FTB 05-20-2018: Quiet Morning for Most, then Evening Showers & Storms in/near the Mountains

Issue Date: 5/20/2018
Issue Time: 12:05 PM

NO flood threat today.

After a very wet last couple of days, the northeastern portions of Colorado should begin to dry out later today, and the mountainous regions of the state should see a round of showers and thunderstorms this evening. The upper-level disturbance that was responsible for some of yesterday’s precipitation is now off to the east (“Yesterday’s Shortwave” in the image below), taking the associated cluster of showers and storms with it. Today’s shortwave is currently situated in northern Arizona and, as indicated by the infrared satellite imagery, is producing little in the way of precipitation or cloud cover. However, as it approaches Colorado from the southwest today, it should tap into the moisture already in place, spawning a fresh round of precipitation.

Upslope flow has persisted this morning in the foothills east of the Continental Divide and the adjacent plains, producing low clouds and drizzle for many in the Urban Corridor. As we progress through the morning, these clouds should lift and burn off, leading to at least some sunshine for many across the state. Meanwhile, showers and storms should start over the higher terrain in the southwestern portion of the state by around noon, with development progressing northward and eastward throughout the day. Rainfall will be light or briefly moderate for most, but favored slopes may see a long-lasting rainfall event today with 24-hour totals approaching 2”. However, given the lower rainfall intensities expected, we do not expect to see a flood threat across Colorado today.

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

Grand Valley, Southwest Slope, San Juan Mountains, Southeast Mountains, Raton Ridge:

Sunny to begin with this morning, but clouds increasing as scattered showers and thunderstorms build during the afternoon. Valleys may remain largely dry, but rain may persist on favored south- and southwest-facing slopes well into the evening.

Rainfall rates: generally 0.05” – 0.20” per hour, may get as high as 0.25” – 0.50” per hour on favored slopes.

Timing for Front Range, Urban Corridor, and Palmer Ridge: Current –11 PM

Southeast Plains and Raton Ridge:

After this morning’s activity winds down, there will be a lull until the afternoon with another round of isolated-to-widely scattered thunderstorms are expected to develop, continuing into the evening hours. A couple of the storms will be strong-to-severe, producing hail, strong winds, and periods of moderate rainfall. Rain rates are expected to remain below flash flood thresholds – max rates: 0.7-1.2 inches/hour

Timing: 1 PM – 11 PM

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

Mostly sunny to start, with increasing clouds as daytime heat and orograophic effects produce bubbling cumulus clouds over the higher terrain. Scattered showers/thunderstorms are expected over the higher terrain, with most eventually drifting over adjacent lower valleys. The best coverage of storms will be north of I-70 where upper-level support is best. Rain rates are expected to stay below flash flood thresholds, with max rain rates as follows:

Northern Mountains, Northwest Slope, Central Mountains, Grand Valley, and Southeast Mountains: 0.4-0.6 inches/hour
Grand Valley, Southwest Slope, San Juan Mountains, and San Luis Valley: 0.1-0.3 inches/hour

Timing: Noon – 10 PM, with a lingering shower or two over the Central and Northern Mountains until Midnight.