Issue Date: Tuesday, August 15, 2017
Issue Time: 11AM MDT
Summary:
It is no secret that heavy rainfall and flood forecasting is an imperfect (though improving) science and sometimes Mother Nature throws us a curveball. Monday afternoon was one of those times. Every morning, we produce a “worst-case scenario” rainfall estimate across the state using our ensemble guidance. Very often, this is an excellent tool for depicting realistic maximum rainfall rates, as well as the locations most likely to be impacted. Yesterday, our guidance for max 3-hour rainfall looked like this:
There was a pocket of higher values across up to 2.25 inches in 3 hours in eastern Colorado where some members of the ensemble indicated a potential convergence zone developing. However, this intensity is not all that noteworthy especially given its presumed low chance of occurrence based on our morning analysis. Also note the higher values just across the border in western KS and NE.
In reality, not only did the convergence zone materialize stronger than expected, it was also significantly farther west than expected, extending almost to Denver metro. Thus, yesterday afternoon’s almost all of yesterday’s individual storms ended up being on the higher side of guidance, or even above it. Moderate rainfall amounts were observed across parts of the higher terrain where up to 0.6 inches of rainfall fell. However, across the northern part of the Palmer Ridge, some much more impressive storms developed. The strongest cell in the state occurred over Kit Carson county as an outflow boundary moved in from NE. Slowly moving cells with access to plenty of “fresh” moist air from the south hung out right along I-70 for a 2-3 hour period. Max 1-hour rainfall rates above 2 inches were common, though the more impressive tally was 4+ inches of rainfall estimated over a roughly 2 hour period. Unfortunately, there were no official gage reports above 1.5 inches though significant street flooding was reported near Vona, which likely required much more than 1.5 inches of rainfall to produce. Another line of storms formed farther west, though rainfall rates with these was significantly lower, up to 1.5 inches per hour. Numerous flash flood warnings were posted for this activity yesterday afternoon.
