So far, this multi-tiered storm outbreak had killed at least 15 people through Mississippi and Georgia - and my fear is that in the next 24 hours, there will be more names added to this list, especially after taking a look at the latest Storm Prediction Center (SPC) forecast, which rarely ever highlights regions of "Moderate" potential for severe weather, not to mention "High".
The significance of the terms "Moderate" and "High" in an SPC Outlook is that it shows extremely high confidence in extraordinarily dangerous weather occurring in a particular location. Really, only in historically significant, near-record tornadic outbreaks do you see a forecast of "High" probability for severe weather.
For instance, if you look above at the Tornado Forecast - You can see a black-hatched area through the "15%" and "30%" chance of tornadoes - indicating that not only is there a 15-30% chance of a tornado within 25 miles of any given point within the southern half of Georgia, the Northern half of Florida, and the Eastern half of South Carolina, there is at least a 10% chance that tornadoes will at least contain the strength of an EF-2 tornado - enough to be extremely destructive.
Looking at the Damaging Wind Forecast, we also notice a black-hatched area, indicating at least a 10% chance of winds in excess of 75 mph - which, in reality, can be be more widespread-destructive than tornadoes. When I see this, I immediately think of a Derecho - which is a line of very powerful thunderstorms - often containing tornadoes and large hail - that also moves at very fast speeds and have been known to cause winds in excess of 100 mph in isolated gusts.
Let's just say...it would be difficult for the Storm Prediction Center to draw anything more dangerous today.
So what makes the Storm Prediction Center so confident in a widespread severe weather outbreak?
Perhaps part of it has to do with the past couple of days of severe weather across the Gulf States. In the Science of Meteorology, understanding the past is key to forecasting the future (it's kind of like having the answer key in front of you, but with some of the pages missing).
What has me concerned? Well, I can't speak for the Storm Prediction Center, but just a quick glance at the atmospheric conditions both at the surface and aloft certainly sheds some light in their forecast.
For example, Dew Point Temperatures over 70 degrees through the Eastern Gulf (yellow), and over 60 degrees through Southern Alabama and Georgia (light yellow) means that there is an excessive amount of moisture available for significant thunderstorm development (and heavy rainfall leading to flooding concerns). A large amount of atmospheric instability is also needed, and we see that just ahead of the very distinct cold front seen below through Alabama and well into the Gulf of Mexico.
The Lifted Index above shows significant signs of atmospheric lapse rates far exceeding the atmospheric "normal", meaning that the low and mid-levels of the troposphere are cooling with altitude much faster than under normal circumstances. This opens a void of unstable air, which, in this case, is filled explosively with rich moisture and warm temperatures capable of severe thunderstorms.
Lastly, a quick look at mid-tropospheric vorticity, winds, and geopotential height above shows a prime environment for atmospheric lift and exhaust. In layman's terms, if the atmosphere wants to sustain severe weather, it must have a means of expelling spent energy while receiving a constant supply of new fuel. The vorticity image above not only shows a strong, negatively-tilted trough of Low Pressure (which brings significant atmospheric instability), but also strong, curving, divergent wind flow over Alabama, Georgia, and Florida, which allows for strong wind shear capable of tilting and rotating severe thunderstorms.
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.
Severe Thunderstorm Forecast Tutorial - January 15th, 2017 West & Central Texas Severe Weather Outbreak
Severe Thunderstorms are expected through Central Texas on Sunday, January 15th, 2017. Watch as I walk though many of the atmospheric dynamics that will likely trigger some very hazardous weather through West & Central Texas.
Originally Posted on DanielSchreiber.org
Join me as I walk through the next 48 hours of forecasting severe weather through Texas and the Southeast. - Dan
Dan Schreiber is an operational meteorologist, with experience