On the evening of January 15th, 2017, a strong Low Pressure system caused a severe weather outbreak to portions of Texas, while widespread freezing rain plagued the Central Plains at the same time.
From Del RIo, Texas Eastward to San Antonio and northward to the Dallas - Fort Worth Metroplex, there were 23 reports of large hail (the largest being softball-size near Bandera, with several reports of baseball-size hail through other parts of Hill Country), 20 powerful wind reports (14 damaging), and two tornado reports (one northwest of Waco near Laguna Park, the other in Grand Prairie).
Several atmospheric variables had to come into play for an outbreak such as this during the dead of winter - but similar things have happened in the past, such as the DFW tornado outbreak the day after Christmas in 2015.
First - there must be a strong, digging Low-Pressure system centered over Far-West Texas and/or South-East New Mexico or Northern Chihuahua, Mexico. A Low Pressure system like this allows for strong atmospheric lift over Central Texas - enough to generate long-lasting, enormous thunderstorms.
Second - There must be access to the moisture from the Gulf of Mexico for several days prior to the outbreak. This typically means that winds must be southerly in nature - or southeasterly closer to West Texas.
Third - it must be somewhat warm the couple days leading up to the weather outbreak. Warm temperatures have the capacity to hold more water. If the winds have been pulling moisture inland from the Gulf of Mexico for several days, warm temperatures will allow for warm dew points - which can allow plentiful moisture for strong to severe thunderstorm development.
Fourth - There must be some sort of cold front, dry-line, or atmospheric trough associated with the Low Pressure over West Texas that allows for destabilization of the atmosphere. This feature will allow cold air to poke into the moisture-rich, warm environment over Central Texas, increasing lapse rates and triggering explosive storm development.
We saw this on Sunday evening.
Above is a storm which persisted for several hours - and was responsible for numerous baseball-size hail reports. It originated in the Serranias del Burro mountains of Northern Coahuila, Mexico, and burst into a severe thunderstorm shortly after crossing the Rio Grande near Comstock, Texas during the late afternoon. After gaining strength shortly after exiting Comstock (it dropped over an inch of rain in 15 minutes in Comstock), it began an eastward track across Hill Country, literally bombing everything in it's path with hail ranging in size from peas to softballs. At several points, there may have been a funnel cloud associated with it, but due to it's existence in a rural area, none was reported.
This same storm can bee seen over two hours later nearing Bandera, still at full-strength. Meanwhile, a tornado warning was issued for southern Kerr County with a strong storm in that region.
Further southwest, a line of strong thunderstorms had just moved through Del Rio just ahead of a cold front. After crossing the Rio Grande just southeast of Del Rio, one storm explosively generated into a severe thunderstorm and reportedly took out electric power through southern Kinney County. It also dropped quarter-sized hail in Brackettville, and radar images indicated extremely strong winds - at one point an individual reported rain and hail being thrown sideways - with this storm through parts of Kinney County.
Further north, a much more pronounced line of storms and heavy rain approached the Dallas-Fort Worth Metroplex - this caused some damaging wind issues, especially associated with a couple of severe storms ahead of this well-defined line. Several tornado warnings were issued - one near Fort Hood, one near Waco, and one splitting hairs between Dallas and Fort Worth, confirmed in Grand Prairie.