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Significant Tornadoes Possible on Wed; Widespread Severe Weather Expected on Thursday

By: Bob Henson 5:45 PM GMT on April 08, 2015

A batch of scattered but potent supercell thunderstorms should erupt late Wednesday afternoon and evening across parts of the central and southern Great Plains into the lower Missouri Valley. At 11:30 am CDT, NOAA’s Storm Prediction Center (SPC) placed a swath from roughly Wichita, KS, to Columbia, MO, under a moderate risk of severe weather for Wednesday, with lesser risk categories extending from northern Oklahoma to West Virginia. Significant tornadoes (EF2 - EF5 on the Enhanced Fujita Tornado Damage Scale) and very large hail (greater than 2” in diameter) are a possibility (see Figure 1).

Figure 1. As of 11:30 am CDT Wednesday, a large part of the central U.S. was under various risk categories for severe weather in the afternoon and evening (top). The crosshatched areas indicate the risk of significant tornadoes (center) and very large hail (bottom). The percentages in these two maps give the odds of a tornado (center) or hail of at least 1” diameter (bottom) occurring within 25 miles of a given point. Image credit: NOAA Storm Prediction Center.

This potential outbreak has been well predicted by forecast models for several days. Ample moisture and favorable jet-stream flow from the southwest have been in place since Monday, leading to a few pockets of severe weather already. SPC logged more than 50 preliminary reports of 1” to 2” diameter hail as far north as southern Minnesota, where moist air from the Gulf of Mexico flowed atop much chillier surface air. One complex of severe storms moved from eastern Missouri to Kentucky on Tuesday afternoon, dropping baseball-sized hail in the central Kentucky town of Garrard and dousing Louisville, KY, with another 1.21” of rain on top of the 14.62” it had already received since March 1. Damage surveys on Wednesday confirmed two tornadoes from this complex southeast of Lexington, KY, and two others were reported on Wednesday night in far southeast Kansas. Thunderstorms continued on Wednesday morning along and near a broad east-west frontal zone extending from central Missouri into southern Ohio, with two severe thunderstorm watches in effect by late morning.

A major upper low that gave much-appreciated snow and rain to California is now making its move into the central states, which will help trigger Wednesday’s main round of severe weather. A piece of energy from the low will sweep across a constellation of boundaries in the OK/KS/MO region. These included a dry line in northwest Oklahoma and the east-west frontal zone, which was pushed into northeast Oklahoma by late-evening storms on Tuesday, then began lifting back north into Kansas and Missouri as a warm front on Wednesday morning.

Scattered thunderstorms should form along or near these boundaries by Wednesday evening, with several rapidly becoming supercells that could spawn tornadoes. The sheer number of storms may be limited at first by a capping layer of warm, dry air several miles high. Weaker storms have formed above the cap in western Oklahoma, which may diminish the risk somewhat along the dry line. However, the amount of instability and wind shear on hand by evening, especially toward southeast Kansas and southwest Missouri, favors the emergence of supercells (long-lived, discrete thunderstorms that produce the lion’s share of stronger tornadoes). The cool low-level air pushed out from any storms that develop could provide boundaries for additional storm formation.

NOAA/SPC has issued a public severe weather outlook for Wednesday’s storms.

Figure 2. Wednesday’s severe weather will be followed by another round on Thursday, April 9. As shown above, the Wednesday 1200 GMT run of the high-resolution (4-kilometer) NAM model predicted that a line of potentially severe thunderstorms would bisect Illinois along a strong cold front at 7:00 pm CDT Thursday, with scattered intense storms from southern Michigan to western New York near a warm front. If overnight storms inhibit daytime heating on Thursday, the storms may be less widespread or severe. Once the Illinois storms form a solid line, the risk of significant tornadoes should lessen there, but a threat for high wind, heavy rain, and large hail would persist with the subsequent squall line into Indiana, Kentucky, and western Tennessee. Image credit: College of DuPage NeXt Generation Weather Lab.

Severe threat shifts to Midwest on Thursday
Tornadoes are also possible on Thursday as the upper low and associated frontal system accelerate northeast toward the Great Lakes. By afternoon, a strong cold front should be plowing east across Illinois, with the east-west frontal zone now sweeping north into Wisconsin and Michigan as a warm front. These boundaries will help focus intense thunderstorms across a broad area, probably more numerous than on Wednesday, with long-lived supercells possible. Wind shear will be stronger than on Wednesday, but it remains to be seen how well the atmosphere manages to recover from the cooling effect of Wednesday night’s storms upstream. If the air does warm up enough to become at least moderately unstable, models suggest that the powerful upper system could trigger a north-south line of fast-moving supercells across Illinois (see Figure 2), eventually becoming a solid line with heavy rain, hail, and high winds. Other dangerous storms may form along the warm front. The overall system’s increasing speed will put much of the Midwest and the Mississippi Valley in line for one or more quick shots of potentially severe weather. As of 12:30 p.m. CDT Wednesday, SPC’s enhanced-risk area for Thursday includes an unusually large swath from northeast Texas to eastern Ohio.

Figure 3. A partial double rainbow took shape near Taylorsville, KY, in the wake of the severe storms that produced at least two tornadoes and baseball-sized hail over central Kentucky. Image credit: wunderphotographer mlongteach.

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