FTB 08-30-2015: Approaching Trough Over Pacific NW Brings Changes to Colorado

Issue Date: 8/30/2015
Issue Time: 9:20 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE GRAND VALLEY, SOUTHWEST SLOPE, CENTRAL MOUNTAIN, AND SAN JUAN MOUNTAIN REGIONS.

The ridge of high pressure (blue “H” and dashed line) that has been overhead the past few days will be nudged eastward today by the trough (red “L” and dashed line) over the Northwest US as it moves over the Northern Rockies. As a result, southwesterly flow will increase over western Colorado through today/tonight, bringing increasing moisture to the area (green arrow). With the influx in moisture, precipitable water values will climb, reaching above 1 inch across the San Juan Mountains, Southwest Slope, and Grand Valley regions, thus the low flood threat in those areas. With that said, all regions across the High Country will see an uptick in thunderstorm coverage today; the main threats will be gusty outflow winds, lightning, and brief moderate-to-heavy rainfall. Rain rates and timing will be broken down in the Zone-Specific discussions below.

08302015_WV

East of the mountains, another hot and mostly sunny afternoon is in store, with temperatures increasing/clouds decreasing from west to east. Adjacent to the mountains, mid- and high-level clouds will increase through the afternoon as thunderstorms build over the mountains. A few isolated thunderstorms will attempt to move overhead, but dry air in the low-levels will mean strong outflow winds are the main threat. Gusts to 45-50 mph are not out of the question, accompanied by virga/light rainfall. By the nighttime hours, mid-level moisture will increase, keeping a low chance of thunderstorms in the forecast along the Urban Corridor and western portions of the Northeast Plains into the early morning hours.

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.

20150830_LFT
Flood Threat Legend

Zone-Specific Forecasts

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

Mostly sunny and hot, with high temperatures pushing into the upper-80s and mid-90s. Adjacent to the mountains, isolated thunderstorms will attempt to move overhead, holding a threat for strong outflow winds (gusts up to 45-50 mph) and virga/light rainfall. Overnight, and into the morning, the Urban Corridor and western portions of the Northeast Plains will hold a low chance (<20%) of one or two isolated thunderstorms as mid-level moisture increases from the west/southwest.

Timing: 2 PM – 8 PM, with the aforementioned isolated thunderstorms possible through the overnight/early morning hours.

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

Scattered showers and thunderstorms expected through the afternoon and nighttime hours, producing bouts with locally moderate-to-heavy rainfall, lightning, and gusty outflow winds to 45 mph. Maximum rain rates will break down as follows:

Front Range, Northern Mountains, Northwest Slope, and Central Mountains: 0.4-0.7 inches/hour
San Juan Mountains, Grand Valley, and Southwest Slope: 0.8-1.2 inches/hour
San Luis Valley and Southeast Mountains: 0.15-0.35 inches/hour

Timing: 11 AM – 10 PM, with a few showers/thunderstorms lingering into the morning hours.