Significant Tornado Threat in TX/LA on Sunday

A worrisome juxtaposition of very high instability and extreme wind shear will fall into place across a large swath of eastern Texas and northern Louisiana on Sunday, setting the stage for a potential round of multiple strong tornadoes. Update (12:30 pm CDT]: The NOAA/NWS Storm Prediction Center has placed the region under a high risk of severe weather (its highest risk category), with a chance of significant tornadoes (EF2 or stronger).


Figure 1. The Day 1 convective outlook from NOAA/SPC issued at 11:27 am CDT Sunday morning.


Figure 2. The probability of a tornado within 25 miles of a given point on Sunday, as estimated by NOAA/SPC in its 11:27 a.m. CDT outlook. The crosshatched area indicates where significant tornadoes (EF2 or stronger) are most likely to occur. Image credit: NOAA/NWS/SPC.

The air mass ahead of a cold front moving across central Texas is expected to be quite volatile by midday Sunday, with very rich moisture flowing north from the Gulf of Mexico beneath seasonably cold air aloft. Instability may vault into or above the range of 2000 - 3000 joules per kilogram, which is more than enough for significant severe weather.

Especially worrisome is the fact that winds in the warm sector over southeast Texas and western Louisiana will be from the southeast but will veer quickly to the south and southwest with altitude, while strengthening markedly. Models suggest that the resulting wind profiles will have a classic sickle shape favorable for storm rotation. Because of the especially deep moisture, cloud bases will be quite low. This will add to the tornado potential (because of the added buoyancy from water vapor condensing within the strongly sheared zone), while also raising the risk that tornadoes will be harder to see.


Figure 3. WU composite of NEXRAD radar as of 8:20 am CDT Sunday, April 2, 2017.

Today’s two main storm modes
A powerful mesoscale convective system (MCS) was already plowing toward the Austin/San Antonio region at 8 am CDT Sunday (see Figure 3 above). This MCS is expected to continue intensifying as it expands north and east through the day, with a risk of embedded tornadoes as well as very strong downbursts (straight-line winds). The HRRR mesoscale model suggests that this MCS could make it through northern Louisiana and southeast Arkansas and on into into northern Mississippi by Sunday night. By late tonight, a solid line of intense storms may extend from the MCS all the way to the Gulf Coast of Louisiana.

Ahead of this MCS, there will be plenty of other thunderstorms over eastern TX and much of Louisiana, likely including the Houston metro area. Any of the strongest storms that can remain more isolated have the potential to be more supercellular, with the highest tornado threat. Tornadic supercells may erupt on the early side for a spring outbreak—perhaps before noon CDT.

A particular zone of concern highlighted by the high-risk area will be near a warm front expected to lie from west to east across east-central Texas and into central Louisiana. In this zone, the low-level winds will have a strong easterly component, maximizing the vertical wind shear and the potential for strong tornadoes. Even though Houston and New Orleans will be south of the warm front, residents should keep on alert as they are not free and clear of Sunday’s tornado threat. The most volatile conditions should remain southeast of the Dallas-Fort Worth area, although DFW could see heavy rain on the north side of the evolving MCS. Very heavy rain will also plaster most of Louisiana; flash flood watches covered the entire state on Sunday morning.


Figure 4. The 12Z Sunday run of the HRRR mesoscale model projected that the significant tornado parameter (STP) would be at very high values, in the range of 4 to 8, over parts of southeast Texas and southwest Louisiana at 19Z (2:00 pm CDT) Sunday, April 2, 2017. The STP values are shown atop projected surface wind plots. The highest STP values can be seen near the warm front, where surface winds have the strongest east-to-west component. STP values indicate where large-scale conditions are supportive for significant tornadoes within a supercell, not where supercells or tornadoes will actually form. Image credit: College of DuPage.

Model-generated indices of severe weather point to the serious nature of Sunday’s set-up. The significant tornado parameter (STP)—a composite index that estimates the likelihood of EF2 - EF5 tornadoes by taking into account instability, wind shear, and how much storms are “capped” by a warm layer aloft—could reach unusually high values of 5 or more in places where the air mass is unaffected by ongoing storms. Most tornadoes rated EF2 or stronger occur where the STP is at least 1, and a study of more than 1000 such U.S. tornadoes found that the average STP value at the time of the tornadoes was 2.2.

Severe weather is expected to continue through the night and on into Monday, as the system quickly progresses into the southeast U.S. If the atmosphere is not heavily suppressed by leftover clouds and rains from overnight storms on Sunday night, a new batch of supercells—potentially including tornadoes—may develop from south Alabama into north Georgia, possibly affecting the Atlanta area. NOAA/SPC has parts of this region under an enhanced risk for severe weather in its Day 2 outlook, with southern Alabama upgraded to a moderate risk for Monday in an update issued at 12:30 pm CDT Sunday.


Figure 5. Top: Damage from the F4 tornado that tore through Paris, TX, on April 2, 1982. Bottom: Approximate track of the Paris tornado (based only on starting and ending points). Top image credit: SPC/Wikimedia Commons. Bottom image credit: NOAA/NCEI.


A somber anniversary: 35 years since the Paris tornado and the first-ever PDS watch
For longtime residents of northeast Texas, Sunday’s tornado threat is bound to bring up memories of a tragic day 35 years ago. On the afternoon of April 2, 1982, the region was hammered by a violent tornadic supercell that plowed through the small city of Paris, killing 10 people and damaging or destroying some 3000 homes. The F4 tornado moved through the heart of Paris (see Figure 5), and the parent supercell spun off other twisters as it charged eastward.

Hailstones up to a phenomenal 6” in diameter fell less than an hour before the Paris tornado struck. A news clip archived on YouTube vividly conveys the aftermath in Paris. Meteorologist Roger Edwards, then a teenager, watched the storm unfolding well to his northeast after he bicycled to a vantage point near his home in Dallas. The NWS office in Fort Worth has a detailed webpage on the event.

The Paris supercell was the “tail-end Charlie” of a devasting line of tornado-spawning storms associated with a very intense upper-level trough and surface low moving through the central U.S. On April 3, 1982, Wisconsin set a state low-pressure record of 28.45” at Green Bay (since eclipsed by another storm in 2010).

Early on April 2, 1982, the NOAA/NWS Storm Prediction Center—then known as the National Severe Storms Forecast Center—issued one of the first convective outlooks (perhaps the very first) to include a “high risk” designation. Later that day, forecaster Bob Johns issued the first-ever tornado watch to be identified as a “particularly dangerous situation” (PDS). This strong wording was borne out by events to come, as 81 preliminary tornado reports were logged from Texas to Ohio in a 25-hour span on April 2-3, 1982. A total of 77 tornadoes were confirmed, with 29 fatalities and 170 injuries.

Bob Henson