Summer heat and thunderstorms have kicked in full-force across much of the contiguous U.S. over the last couple of weeks, with a generous helping of heat advisories, severe thunderstorm and tornado watches and warnings, and flood watches and warnings scattered throughout the country. A large portion of the Great Plains states in particular have had an active past week or so, as last weekend the remnant moisture from Hurricane Bud was carried from the Baja California, across the Four Corners region, and through the northern Plains, where rainfall of 1-4″ was fairly widespread over the course of several days of showers and thunderstorms. The influx of tropical moisture also ushered in rounds of severe weather for many, with back-to-back days of damaging hail up to 3″ in diameter occurring in Colorado. While the hail damage may end up being substantial, storms have also produced pockets of flooding across Texas, Colorado, and the upper Midwest, which, on the aggregate, are likely to be the most damaging weather events of the week.

Flooding across the Upper Midwest kicked off Sunday and Monday as thunderstorms associated with Bud’s remnants dropped up to 7″ of rain across Minnesota¬†and Wisconsin. Simultaneously, a separate area of tropical moisture associated with an open tropical wave was beginning to push into Texas on Sunday night, producing disorganized but very heavy thunderstorms along much of the coastal plain. Repeated bursts of heavy rain on each of Monday and Tuesday culminated in flooding on Wednesday morning, unfortunately inundating many of the same cities and towns that were devastated by Hurricane Harvey back in August. Some of the thunderstorms that occurred Tuesday night into Wednesday morning are apparent in the infrared satellite loop (Figure 1), which shows the growth, decay, and relatively slow motion of thunderstorms across the area. The red and black areas indicate the coldest cloud top temperatures, and therefore the deepest and strongest thunderstorms. While the total rainfall across south Texas was nowhere near as extreme as witnessed during Harvey, 3-day rainfall totals from Sunday to Wednesday morning (Jun 17-20) approached 18″ in spots (Figure 2), and were widely in excess of 3″ from Beaumont to Brownsville.

coastal TX 17 20jun2018 rainfall

Figure 2. Total rainfall as estimated by MetStormLive over the 3-day period from 12:00 UTC (7am CDT) on Sunday, June 17th, 2018, to 12:00 UTC on Wednesday, June 20th, 2018.

Also apparent in the IR satellite loop above is an area of much more organized deep thunderstorms covering much of Kansas and southern Nebraska. This storm complex was spawned by the growth and merging of severe supercells that formed over Colorado on Tuesday afternoon, which produced numerous large hail reports and a handful of tornadoes. As storms progressed eastward after dark, they encountered deeper and deeper moisture, which enabled them to grow into a pair of mesoscale convective systems. MCSs are common in the Great Plains during the summer months, but to have two separate complexes in such close proximity is far less common. As a result, some places in northern Kansas experienced hours of moderate to heavy rain as both complexes passed over in succession. The radar image below illustrates the two separate complexes nicely (Figure 3), with each complex having a leading line of heavier rainfall (reds and oranges on radar) and sprawling areas of light-to-moderate rainfall spread out for dozens of miles to the north and west (greens and yellows on radar).

Plains radar 20180620 0337z

Figure 3. Composite radar image of the thunderstorm complexes across Kansas and Nebraska around 10:37pm CDT on June 19, 2018.

Some staggering rainfall totals emerged from this region on Wednesday morning, with most of the northwestern 2/3rds of Kansas and southeastern half of Nebraska having received at least some heavy rain from a thunderstorm during the previous night (Figure 4). Pockets of 3″ to as much as 8″ of total rain were scattered along the KS/NE border, particularly in places that were hit by both MCSs. Fortunately, the area is somewhat sparsely populated, and major flood impacts were not reported.

KS NE mcs 19 20jun2018

Figure 4. Total rainfall as estimated by MetStormLive for the 24-hour period ending at 13:00 UTC (8am CDT) on Wednesday, June 20, 2018.

Today, June 21st, marks the summer solstice, which means that the season of summer thunderstorms is really only beginning. As always, MetStat will remain vigilant for these heavy rainfall events and will share updates on major storms soon after they happen. Thanks for reading, and be sure to check back with us soon!