FTB 08-31-2015: Monsoonal Moisture Plume Stretching Across Colorado

Issue Date: 8/31/2015
Issue Time: 9:20 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, RATON RIDGE, CENTRAL MOUNTAINS, NORTHERN MOUNTAINS, GRAND VALLEY, SOUTHWEST SLOPE, AND SAN JUAN MOUNTAINS.

A late-summer monsoon day is in store for much of the state as deep moisture continues to overspread Colorado due to southwest flow aloft. The IPW graph below shows the increase over the last day or so, first for Grand Junction (green line) and then for Boulder (blue), Pueblo (pink), and Shriever AFB (red). Grand Junction has moderated just above 1 inch this morning, but that will uptick a bit again around lunchtime with the arrival of a new disturbance from the southwest. Additionally, the other three stations will see their moisture levels climb throughout the afternoon, as well.

IPW_08312015_2

Overall, isolated-to-scattered thunderstorms are expected across much of the state, with the best coverage over the Northern Mountains, Central Mountains, Front Range, San Juan Mountains, Palmer Ridge, Urban Corridor, and Northeast Plains. Storms are expected to maintain a good pace of motion, limiting the flood threat, thus only a low flood threat is warranted. For today, burn scars, low-lying areas, and urban areas susceptible to drainage issues will command the most relative attention, but the low flood threat will cover a good chunk of the state. For more details on rain rates and timing, 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

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

Isolated thunderstorms are expected over the Northwest Slope; isolated-to-scattered coverage over the Grand Valley, San Luis Valley, and Southwest Slope regions; and scattered coverage over the Northern Mountains, Central Mountains, Front Range, and Southeast Mountains. Areas that received good rainfall yesterday have been included in the low flood threat due to saturated soils exacerbating moderate rainfall rates. Maximum rain rates will break down as follows:

Front Range, Southeast Mountains, and Central Mountains: 1.0-1.6 inches/hour
Northwest Slope, Northern Mountains, Grand Valley, and San Luis Valley: 0.5-0.8 inches/hour
San Juan Mountains and Southwest Slope: 0.6-1.0 inches/hour

Timing: 11 AM – 10 PM, with a couple of showers lingering into the early morning hours.

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

Scattered showers and thunderstorms expected this afternoon and evening, with the best relative coverage for areas adjacent to the mountains, and across the Palmer Ridge and Northeast Plains. With moisture climbing, favorable position of an upper-level jet streak, and a mid-level disturbance, locally heavy rainfall and lightning will be the main storm threats today. As stated above, burn scars and urban areas susceptible to drainage issues will command the most relative attention with respect to flooding/ponding issues. Maximum rain rates will be 1.2-2.0 inches/hour, with locally heavier rain rates across the Palmer Ridge at 1.8-2.2 inches/hour. Storm motions will help limit the flood threat, so only the low flood threat is warranted.

Timing: 1 PM – 10 PM will be prime time, ending from west to east as drier air works in. Thunderstorms will continue until about midnight across the plains as they work their way into KS/NE.