The overall setup has not changed much, but there appears to be better confidence in mesoscale details. I think that the potential for tornadoes, including significant tornadoes, has increased. Earlier there were concerns about low level wind fields, warm sector details, etc., but the model progs are basically unanimous this morning with a volatile setup unfolding Monday afternoon across northern Alabama and adjacent portions of northeastern Mississippi and lower Middle Tennessee. This is evident in 12z convection allowing models, which all show semi-discrete and discrete supercells Monday afternoon in the area.
Thermodynamics:
Some of the early NAM progs for exceptionally steep mid-level lapse rates (nearing 8 C/km) were indeed overdone, but the models have converged on a consensus that is both seasonally impressive for the Tennessee Valley and supportive of significant severe weather. The model consensus is for these lapse rates to be around 7 C/km during peak heating with most of the enhanced risk zone in the 6.5-7 C/km range. This, when combined with surface heating will likely create unseasonably large CAPE profiles. The high resolution models show ML/SB CAPEs in excess of 2000 J/kg throughout much of the risk area.
Kinematics/upper level support:
A compact shortwave/closed low is progged to transverse across southern Missouri Monday afternoon, placing the right front quadrant of the mid/upper level jets over the Tennessee Valley. Timing was waffled back and forth as earlier it looked like the system was about 3-6 hours too fast for a significant outbreak. However, model runs since last night had shifted back west a bit, increasing confidence in the forecast. Deep layer shear will be more than adequate for supercells (>50 knots) and low level shear will also be favorable for tornadic supercells.
In general, I think this has the makings of a local severe weather event (low-end outbreak) on Monday. Not a large scale outbreak, but I would be surprised if there were not numerous tornadoes and at least a few reports of significant hail. Mesoscale details still need to be fine-tuned a bit with respect to strong tornado potential, but I'd keep closest watch on north-central/northwestern Alabama and far southern Tennessee. The 3km NAM and especially 12z HRRRX show some morning convection re-enforcing a NW/SE oriented warm front across northern Alabama. This keeps the wind flow relatively backed across the region, meaning we can expect more of a S to SSE near-surface wind profile, as opposed to some earlier concerns of more of a SSW low level flow.
Convective mode appears to be mixed over Tennessee, where ongoing convection may lead to cell mergers and limited windows for long-lived discrete supercells. Cooler temperatures aloft will help offset somewhat less impressive boundary layer heating, but minimal capping means mid to late morning convection may simply continue, while the surface low catches up to the warm air advection activity. The bet shot at a discrete supercell or two will probably be in far lower Tennessee, where surface heating will be stronger and ongoing convection may be limited around the time of early afternoon, particularly near the warm front.
Discrete/semi-discrete supercells seem probable from northeastern Mississippi into northern Alabama in two regimes. The early day convection will likely help establish the moisture/CAPE gradient in the region and could result in throwing an outflow boundary or two into the mix as well, that may result in a few pre-frontal supercells in vicinity of the warm front. Some relatively dry air aloft should work to cool mid-level temperatures and result in a modest cap, which will be breakable by mid-afternoon as a cold front approaches. The importance of the subtle cap is to limit midday/early afternoon convection and allow surface heating to boost CAPE values.
A conditional threat for supercells may exist farther south from east-central Mississippi into central Alabama. Here, forecast soundings show a sizable cap that may not be breached until late afternoon. Although the warm sector will be largely undisrupted here, it is unlikely that any discrete cells will form until stronger forcing arrives with the cold front. While low level flow will be veered somewhat in comparison to areas farther north, buoyancy will help offset the somewhat smaller hodographs and can still support large hail and isolated tornadoes.
In summary, I would expect a broken line of supercells, some intense, to develop by mid-afternoon from lower Middle Tennessee into northwestern Alabama and northeastern Mississippi. Out ahead of this activity, there may be embedded supercells in the vicinity of ongoing convection in Tennessee, as well as the threat of a supercell or two in north-central to northeastern Alabama, near the warm front. By late afternoon, watch for supercell potential farther south into west-central Alabama, although this threat is more conditional and becomes less certain with southward extent. The best overlap of parameters and highest probability of discrete supercells should be in the northern Alabama vicinity. A tornado threat may continue into the evening to the east, but storms should tend to outrun favorable thermodynamics into eastern Tennessee and northern Georgia, so damaging winds and brief spin-ups may become the main threats after dark.
The event is only about 24 hours away, so do not expect much change on the synoptic level. The closed low could trend to slow down just a tad and honestly, any minor slow-down would only enhance the significant severe threat. The most volatile analogs do show about a 6 hour slow down in comparison to the 12z Sunday modeled position of the system at 00z Tuesday, but I will be clear that even as currently advertised, this has the look of a low-end severe outbreak over the Tennessee Valley.
Chasing prospects are mixed, as the terrain and road networks over parts of northern Alabama are workable. The area is less favorable just to the west (northeastern Mississippi and far northwestern Alabama) and to the east (approaching the Appalachians). The road network is not too friendly over Middle Tennessee, but I am sure locals may have a few spots in mind there.
I've gone back and forth on chasing and will wait until later today to make a call, but the latest model trends seem to be leaning toward a more significant event here.
