FTB 09-30-2017: Widespread Showers & Weak Storms For Higher Terrain, But Rain Rates Limited

Issue Date: Saturday, September 30, 2017
Issue Time: 10:30AM MDT

— Flooding is NOT expected today

NOTE: This is the last scheduled Flood Threat Bulletin of the 2017 season. However, additional Bulletins may be necessary next week to cover the heavy rainfall threat anticipated in southern/eastern Colorado.

A broad upper-level trough continues to be positioned over the western half of the United States, as shown in this morning’s water vapor image, below. Despite the many, rather disorganized smaller-scale features, it is apparent that plenty of upward motion is currently occurring east of the trough’s axis in UT, western CO and northern New Mexico. In fact, showers and isolated weak thunderstorms are already occurring in the San Juan Mountains, showing that the system is bringing plenty of dynamical support. From the standpoint of heavy rainfall, the main limiting threat will once again be moisture, along with fast steering winds. This morning’s Precipitable Water (PW) measurements at Grand Junction and Denver are a rather subdued 0.54 and 0.48 inches, respectively. Higher values approaching 1 inch are found just east of the Colorado border into KS and NE. However, with only a weak easterly component of low-level flow, PW is expected to remain generally steady across the state.

With plenty of dynamical support, we expect a round of numerous to widespread shower and storm activity coinciding with peak heating this afternoon. Highest coverage will be over the higher terrain, mainly west of the Continental Divide. Rain rates are expected to remain well below flood threat thresholds over most of the state. Isolated higher rain rates are possible with storms that come off the Palmer and Cheyenne ridges. However, we expect that even these will stay just below flood threat intensity. Thus, flooding is not expected today. Nonetheless, widespread precipitation totals above 0.25 inches are expected over the central and western part of the state.

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, scroll below the map.

Zone-Specific Forecasts:

Northeast Plains, Southeast Plains:

Mostly sunny this morning then becoming partly cloudy and breezy with isolated to scattered showers and thunderstorms this afternoon and evening. Highest coverage will be over western areas, towards the foothills. Max 1-hour rain rates up to 1.2 inches possible with the strongest storm cells, along with gusty winds up to 55 mph. Flooding is not expected today.

Primetime: 2PM to 9PM

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

Mostly cloudy and cool with numerous shower and weak thunderstorms increasing in coverage this afternoon. Max 1-hour rainfall up to 0.4 inches (north) and 0.7 inches (far south, along NM border). Snow levels 10,000 feet (north) and 11,500 (south). Gusty winds and small hail could accompany the strongest storms. Flooding is not expected today.

Primetime: 11AM to 10PM, with light rain and snow showers continuing overnight

Southeast Mountains, Raton Ridge, Front Range, Urban Corridor, Palmer Ridge:

Mostly sunny this morning, then increasing clouds with scattered to numerous showers and weak thunderstorms developing by early afternoon. Highest coverage will be over the foothills and higher terrain. Max 1-hour rain rates up to 0.75 inches (below 6,000 feet) and 0.6 inches (above 6,000 feet). Snow level will fluctuate around 11,000 feet, lowering later tonight. Gusty winds up to 50 mph and small hail could accompany the strongest cells. Flooding is not expected today.

Primetime: 2PM to 9PM, with light rain and snow showers continuing overnight