Slope, and Northern Mountains regions.

Timing: 11 AM – 11 PM will be the prime time, but scattered showers/thunderstorms will continue into tomorrow morning. For that reason, the flash flood threat areas will be valid through 11 AM tomorrow.