Southern Plains Bracing for Potential Tornadoes, Huge Hail

Damaging tornadoes--and hailstones larger than baseballs--may crop up later Tuesday along a swath from southern Nebraska to central Texas, as a long-predicted outbreak of severe weather takes shape. NOAA’s Storm Prediction Center (SPC) has draped a moderate-risk area--the second-highest of SPC’s risk categories--from southern Nebraska into northern Texas. Despite the high confidence that widespread severe weather will occur, there remains uncertainty over exactly where the most dangerous convection (thunderstorms) will develop.


Figure 1. The NOAA/SPC outlook for severe weather issued early Tuesday morning shows a large slight risk area over the Great Plains, with increasing levels of threat centered on central KS and OK. A few severe storms are also possible across the mid-Atlantic, including the Washington, D.C., area. In its update at 11:30 am CDT, SPC extended the moderate risk area into northern Texas.


Here are some key points as of midday Tuesday:

--The strongest storms are expected in two zones: one near a warm front that will be moving slowly north across northeast Kansas and northern Missouri toward southern Nebraska, and the other along and ahead of a dry line that should be located roughly 50-100 miles west of the Dallas-Fort Worth, Oklahoma City, and Wichita areas by late afternoon.

--The vertical wind structure of the atmosphere is not ideal for a widespread tornado outbreak. Although winds do strengthen with height, and upper-level winds are somewhat stronger than expected, the upper-level storm moving into the Plains has dug far enough south that winds will have a strong southerly component at all levels. This reduces the amount of directional shear (variation of wind direction with height) that can help enhance storm rotation. In addition, storms that are moving parallel to the dry line will tend to coalesce more quickly over time, rather than being more scattered or isolated supercell storms. Still, there will be local variations in the wind profile, so tornadic supercells are a distinct possibility, including the potential for strong tornadoes. SPC's late-morning tornado outlook reflects the overall picture, with fairly low tornado probabilities spread across a large area. Models were not yet in strong enough agreement to pin down smaller areas of higher tornado probability, but those could emerge later in the day.

--Hail will be widespread, and some pockets of large hail are a near-certainty with this event, thanks to extreme instability (cold air aloft overtopping very warm, moist air at low levels). SPC expects at least a few reports of giant hailstones measuring at least 3” in diameter. This NWS reference list of hail sizes relative to common objects may come in handy. Golf-ball-sized hail is around 1.75” in diameter; baseball size, 2.75”; grapefruit size, 4.00”; and softball size, 4.50”.

--A layer of warm air several miles above the surface should serve as a cap to inhibit thunderstorm development until early to mid-afternoon. Winds above the surface should become more favorable for tornado development over time, so the longer it takes for storms to develop, the greater the risk of tornadic supercells, especially within a window from late afternoon to just after dark.


Figure 2. The high-resolution HRRR model run from 14Z (9:00 am CDT) Tuesday, April 26, 2016, projects a broken squall line to be plowing across parts of MO, KS, OK, and TX by 05Z Wednesday, April 27 (midnight Tuesday night CDT).


--High-resolution short-range models indicate that individual storms should congeal within a few hours into one or more squall lines with severe winds and large hail along the advancing dry line/cold front. This transition may occur just west, just east, or somewhere close to the DFW/OKC/Wichita corridor.

--Additional severe storms, including tornadoes, are possible on Wednesday, especially toward Arkansas and Louisiana, depending on the evolution of this squall line and how quickly the atmosphere recovers ahead of it. Another multiday stretch of severe weather is expected to start late Thursday night over north Texas and extend into the weekend, with excessive rains and flooding very possible around northeast TX, southeast OK, southwest AR, and northwest LA.

25 years ago today: the “overpass video”
Tuesday’s date has added resonance for long-time followers of U.S. severe weather. Tuesday is the 25th anniversary of a major tornado outbreak that pummeled the Southern Plains on April 26, 1991. A total of 55 tornadoes were confirmed, including an F5 twister that moved from near Wichita, KS, to demolish a mobile home community in Andover, KS, killing 13 people there. A subsequent tornado from the same supercell moved over a Kansas Turnpike overpass while a team from Wichita’s KSNW-TV were huddled beneath the girders. The resulting footage--one of the first truly viral tornado videos, even though it preceded smartphones, Facebook, and Twitter--left many with the impression that overpasses were suitable shelter from tornadic storms.


Figure 3. Overpasses are a terrible place to shelter, whether it’s attempting to shield your car from hail or to protect yourself from a tornado. Image credit: NWS/Norman, OK.


As it turns out, the KSWN team was largely lucky: the tornado that struck them was relatively weak, and many overpasses lack the girder structure that helped protect the crew. Subsequently, three people were killed near or beneath highway overpasses by tornadoes in the Oklahoma City area on May 3, 1999. The NWS now strongly advises against using overpasses as shelter, and the case for the danger of this practice is well made in this NWS slide presentation. (Needless to say, parking beneath a highway overpass on the highway to avoid hail, as shown above, is equally ill-advised, as it quickly blocks traffic and could endanger many other people.)

April 26: a date of tornadic infamy
As shown in Figure 4 below, a total of 300 tornadoes rated at least F1/EF1 have been recorded on April 26 in SPC records going back to 1954, as compiled by Harold Brooks (National Severe Storms Laboratory). These include the 1991 outbreak as well as more than 50 tornadoes on April 26, 2011--the day before the horrific 2011 Super Outbreak--and a round of tornadoes that caused 11 deaths in Oklahoma on April 26, 1984. The only dates with a larger total than April 26 are April 3 (317 tornadoes, nearly all from the infamous 1974 Super Outbreak) and April 19 (340 tornadoes, the result of major outbreaks in 1973, 1996, and 2011). The two-day total of 457 on April 26-27 is the largest for any two-day period--largely due to the 2011 Super Outbreak, whose worst day by far was April 27. Of course, tornadoes can strike on any day when conditions are favorable, and Figure 4 reveals that every date in April and May has produced dozens of twisters over the years.

Follow the severe weather on our liveblog
We will be tracking today’s events on a WU liveblog accessible from the WU front page and from this direct link.

Bob Henson


Figure 4. The number of F1/EF1 tornadoes reported on each calendar date across the period 1954 to 2015. Image credit: Harold Brooks, NOAA/NSSL; data from NOAA/SPC.