Above: Tornado damage in Monroe, Louisiana, on Easter Sunday, April 12, 2020. (Twitter/@CityofMonroe)
Update (3 am EDT Monday): Intense thunderstorms—some packing giant hail, severe downburst winds, torrential rain, and strong tornadoes—barrelled across the Southeast U.S. on Sunday in an Easter-weekend outbreak of severe weather that took at least eight lives and damaged or destroyed hundreds of homes. More severe weather, including the risk of strong tornadoes, will extend into the mid-Atlantic states through at least midday Monday.
Computer models on Saturday night agreed on the potential for severe storms across a broad area—and on a more concentrated threat of long-tracked, tornadic supercells—but the precise timing and extent of the most dangerous storms remained in some doubt. Among other things, it’s possible that less-intense showers and storms will be widespread into Sunday afternoon, which would reduce instability and tamp down the full mischief-making potential of this otherwise very potent scenario.
Update (12:30 pm EDT Sunday): A well-defined complex of thunderstorms (convection) was moving quickly across southern Arkansas, northern Louisiana, northern Mississippi and Alabama, and western Tennessee. This complex produced several confirmed tornadoes over northern Louisiana on Sunday morning, including one in the Monroe area, where damage to homes has been reported. A research radar based at the University of Louisiana Monroe detected 111-mph winds just above the surface, and about two miles to the southeast, the airport weather station in Monroe reported gusts to 60 knots (69 mph).
In its 12:30 pm EDT Sunday discussion, SPC said: "There is still enough uncertainty with regard to convection outpacing stronger surface-based instability (especially with northeast extent), as well as the overall convective mode, to preclude an upgrade to High Risk."
The two key ingredients on Sunday are a strong, compact upper low being kicked out into the Southeast and a plume of very warm, moist low-level air from the Caribbean and western Gulf of Mexico. Sea surface temperatures across the Gulf last week were at record-warm levels for this time of year, which reflected weeks of unusually hot, dry weather. In turn, the unusually toasty Gulf is helping to warm and moisten the surface air flowing into Sunday’s thunderstorms.
The strongest upper winds and the muggiest low-level air are expected to meet up across the lower Mississippi Valley and Deep South. There, the juxtaposition of extremely strong vertical wind shear and moderate to high instability will likely lead to one or more long-lived tornadic supercells capable of strong, long-track tornadoes. Short-range models and observations on Sunday should help narrow down the threat area, although these models could be hobbled somewhat by a shortage of frequent upper-level wind data owing to the sharp dropoff in flights due to the coronavirus pandemic.
On Saturday night, as the upper low began its trek from Southern California—where it had lingered for days, producing a record-long stretch of wet days in Los Angeles—the upper-level storm triggered severe weather in far southwest Texas. A storm moving across the Rio Grande on Sunday night spat out baseball-sized hail near Del Rio and softball-sized hail just to the east in Brackettville. Eric Webb (tropicaltidbits.com) pointed out that the radiosonde launched from Del Rio at 7 pm EDT Saturday measured winds of 160 knots (184 mph) at the jet-stream level of 250 mb—a sign of the ferocity of the upper-level environment approaching the Southeast.
By Sunday morning, this upper energy will punch out atop increasing Gulf flow in Louisiana, where tornadic storms could develop before noontime.
The greatest threat for strong tornadoes will be from northern Louisiana across much of Mississippi into northern Alabama on Sunday afternoon and evening, near and just south of a warm front. Any supercells here could also generate very large hail and severe downburst winds.
As the severe thunderstorms barrel eastward—perhaps moving at 50 mph or more—they may tend to consolidate into short lines or clusters that could still produce embedded tornadoes long into the nighttime hours. Residents will need to be on particular guard as the storms march into Georgia and north Florida overnight, and eventually into the Carolinas by early morning, as such tornadoes are typically brief and difficult to spot at night yet can still be destructive.
Powerful winds from the upper low will mix down into some of the thunderstorms as the system heads for the East Coast, which could lead to stretches of destructive downburst wind all the way to the mid-Atlantic and Carolina coastlines by early Monday afternoon. Wind shear east of the Appalachians will be more than ample for tornadic supercells, even during the morning hours, assuming that the storms do not progress eastward as a squall line. Some severe storms are even possible as far north as southern Pennsylvania and the Washington, D.C., area, as highlighted by SPC in its Day 2 outlook for Monday.
Along and north of the warm front, a batch of intense thunderstorms could drop rains of 4” to 6” in “training echo” fashion over and near the Tennessee Valley and southern Appalachians. The NOAA/NWS Weather Prediction Center placed much of this area under a moderate risk of excessive rains leading to flash flooding in its Day 2 outlook issued late Saturday and valid from Sunday into early Monday.
For more on the Sunday/Monday severe outlook, see the frequently updated article at weather.com.
A major high-wind and lakeshore-flood event likely in western New York on Monday
This weekend’s powerful storm system is expected to generate a slew of other high-impact weather, including snowfall on its north side from Colorado to Upper Michigan. Perhaps the biggest non-thunderstorm threat is the likelihood of significant flooding on the east shores of Lakes Erie and Ontario, including the Buffalo area, by Monday.
A surface low may intensity at a “bomb cyclone” rate of 24 millibars in 24 hours as it tracks from Arkansas to eastern Canada. Shrieking west winds passing over the lakes, which are at or near record-high levels for this time of year, will push high water against the east shores in the form of a seiche, or standing wave, sloshing from one side of the lake to the other. The NWS office in Buffalo warned that winds could reach 60-70 mph or more across parts of western New York, calling the system a once-in-a-20-to-30-year event.
In an email, WU co-founder and Cat 6 founder Jeff Masters pointed out: “Water levels on Lake Erie, Lake Huron, Lake Michigan and Lake St. Clair all broke records that were set in 1986 in March. Lake Erie is currently above its April record monthly average by 4 inches. Lake Ontario is not record high, but still well above average.”