An Overview of Colorado's Extreme Rainfall Events from 2023

The eastern half of Colorado experienced several convective rainfall episodes from May through August of 2023 which contributed to the wettest 4-month period on record for many locations. Eight days experienced a 100-year average recurrence interval exceedance (at 1-, 3-, 6-, or 24-h intervals) estimated from Stage IV precipitation analysis, on which there were 61 flood/flash flood local storm reports. This work synthesizes both the large-scale pattern and the mesoscale ingredients for these individual events. The synoptic conditions for the heaviest rainfall individual events were much like past extreme rain events for Colorado, with a mid-level trough approaching, a moist post-frontal airmass, and easterly upslope low-level flow across the plains. These conditions persisted due to the influence of a blocking mid/upper-level pattern which resulted in the continued presence of an upstream trough and anomalously high moisture. Slowly evolving conditions served to produce 1) many consecutive days of rainfall, and 2) for the heaviest individual rainstorms, to produce multiple rounds of various convective modes (i.e., backbuilding and/or training multicells, supercells, squall lines). The most impactful days occurred mainly in June, during a period of very wet soil conditions. This prompts consideration of the role of local evaporation on low-level moisture and precipitation errors from the High-Resolution Rapid Refresh (HRRR) model, which has been noted to have a dry bias during the warm season in some regions of the CONUS.