FTB 07-31-2015: Monsoon Feed Will Support Heavy Rainfall For Some Areas

Issue Date: July 31, 2015
Issue Time: 10:20AM

— Moderate flood threat for San Juans and Southeast Plains
— Low flood threat for Palmer Ridge, Southwest Slope, Southeast Mountains, San Luis Valley and Raton Ridge

This morning’s water vapor image, below, shows the continuation of a subtropical moisture feed that has elevated precipitable water (PWs) values to well above an inch in the Four Corners region. In addition to this feature, a strong jet is noted just northeast of Colorado. The presence of this jet allowed for very strong wind gusts to be transported from the upper atmosphere down to the surface yesterday as intense thunderstorms crossed the Northeast Plains. Surface dew points continue to run high, in the 50s to lower 60s across most of the state, with lower values in the northwest. PWs ranged from 0.6 (and climbing) inches in Grand Junction to near 1 inch in Boulder to 1.3 inches in Pueblo. Thus, there is ample moisture for a heavy rainfall threat.

watervapor_20150731For today’s weather evolution, we expect mostly sunny skies this morning to lead to thunderstorm activity by early afternoon. Most action today will be focused on the higher terrain, south of I-70 with some spillover onto the Palmer Ridge. High moisture content and adequate instability will support heavy rain rates capable of flash flooding. A Low flood threat has been posted for much of the higher terrain across southern Colorado. A Moderate threat is in place for the San Juans due to higher storm coverage. While most activity should subside by evening, a complex of storms may be ongoing across northeast New Mexico that could clip southeast Colorado. Very heavy rainfall, up to 2.8 inches per hour will be possible with these storms, warranting a Moderate flood threat for that region as well.

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

San Juans, Southwest Slope, Southeast Mountains, San Luis Valley, Palmer Ridge, Raton Ridge:

Mostly sunny early with scattered to widespread thunderstorms forming by early afternoon. Highest coverage will be in the southwest. Max 1-hour rain rates up to 1.3 inches will be possible, leading to isolated flash flooding, mud flows and debris slides. In addition, gusty winds up to 50 mph will be possible with the strongest storms. Activity will subside shortly after sunset. Due to higher coverage, a Moderate flood threat is in place for the San Juans, with a Low threat elsewhere.

Primetime: 1PM to 8PM

Southeast Plains:

Sunny early, then scattered thunderstorms developing by mid afternoon. Closest to the mountains, these storms will be capable of rain rates up to 1.3 inches per hour. Isolated flash flooding may be possible, warranting a Low flood threat. In the southeast corner, ongoing storm activity will likely spill over from New Mexico by early evening, and continuing through around midnight. With PW values as high as 1.6 inches, these storms could produce rain rates up to 2.8 inches per hour, and 4 inches per 3 hours. A Moderate flood threat is in place.

Primetime: 1PM to 8PM near the mountains, 3PM to 1AM in the far southeast

Northeast Plains, Front Range, Urban Corridor:

Mostly sunny early, then partly cloudy with isolated thunderstorms possible. Max 1-hour rain rates up to 0.7 inches are possible, along with gusty winds up to 45 mph. Flooding is not expected today.

Northwest Slope, Northern Mountains, Grand Valley, Central Mountains:

Mostly sunny early, then partly cloudy by mid afternoon. An isolated thunderstorm cannot be ruled out, especially farther south in the Central Mountains. However, max 1-hour rain rates will be limited to about 0.6 inches. No flooding is expected. Gusty winds up to 45 mph may accompany the strongest storms.