FTB 08-27-2015: Plenty of Moisture Available, but Timing of the Trough Proves Crucial

Issue Date: 8/27/2015
Issue Time: 9:05 AM


The IPW graph below certainly shows that there is plenty of moisture available to showers/thunderstorms. Grand Junction, after peaking above 1.35 inches yesterday evening, has come back down slightly to just over 1.2 inches. As the monsoonal disturbance has rotated across Colorado, IPW has climbed at the other three of the normal reporting stations (Boulder, Pueblo, and Shriever AFB), as well. Moisture is definitely out there.

Fortunately, that amount of water is not coupled with proper timing of the mid-level trough currently moving across the Central/Northern Rockies. The trough axis sits roughly along the Continental Divide at this time, with the best forcing for showers/thunderstorms already across Eastern Colorado. The trough/associated forcing will continue to push east, targeting central NE/KS and points further east for stronger thunderstorms this afternoon/evening. Behind that trough is a weakly subsident airmass, designed to try and suppress strong convection. This will limit what storms can do along and north of I-70, with a more neutral environment south of I-70.


The best opportunity for storms will be over the higher terrain, favoring the Continental Divide and mountains to the east, near the mountains along the Urban Corridor and western extents of the Northeast/Southeast Plains, and wherever sunshine can produce enough daytime heating to break the cap across the Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge. Rain rates are not expected to be great enough to warrant a flood threat, but street/field ponding are expected under isolated, stronger thunderstorms. For more information on timing and rain rates, please jump below the map for zone-specific discussions.

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

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

Early morning showers and thunderstorms continue to wind down across the area at this time. By this afternoon, expect coverage to uptick again, with the main focus of scattered showers/thunderstorms along and east of the Continental Divide. There will be isolated showers/thunderstorms elsewhere with partly sunny skies. Instability will be lacking, which will limit rain rates, thus no flood threat is warranted. Maximum rain rates will be 0.3-0.7 inches/hour for locations west of the Continental Divide, and 0.6-0.9 inches/hour along and east of the Continental Divide.

Timing: 11 AM – 9 PM, with a few weak showers lingering into the overnight hours.

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

Isolated-to-scattered showers and thunderstorms expected, with the best coverage along the Urban Corridor, Palmer Ridge, and western extents of the Northeast and Southeast Plains. Main storm threats will be lightning and gusty winds, but locally moderate-to-heavy rainfall will be attend stronger storms. Under the stronger storms, street/field ponding will likely occur, but nothing to warrant a flood threat, as storm motions will limit the impact. Maximum rain rates are as follows:

Urban Corridor and Palmer Ridge: 0.7-1.4 inches/hour
Northeast Plains and Southeast Plains: 0.8-1.2 inches/hour
Raton Ridge: 0.4-0.8 inches/hour

Timing: 2 PM – Midnight, with a few lingering showers/thunderstorms over the Southeast Plains into the early morning hours.