FTB 09-29-2016: Colorado Contrast

Issue Date: 9/29/2016
Issue Time: 9:40 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE NORTHWEST SLOPE, GRAND VALLEY, AND SOUTHWEST SLOPE.

Stuck between an upper-level ridge over the central US and an upper-level trough off the West Coast, the flow across Colorado will be southerly today/tonight. This will continue to bring in subtropical moisture (associated with the weakening upper-level low) to western Colorado, while drier air remains in place for areas east of the mountains. This stark contrast between the two regions is illustrated well by the IPW graph below, showing the dramatic increase in moisture over Grand Junction (green line), while measurements from Boulder (blue line) and Schriever AFB (pink line) have remained fairly steady for the past few days.

ipw_20160929

The continued stream of subtropical moisture into western Colorado will lead to scattered showers/isolated thunderstorms throughout the day/night, producing mainly light-to-moderate rainfall. During the afternoon and early evening hours, thunderstorms will produce their most efficient rainfall. However, storms will be moving northward at a good pace, minimizing the flooding threat. Overall, a low flood threat is warranted, but it is important to note that any flooding issues will likely be localized ponding on roadways and in poorly drained areas.

Meanwhile, due to the southerly flow aloft, showers/storms are expected to stay confined along and west of the Continental Divide, with perhaps one or two spill-over showers into the Front Range/Southeast Mountains. East of those regions, the lower elevations will be controlled by high pressure and a stable air mass, keeping conditions dry and mostly sunny. For more details, including rain rates and timing, 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.

Flood Threat Legend

Zone-Specific Forecasts

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

Sunny and dry will once again be the main weather story today, with high temperatures pushing a few degrees above climatological average. Winds will be a bit gustier than previous days, so keep that in mind if you plan on being outdoors.

Front Range, Southeast Mountains, and San Luis Valley:

Mostly sunny skies and mainly dry conditions will be the name of the game today, with only one or two “spill-over” showers possible over the higher terrain. Even so, with near-surface levels remaining dry, any shower activity will be high-based, producing gusty winds and a sprinkle or two.

Timing: Noon – 10 PM

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

Scattered showers/isolated thunderstorms are expected throughout today/tonight, with the best coverage occurring over westernmost regions. Rainfall will be most efficient during the afternoon and early evening hours, bringing the potential for localized flooding issues on roadways and poorly drained areas. Overall, the flood threat can be characterized as a “low-end, low flood threat.” Maximum rain rates are as follows:

Northwest Slope, Grand Valley, and Southwest Slope: 0.6-1.0 inches/hour
Central Mountains, Northern Mountains, and San Juan Mountains: 0.4-0.7 inches/hour

Timing: 11 AM – 11 AM