The past couple of days in Colorado have been quite dramatic in terms of the weather, with temperatures in the mid 70s to low 80s earlier in the week replaced by temperatures in the low 40s, well below normal for this time of year. The dynamics of the atmosphere made for some stark differences across the United States. Above normal temperatures were present across most of the Eastern U.S. with widespread heavy rainfall and severe thunderstorms from the Southern Plains through the Upper Midwest and much cooler temperatures and snow for the western U.S.

Figure 1: A home in Estes Park, Colorado, covered in 25 inches of snow Friday. (NOAA ESRL)

A strong upper level low moved into western Colorado on Thursday morning from the northwest. It initially brought snow for higher elevations and rain for lower elevations. As the day went on, snow began to accumulate in the mountains of Colorado and Wyoming and rain became mixed with snow in areas of the front range. A frontal boundary situated from the Southern Rockies to the Great Lakes region produced widespread severe storms, prompting the SPC to issue an enhanced to high risk thunderstorm outlook that included south-central Kansas and northwestern Oklahoma. There were even a handful of severe storm reports in southeastern Colorado, including a tornado. Several reports of severe thunderstorms, tornadoes, and flash flooding were reported Thursday afternoon and evening across the Southern and Central Plains.

Figure 2: Convective Outlook issued by the Storm Prediction Center for Thursday 5/18/17.

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The meteorological setup across the U.S. is shown in Figure 3. This was a cold, slow moving system, perfect for large amounts of snowfall in Colorado. The heaviest amounts were reported in north and northwest portions of the state, ranging from feet in the mountains to several inches in the plains of northeastern CO. Winter storm warnings were issued late Wednesday afternoon and again early Thursday morning for central and north central Colorado as rain began to turn to snow. The official snowfall total reported at Colorado State University in Fort Collins was 5.8”, which is a daily record for May 18th (old record just a trace in 1915 and 1960), and the highest daily total for any day after May 10th. The daily precipitation of 2.77″ is a daily record for May 18th (old record 1.83″ from 1915), more than average during the entire month of May, and the 3rd largest single May day precipitation on record.

Figure 3: Forecast for Thursday 5/18/17 issued by the National Weather Service

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Several areas experienced large amounts of snowfall, while other areas only received a trace. Largest amounts were in Ward, CO with an astounding 41.7 inches of snow; however, Denver officially received only a trace. Areas that experienced heavy snowfall were heavily impacted. Tree damage was widespread, as leafed-out branches broke under under the weight of the very dense, sticky snow. Schools were closed, roads became dangerous due to slick conditions, and several graduations were postponed as a result of this storm.

Figure 4: Accumulation amounts for Northeastern Colorado over a 72-hour period.

Figure 4 shows accumulation amounts for Northeastern Colorado. Because this was a snow event, the amounts reflected in Figure 4 are water equivalent amounts with a maximum amount of 3.47 inches and largest amounts east of the I-25 corridor between Denver and Greeley. This snow storm brought wet, heavy snow and the amounts reflected in Figure 4 indicate a substantial water accumulation in a brief period of time.

Spring snowstorms are not unusual in Colorado, but this is remarkably late in the year. The unusually cold temperatures are underscored by a freeze warning issued by the National Weather Service for Friday night into Saturday morning for Central Colorado, with impacts to crops and vegetation likely. The system has already begun winding down as it continues to move out of the area, but residents in the Southern Plains should once again brace for another day of severe weather. For more on extreme precipitation events across the U.S., please continue to monitor this space from MetStat.