A powerful storm system will generate life-threatening severe thunderstorms and an expansive shield of heavy downpours across vast swaths of the South-Central US this weekend. Flooding rains will target areas from northern Texas and Oklahoma to the central Mississippi Valley. South of the shield of flooding downpours, portions of the lower Mississippi Valley and Arklatex will be targeted by long-lasting thunderstorms capable of producing flash flooding, tree-toppling wind gusts, golf-ball sized hail, and violent tornadoes strong enough to flatten any structure in its path.
Low pressure will develop over southern Texas Friday night and trek northeastward with a vigorous upper-level trough propagating across the southern Plains and Ozark Plateau. The trough will be dragging cold, dry air all the way from southern Canada. Meanwhile, a warm and soggy airmass will surge ahead of the trough in eastern Texas, Oklahoma, and the Mississippi Valley. This is a recipe for disaster.
Friday night, the warm front leading the air mass from the Gulf of Mexico will collide with a much cooler airmass over the southern Plains and middle Mississippi Valley. The warm air will contain copious quantities of moisture that will be released after the collision. The result will be the development of torrential downpours from the Texas Panhandle eastward across northern Arkansas. Unfortunately, the warm front will make little northward progress until mid Saturday afternoon, when the trough finally changes heading to the northeast. This will keep the downpours falling for hours on end without little intermission.
The heavy rain will eventually depart from the Texas panhandle late Saturday afternoon, but not before a mix with or complete changeover to heavy snow. A swath of 1-2″ of snow may fall over north Texas and far western Oklahoma before precipitation finally ends there. Meanwhile, the deluge will intensify over Arkansas while just starting over southeastern Missouri and the southern Ohio Valley. The downpours will continue until late evening in Arkansas and until early Sunday afternoon in the Ohio and Tennessee Valleys.
Some of the heaviest rain will fall over the flood-ravaged Mississippi Valley. Much of the lower Mississippi River is still observing minor or moderate flooding from a slew of recent soakings and upstream snow melt. Parts of the region will pick up over 4″ of rain over a matter of hours. The watershed will not be able to keep up with all the runoff, which will end up ponding on roads and soil. Much of the runoff will eventually make its way toward tributaries of the mighty Mississippi, including the river itself. The floodwaters will submerge basements and low-lying areas near waterways and will make for life-threatening driving conditions. Floods make driving in rain especially dangerous since it is impossible to gauge the depth of the water. It only takes two feet of water to carry away most vehicles, including SUVs and pickup trucks.
South of the warm front, a highly unstable airmass will be matched by a vigorous low-level jet stream in eastern Texas, Louisiana, southern Arkansas, and western Mississippi. The winds will veer clockwise up to an incredible 90 degrees and accelerate to 70 mph just within the lowest 1km of the atmosphere. These volatile environmental conditions will be ripe for severe weather.
Morning clouds and a temperature inversion will initially keep a lid on convection until the sun burns them away. But when upward motion is realized during the afternoon, the low level jet stream will energize the storms that eventually develop. The strong clockwise directional and speed shear will separate storm updrafts from downdrafts, allowing them to grow larger and last longer than typical thunderstorms. Many of these storms will develop rotating updrafts and become sueprcells. And with much of the change in wind in the lowest levels of the atmosphere, tornadoes will have little trouble forming beneath the supercells.
There is a strong chance that a few of the tornadoes could become particularly violent, capable of flattening trees and destroying most structures in their paths. Given the widespread nature of the thunderstorms, there is a possibility that some communities will find their surroundings completely unrecognizable when the barrage ends. Residents of east-central Texas, northern Louisiana, and northwestern Mississippi ought to pay extra close attention to the storms Saturday afternoon and evening, as this is where tornadoes will be most favorable.
Tornadoes won’t be the only threat posed by the thunderstorms. Golf-ball sized hail, hurricane-force straight-line winds, and flash flooding will be additional concerns that the lower Mississippi Valley must confront. Some supercell thunderstorms could even generate all four modes of life-threatening weather.
Thunderstorms will become more numerous with increasingly widespread impacts as evening approaches in Louisiana, southern Arkansas, and Mississippi. Severe weather may become completely unavoidable for some areas within this region. Eventually the storms will merge to form large clusters of dangerous storms. Some of the clusters will become linear as they accelerate across Mississippi, Alabama, and southwestern Tennessee later into the evening. Damaging straight-line winds will be the primary threat here, but any storms that develop ahead of the line earlier in the afternoon and evening will also be capable of producing damaging straight-line winds, hail, and possibly a tornado.
A cold front will sweep across the Arklatex to end the madness Saturday evening west of the Mississippi River. Along and east of the river in western Tennessee, Mississippi, and Alabama, the cold front will be too far behind to curtail the severe weather. Only the loss of day-time heating can work against the storms here.
Sunday the trough will cross the Eastern US. It will have lost some of its strength, but it will nonetheless be capable of provoking havoc. The severe weather threat will extend from the Ohio Valley, across the western Mid-Atlantic into the Southeast. The most favorable corridor for severe thunderstorms will be from southern Columbus to the Atlanta metropolitan area, where modest instability will be intersected by the most vigorous portion of the low-level jet stream. The severe weather won’t be as destructive here, but the storms will have enough of a jolt from the jet stream to produce fairly widespread damaging wind gusts and perhaps a few tornadoes.
Life-threatening conditions are poised to develop over portions of the South-Central US Saturday. Destructive weather could occur anywhere in northern and eastern Texas, Oklahoma, and the lower Mississippi Valley. Communities of east-central Texas, northern Louisiana, southeastern Arkansas, and northwestern Mississippi need to be on especially high alert as this is where thunderstorms will be widespread and where tornadoes will be most probable. Be sure to heed warnings from the National Weather Service and from local law enforcement. If a tornado warning is issued immediately seek shelter under sturdy furniture in a basement or in an inside room without windows on the lowest floor. Closets, bathrooms, and central hallways are excellent choices. Don’t forget about pets! Prepare emergency kits stocked with supplies including first-aid kits, prescription medications, matches, blankets, dust masks, batteries, flashlights, a battery-powered or hand-cranked radio, a wrench to shut off utilities, a manual can opener, three days worth of non-perishable food, and one gallon of water per person per day for at least three days.