Above: The severe weather outlook for Sunday, April 12, 2020, issued early Saturday by the NOAA/NWS Storm Prediction Center. Shown is the overall risk level (left) and the probability of tornadoes (right) within 25 miles of any given point. The hatched lines indicate where there is at least a 10% chance of significant tornadoes (EF2 or stronger). (NOAA/NWS/SPC).
Conditions are aligning for a volatile weekend of severe weather from Texas across the Southeast, including the potential for strong tornadoes on Sunday. Update: In its Day 2 outlook for Sunday updated at 12:30 p.m. CDT Saturday, the NOAA/NWS Storm Prediction Center placed the area from northern Louisiana across most of Mississippi into western Alabama under a moderate risk for severe weather, the second highest of the five risk categories maintained by SPC.
"Strong tornadoes, potentially widespread damaging winds, and large hail are all possible," SPC warned. "There is too much uncertainty to upgrade the ongoing outlook at this time," the center added, implying that an eventual upgrade to high risk could occur on Sunday if the ingredients come together as they might.
The potential outbreak will emerge from the juxtaposition of a winterlike upper-level storm system with a flow of summerlike low-level air. The Gulf Coast region has been simmering under relentless heat the last few weeks, leading to the warmest March on record for a number of U.S. locations (as well as many parts of Mexico on the other side of the Gulf). The developing storm system this weekend will pull another rich feed of warm, moist air into the Southeast. Because of the powerful winds and high water-vapor content expected with this moisture channel, it may end up qualifying as a strong atmospheric river—the type of narrow corridor that can fuel very heavy rainfall.
Meanwhile, a cold upper-level low that has been dumping impressive amounts of spring rain and mountain snow over Southern California for most of the week—including 9.85” of rain at Lytle Creek and 34” of snow at Mountain High ski resort—will get kicked eastward this weekend by an impulse in the polar jet stream that will be diving through the Pacific Northwest. The upper low will race across Texas on Sunday, with fierce southwest winds ripping ahead of it. The low will become positively tilted (oriented from northwest to southeast), which tends to favor diverging winds aloft and especially strong upward motion just to its east.
At the jet-stream level (250 mb or about 34,000 feet), a pocket of winds exceeding 160 mph may develop over Texas early Sunday and spread into the Deep South later that evening.
These ingredients will likely combine to create a high-end environment for rotating supercell thunderstorms that could spawn long-lived tornadoes. The exact timing and location can’t be pinned down this far out, but here’s one plausible scenario (updated early Saturday):
—Several clusters of severe thunderstorms could move across Texas and western Oklahoma on Saturday, perhaps generating very large hail.
—A more intense complex of severe storms may erupt later Saturday night across southwest Texas and move quickly eastward, reaching southeast Texas early Sunday morning. These could be unusually potent storms for the overnight hours. Strong downburst winds and very large hail will be possible, and embedded tornadoes aren’t out of the question. SPC has issued a Day 1 enhanced risk of severe weather across parts of central and south Texas for this threat.
—Supercells with the potential for strong long-lived tornadoes could develop as soon as late Sunday morning in parts of Louisiana, well ahead of a strengthening cold front and near and south of a warm front, as lower- and upper-level winds intensify and instability increases. The zone of greatest concern will shift during the day into Mississippi and western Alabama. By late in the day, storms may tend to consolidate into intense lines or clusters as they race across eastern Alabama into Georgia. If so, these would be less likely to spawn long-lived tornadoes but could still produce brief, damaging twisters that are difficult to warn for, as well as large hail, torrential rains, and extremely strong downburst winds. Note that tornadic supercells will still be possible in this region, even well into Sunday night.
"The environment for this event is comparable to the most severe weather outbreaks in Georgia history," warned the NWS office serving the Atlanta area on Saturday afternoon.
—Update (6 pm EDT Saturday): Models are increasingly suggesting that conditions may remain favorable for widespread damaging winds and possible tornadoes during the predawn and morning hours Monday across much of the Carolinas, as the unusually warm and moist air mass flows northward beneath the extremely intense upper winds. Severe weather could even extend to the mid-Atlantic, including the Washington, D.C. area, by early Monday afternoon, as reflected in an unusual midday update to the Day 3 outlook issued by SPC for Monday.
North of the warm front, a large complex of intense thunderstorms will sweep across the Tennessee Valley. Powerful winds pushing upslope against the Southern Appalachians will squeeze out plenty of moisture. Though these will be fast-moving thunderstorms, any “training” of echoes could produce corridors of 3” to 6” of rain near and north of the warm front from northern Mississippi to western North Carolina. On Saturday afternoon, the NOAA/NWS Weather Prediction Center placed this area under a moderate risk of flooding for Day 2 (Sunday into early Monday).
"Both precipitable water and Integrated Vapor Transport values are forecast to be at or near record levels for mid-April," WPC noted.
What could go wrong (for severe)?
Although this weekend’s set-up appears likely to have all of the ingredients needed for a high-end severe outbreak, we can always root for “failure modes” that might diminish the potential. For example, the rich infusion of moisture might lead to widespread thunderstorms early in the day Sunday that defuse the potential for stronger storms later on. Computer models suggest that a very warm layer of air about one to two miles high will sweep across the risk area, capping off development until instability is maximized later in the day, but such caps are difficult to predict accurately this far out.
One other thing to keep in mind: computer models—especially the short-range ones that are updated hourly—may struggle more than usual in depicting the weekend’s severe weather modes, owing to the current lack of upper-air data from the massive reduction of air travel prompted by the coronavirus. For more on this topic, see the weather.com article I cowrote with Chris Dolce.
Don’t let the coronavirus stop you from seeking tornado shelter
Debate has spiked in recent days over the challenges of group sheltering from severe weather while the coronavirus pandemic is still raging across the United States. Despite concerns around social distancing in a shelter environment, the emerging consensus is that taking the safest form of shelter in prudent fashion should be the top priority if you find yourself in a warning.
The American Meteorological Society weighed in on Thursday with an official statement, “Tornado Sheltering Guidelines during the COVID-19 Pandemic”. The society stressed: "Do not let the virus prevent you from seeking refuge from a tornado…If a public tornado shelter is your best available refuge from severe weather, take steps to ensure you follow CDC guidelines for physical distancing and disease prevention."
The statement also stresses this important point: “Many communities have announced that they will not open public storm shelters during the pandemic. If you rely on public shelters, like schools, stores, or community facilities, determine if that shelter will be available during the COVID-19 pandemic. This information can be found through websites and official social media accounts or by contacting your local emergency management agency.”
For more on how state and local officials are handling this convergence of public health issues, see the weather.com article by Jan Wesner Childs. (Disclosure: As a member of AMS Council, I assisted with finalizing the AMS statement.)
A fresh look at F5/EF5 tornado history
In a post on Thursday, WU weather historian Christopher Burt put together a definitive summary of more than 100 tornadoes that experts have rated as F5 or EF5 on the Fujita and enhanced Fujita scales, going back to 1880. It’s well worth checking out if you haven’t seen it yet. Among the noteworthy statistics: just 20 states have generated all of the nation’s F/EF5 tornadoes on record. Can you guess which states those are? (See the post for the answer.)