A Prolonged Series of Severe Threats—But How Severe?

As storm systems sweep across the country over the next week in classic late-March fashion, we can expect near-daily doses of severe weather over parts of the south-central and southeast United States. A parade of moderately strong upper-level lows will be pulling in a steady stream of warm and moderately moist air from the Gulf of Mexico. Most of the resulting severe weather will plow through the regions most favored for stormy conditions in early spring, from Texas and Oklahoma across the Mississippi Valley into the Southeast states. Fortunately, no sign of a major tornado outbreak is rearing its head right now.


Figure X. The NOAA/NWS Storm Prediction Center’s convective outlooks issued on Thursday morning, March 23, 2017. Each of the next three days (March 23-25) features a slight-risk area.

A warm Gulf of Mexico, yet a modest supply of moisture
The extended sequence of severe weather will kick off with a line of storms expected to form by around sunset Thursday (see Figure 3 below) along a sharp dry line that will extend from west Texas to central Nebraska. There will be strong dynamics and ample wind shear. However, the low-level flow from the Gulf hasn’t yet been fully recharged with moisture throughout the lowest mile or so of the atmosphere. Even though the western Gulf is much warmer than normal for late March, a condition that typically favors severe weather (see this Capital Weather Gang overview from Wednesday), the atmospheric dynamics in recent days haven’t been optimal for water vapor to evaporate from the toasty sea surface into the atmosphere.

The result is an air mass with fairly high surface dew points but a shallow vertical extent of moisture. Even at Corpus Christi, TX, the 12Z Thursday sounding showed a mere 1.66 cm (0.65”) of precipitable water (moisture in a column above the surface). This is well below the average for the date of about 0.90”. Soundings from Brownsville, TX, and Lake Charles, LA, were also less moist than average for this time of year.

It may take several more days before the Gulf flow will be able to bring a richer supply of moisture into the severe weather belt. Moreover, the upper-level features moving through the country over the next few days aren’t especially cold aloft. This will cut down on the contrast with the warmer, moister air below, which in turn will keep the air from being as unstable as it could be.


Figure 2. Winds at the 250-mb level, about xx miles above the ground, will be howling at more than 100 knots (115 mph) in a pocket approaching the Southern Plains at 6:00 pm CDT Thursday, according to this 12-hour forecast from the 12Z Thursday run of the GFS model. Image credit: tropicaltidbits.com.


Figure 3. A line of strong thunderstorms was projected by the 12Z Thursday run of the 3-km NAM model to be in place across western TX/OK/KS at 8:00 pm CDT Thursday, with more isolated storms over northwest Kansas and southwest Nebraska. Version 4 of the NAM, with an upgraded horizontal resolution of 3 kilometers (roughly 2 miles) across the entire contiguous U.S., was introduced on March 15, 2017. Image credit: tropicaltidbits.com.

Day by day
The relatively modest instability on Thursday will favor storms with strong downbursts along the dry line but will make it tougher for intense supercells to consolidate, which greatly reduces the threat of a widespread tornado outbreak. I would expect the best odds for a sustained supercell, perhaps with one or more tornadoes, in northwest Kansas or southwest Nebraska, where the nose of the moist low-level return flow will wrap around a surface low beneath strong upper-level forcing. On the other side of this low, wind-whipped snow may create blizzard conditions by early Friday across parts of eastern Colorado just east of Denver.

Another round of storms is expected on Friday from eastern OK and TX into Arkansas and Louisiana. It remains to be seen how the leftover storms and cloud cover from Thursday night will affect the prospects for Friday. Again, the most likely outcome is a squall line with pockets of high wind and large hail. The best odds for any tornadic supercells may be toward the north end of the risk area, from southeast Kansas into northern Arkansas and southern Missouri, if enough unstable air can make it to this region. Low-level winds will be backing toward a south-southeast direction across this area, which should enhance wind shear. A modest severe threat should again emerge on Saturday, this time across Mississippi and Alabama, as the upper-level low and surface features continue sweeping eastward. As shown in Figure 1, NOAA/SPC’s Storm Prediction Center has slight risk areas in effect for Thursday, Friday, and Saturday, with the highest localized odds of severe weather at just 15% on each day. (Day 1 tornado odds are even lower, peaking at 2% along the dry line).

The next sequence of severe weather should kick off Sunday in deja-vu fashion across northern Texas and Oklahoma as a second upper-level low moves across, a bit weaker and further south than Thursday’s upper low. It’s not yet clear how much moisture will have returned by then, but a compact severe weather threat late Sunday is certainly possible—as suggested in SPC’s Day 4 outlook—perhaps expanding on Monday into the Mississippi Valley.

Two more upper-level lows are projected to move across the Southern Plains later next week. Models remain in disagreement on how these will evolve, but they may be even further south than their predecessors, so the most focused area for severe weather by that point could end up being Texas. Several bouts of heavy rain appear likely from parts of KS to TX and points east, which might not be a bad thing at all. This week’s U.S. Drought Monitor shows that drought conditions now cover 64% of the south-central U.S., up from 55% last week.

We’ll be back with a new post on Friday.

Bob Henson


Figure X. Total precipitation projected by the GFS model run from 06Z Thursday, March 23, 2017, for the 8-day period ending at 06Z (1:00 am EDT) Friday, March 31, 2017. The exact locations of heavy precipitation over the Southern Plains are likely to vary in subsequent model runs. Image credit: tropicaltidbits.com.