Late-Winter Severe Weather on Tap Across Southeast; Strong Tornadoes Possible

A compact, potent streak of upper-level energy diving into the Southeast on Tuesday is expected to trigger widespread severe weather from Tuesday afternoon into Wednesday, with the potential for a regional tornado outbreak across the Deep South late Tuesday. The set-up has some of the earmarks of earlier outbreaks this winter that have brought intense storms to the central Gulf Coast and Florida, but in this case the dynamics in play are especially strong. At midday Tuesday, NOAA’s Storm Prediction Center (SPC) was calling for a moderate risk of severe weather from far eastern Louisiana across southern Mississippi and Alabama into far southwest Georgia and the western Florida Panhandle. Strong tornadoes are possible across the moderate-risk area. A larger slight-risk area surrounds the moderate risk (see Figure 1).

Severe weather had already kicked off just after noon CST Tuesday in southeast Louisiana, as an apparent tornadic waterspout formed over Lake Ponchartrain. Waterspouts often develop from relatively modest lines of cumulus clouds, typically moving quite slowly, but they can also emerge from severe thunderstorms in much the same way as land-based tornadoes do. These tornadic waterspouts can move quickly onshore and pose a much greater threat to coastal communities. A tornado watch was issued late Tuesday morning across most of southern Louisiana, with more watches sure to follow further east.



Figure 1. Areas of severe weather anticipated by NOAA’s Storm Prediction Center for Tues., Feb. 23 (left, extending through 7:00 am EST Wednesday) and Wed., Feb. 24 (right, extending through 7:00 am EST Thursday). The strongest activity is expected late Tuesday near the upper Gulf Coast and on Wednesday afternoon over eastern North Carolina. These outlooks were issued at 11:30 am EST (left) and 12:30 pm EST (right) on Tues., Feb. 23. Image credit: NOAA/SPC.


Today’s watchword: dynamics
The upper-level feature heading toward the Gulf Coast is a small but very energetic lobe at the base of a larger upper trough moving across the central states into the Midwest. A ribbon of jet-stream-level winds exceeding 120 mph, over the central Rockies early Tuesday, will be diving into the southeast US by late Tuesday. The lobe will spin up a powerful surface low expected to track from Louisiana on Tuesday afternoon to western Ohio by Wednesday, pushing a cold front across the Southeast. Warm, unstable air from the Gulf of Mexico will struggle to make it very far northward before the front arrives, but moderately unstable air with dew points above 60°F should envelop most of Mississippi and Alabama by Tuesday night. A narrow wedge of instability will shoot up the East Coast on Wednesday, perhaps making it north of the Washington, D.C., area by evening.


Figure 2. Jet-stream winds at the 250-mb level (about 34,000 feet) predicted by the GFS model in its 24-hour forecast valid at 7:00 am EST Wednesday, Feb. 24, 2016. Very strong winds will be in place across the severe weather risk area late Tuesday night. Upward motion favoring supercells will be maximized near the upper-level jet streak, or across southern Mississippi and Alabama and the western Florida Panhandle late Tuesday night. Image credit: Levi Cowan, tropicaltidbits.com.


In some cases, the modest amounts of instability on Tuesday and Wednesday might produce only garden-variety thunderstorms. However, in this case the relatively low instability will be more than compensated for by extremely strong dynamics. It is rare for an upper-level feature this strong to travel so far south across the upper Gulf Coast. As the surface low organizes well to the northwest, vertical wind shear will dramatically increase across the moderate risk area, as winds strengthen at all levels and veer from southeasterly near the surface to southwesterly just aloft. The wind shear will be approaching levels observed in some of the stronger Gulf Coast severe weather outbreaks on record. WU contributor Lee Grenci has a new post explaining why the low-level jet stream is especially important on a day like today in generating storm rotation.

At midday Tuesday, Weather Channel’s Dr. Greg Forbes raised the TOR:CON level to 8 across the SPC’s moderate-risk area. This means there is a 80 percent chance of a tornado within 50 miles of any location in the specified areas.

Residents of the upper Gulf Coast within the moderate risk area who lack adequate shelter at home would be well advised wherever possible to identify potential safe-shelter areas before nightfall. Storms will be moving and evolving at an increasingly faster pace as this evening unfolds. The most dangerous tornadoes are most likely to occur in any isolated supercell thunderstorms that develop ahead of the main front. However, strong winds and brief tornadoes are also possible with a squall line or line segments that should develop near the front later tonight.


Figure 3. Simulated radar reflectivity from the 14Z Tuesday run of the HRRR model, valid at midnight EST Tuesday night, Feb. 23, 2016. Even at this late hour, powerful thunderstorms should be rumbling across parts of Alabama, western Georgia, and the Florida Panhandle. Image credit: NOAA/ESRL.

Eastern NC at risk of strong tornadoes on Wednesday
Wednesday’s severe potential will depend on part on how extensive and long-lived Tuesday night’s storms end up becoming. However, there is some confidence that eastern North Carolina will be under the gun, given the timing of the system (it will be approaching eastern NC by mid/late afternoon, when surface temperatures and instability would normally be peaking). In its Day 2 convective outlook for Wednesday updated at 1:30 pm EST Tuesday, SPC introduced a risk area of significant tornadoes over eastern NC.

Follow the severe weather on our live blog
We’ve launched a WU live blog to provide frequent updates on the severe weather this afternoon and evening into Wednesday. You can get to the live blog from this link and from the “Tornado Risk” box on the right hand-side of the WU home page.

Bob Henson