The intense midlatitude storm that swept across the Great Plains on Wednesday, dropping several tornadoes across central Iowa, may produce the worst flooding in decades along the southeast shores of Lake Michigan on Thursday. The powerful surface low at the heart of the storm is being energized by a pocket of extremely strong winds at upper levels. The radiosonde launched at 6:00 pm EDT Wednesday night from the Springfield, Missouri, office of the National Weather Service sampled west-southwest winds raging at 155 knots (178 mph) near the 300-mb level, or about 30,000 feet above ground level. See the YouTube clip at bottom for a taste of what it was like to launch this radiosonde in surface winds gusting to 40-50 mph.
Figure 1. WunderMap composite radar display from 0412Z Thursday, November 12, 2015 (10:12 pm CST Wednesday), as the line of thunderstorms that swept across Iowa earlier in the day reached the Chicago area. Other scattered storms extend along the front all the way to Texas.
Many towns and cities across the Midwest experienced tree damage and scattered power outages on Wednesday due to widespread high winds. As a narrow ribbon of unstable air flowed north ahead of the surface low and associated front, a crop of small supercells emerged across south central Iowa, leading to a mini-outbreak of tornadoes between about 2:00 and 5:00 pm on Wednesday afternoon. A total of 10 tornado reports were received by the NWS Storm Prediction Center (SPC). No major destruction was reported, but a few homes and industrial buildings suffered serious damage. This weather.com story provides a detailed roundup of the tornado and multi-state high-wind damage from Wednesday and early Thursday. As noted by the Weather Channel’s Stu Ostro, the coverage of high wind reports across Iowa, Missouri, and Illinois qualifies the event as a serial derecho, one of two main types of derechoes.
Perhaps the closest call of the day on Wednesday was a tornado that developed near the Des Moines International Airport around 4:30 pm. The twister was visible to staff at the nearby NWS office, as noted on a special observation that you’ll find in the hourly weather history section of the WU daily almanac.
High winds, high lake levels team up for flood risk Strong winds were spreading across the Great Lakes region on Thursday, already gusting to 55 mph in parts of southwest Michigan. The winds are paving the way for an unusually strong episode of coastal flooding along the east shore of Lake Michigan. The storm has prompted the first lakeshore flood warning from the NWS Grand Rapids office since the 1990s. On top of an expected 6-to-12-inch storm surge, waves could peak as high as 16 feet, with the largest waves expected in the Holland/Saugatuck area of southwest Michigan. Beach and dune erosion may be “severe,” warns the NWS. Further east, the high winds are expected to produce a "seiche" that pushes up water levels up on the east end of Lake Erie by several feet; see Lee Grenci's WU blog post for details
Thursday’s storm is indeed a strong one--high wind warnings are in effect from southern Michigan and northern Indiana all the way to western New York, for gusts as high as 60 mph--but there is another factor at work. This year, Lake Michigan and the other Great Lakes are seeing water levels significantly above the long-term average for the first time in the 21st century. After hitting a record-low level in January 2013, the height of Lakes Michigan and Huron (considered a single unit by the U.S. Army Corps of Engineers) has risen more than three feet. The reason: Relatively wet, cool conditions have predominated over the region during the last two years, sending more streamflow into the lakes as well as cutting down on the loss of lakewater through evaporation. Extensive ice cover during the winter of 2013-14 also helped cut back on evaporation rates. In contrast, between 1999 and 2013, the balance was tilted toward warmer air and water temperatures, less ice cover, and greater evaporation. The lakes are prone to large multidecadal variations in water height.
Along with these factors, the amount of water drawn from the Great Lakes for industry, agriculture, and consumer use is now on the decline, thanks in large part to the Great Lakes Compact, a U.S.-Canada agreement that went into effect in 2008. Withdrawal of Great Lakes water by the United States has been dropping by about 4 percent a year since 2007.
The NOAA Great Lakes Environmental Research Laboratory is hosting a press conference on November 19 exploring the potential impacts of El Niño on Great Lakes water levels. Slides will be posted on the GLERL website after the event.
The howling winds across the Great Lakes on Thursday and Friday will also lead to some lake-effect precipitation, with rain and snow showers in New York and up to 7” of snow across parts of the Upper Peninsula of Michigan. In his latest post, WU blogger Steve Gregory weighs in on the recurrently storm pattern expected to persist over the contiguous U.S. for the next week or two. NOAA/SPC is already highlighting the possibility of severe weather, including tornadoes, from Texas into Louisiana next Monday and Tuesday.