Conditions on Friday night into the early morning hours on Saturday were prime for thunderstorms to develop in eastern Montana. A north-south oriented trough coupled with a surface stationary front instigated these thunderstorms, which ended up producing heavy rain, hail, and strong winds in southeastern Montana (see Weather Prediction Center’s surface analysis map below). The Storm Prediction Center issued a severe thunderstorm watch over the eastern half of the state from 4pm-midnight, with the southeast corner of the state in a flood warning.

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MetStorm™’s 24-hour analysis estimated that an astounding 7.72″ of rain fell just to the east of Broadus, Montana. While there were no rain gauges in the storm center, Weather Decision Technologies’ (WDT) Polarimetric Identification System (POLARIS) Quantitative Precipitation Estimate (QPE) from mosaicked and quality controlled radar data (used in MetStorm™) along with the National Weather Service’s estimated precipitation indicate this amount was valid. Over 7 inches of precipitation in 24-hours is an exceptional amount of rain to fall in such a short time. We were left asking ourselves how rare this event actually was.

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MetStorm™’s Average Recurrence Interval (ARI) product can tell us just how uncommon this amount of precipitation was over this duration for this area. As of June 2015, MetStorm™’s maximum frequency is 1000-year, so if the maximum ARI for a storm is 1000.5 years, it indicates that the storm was over the 1000-year threshold MetStorm™ has. Below is the 24-hour ARI map for this storm. The storm cells to the east of Broadus and south of Powderville have over a 1000-year return period associated with them, meaning that they have less than 0.1% chance of occurring in any given year and that this storm was remarkably rare.

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Please note that the maps presented here are preliminary and will be updated when new data become available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at http://metstat.com/contact-us/.

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-MetStat Team