2018-03-19 EVENT: TN/MS/AL/MO

[Dan Robinson]

A potent event seems evident on the latest 78-84 hour output on the NAM/GFS. A fast-moving negatively-tilted shortwave embedded in the broader westerly flow ejects across Dixie with a 90kt midlevel speed max overspreading mid-60s dewpoints. Good low-level turning and MLCape shown approaching 3,000 across MS and AL by mid-afternoon results in an evidently volatile environment ahead of the cold front.

There are not a whole lot of negatives about this setup (other than its overall geographic location) except for a rather anemic-looking EML that might result in an early-day junkvection-fest.

The moisture axis is shown wrapping all the way back into the surface low in Missouri, which might offer a viable secondary target if breaks in the clouds allow a little sunlight early in the day.

Right now the distance to the primary target area is my main hesitation, but that surface low play may prove tempting enough to get me out the door on this one.

 

[Ben Holcomb]

I've been watching this on the GFS for a couple days and it looks like a classic southeast outbreak. Instability is limited into Tennessee which may limit the huge tornado potential, but MS/AL during the afternoon hours looks pretty brutal. I'd expect a somewhat early show as well.

I have the day off Mon and Tue and want to play out there but will be unable to make this setup due to vehicle situation. Right now I'd be starting around Memphis at noon on Monday.

 

[Justin Bailey]

Yes, this mornings 12Z NAM caught my attention and immediately called over a colleague to discuss as we both live in middle TN. Needless to say, going to be watching future model runs very closely. As of right now I'm seeing deepening negatively tilted system, rapidly intensifying low-level jet, strong wind shear profiles, abundant sfc moisture, mid-level drying owing to breaks in cloud-cover, CAPE1500-3000, helicity values >500, 0 cinH...I have to keep telling myself this is just one model run, however, if this were to verify it has the potential to be a rather potent event. If I were to attempt this, Id drive south on I-65 into N Alabama and attempt to chase in the relatively flat and barren northern portion of the state. If it slows down some then Mississippi River flood plain may come into play, however, the swath of chaseable terrain east of the river is quite narrow and then its rather miserable from there until you cross the TN River and get into some sections of the Nashville Basin or up onto the Cumberland Plateau. There is a section of the plateau in South Central KY that is a pretty decent option as well as it is quite flat from Hopkinsville to a little east of Bowling Green.

 

[Brandon Centeno]

Far from a certainty or slam dunk. Upstream convection tearing apart the EML, early convection, and some degree of uncertainty regarding low level mass response and associated wind fields all seem like flags to me from a chasing or forecasting perspective. Many details to be resolved. High end potential, sure, but also the potential that mitigating factors turn this into a run-of-the-mill spring severe event in the southeast.

Also have to keep in mind that the 12z NAM suffered big time from convective feedback, which made those composites, parameters, wind fields etc. entirely over-done. 18z NAM not nearly as potent, with wind profiles that aren't quite as impressive. I would think something closer to the 12z NAM verifies than does the 18z NAM, just based on synoptic pattern/evolution, but we shall see.

 

[Quincy Vagell]

I'm trying my best not to over-analyze this setup, since it is still three days out and the forecast has been evolving. Regardless of any model noise (12z vs. 18z), it currently has the look of at least a notable March severe weather event across Dixie. Just look at the big picture, with a compact closed low punching east across the Arkansas vicinity and a ~996mb low ejecting into Tennessee. It won't take much to produce with that setup, barring complete convective destruction of the environment, but there are reasons to believe there will not be substantial convective overturning, especially south of the Tennessee border into Alabama and surrounding areas.

The NAM is extremely bullish with the EML progs, as it shows 7 to 8 C/km mid-level lapse rates across a sizable portion of AL/MS/TN, Dry mid-level air seems to be a factor, allowing for the profile to cool in the mid and upper levels, supporting enlarged CAPE values.

Just eyeballing some rough data from the 12z Euro supports an extensive area of 6-6.5 C/km lapse rates, which is closer to the GFS. I'd take a blend of the Euro/GFS and NAM to get a better idea of thermodynamics (can't just rely on global models this far out, but the NAM at 72-78 hours out also needs to be taken with a grain of salt). Even so, unless there is a massive MCS Monday morning, it seems probable that much of Alabama and surrounding areas will see CAPEs into at least the 1000-2000 J/kg range by early afternoon. Given the expected wind fields, that is more than enough to support severe.

Even if there are a few wrenches in the mesoscale details, I think severe hail, some significant, will be the most obvious concern, given the cold air aloft and even the more conservative lapse rate progs. About 20% of the CIPS analogs featured significant severe hail in northern Alabama.

More than half of the analogs show a severe weather event (above the March garden variety) and a few do suggest outbreak potential. With that said, hail probabilities (strictly from analogs and thermodynamics) appear much greater than tornadoes. I'm not saying the forecast soundings don't support tornadoes, they certainly do, but I think we'll see fairly widespread severe hail if the latest model progs are close to right, along with at least a few tornadoes. Indeed, of the analogs had a few tornadoes, but not many were downright outbreaks. The difference between an ENH/MDT risk-type tornado event and something higher end will likely boil down to how large of an area is able to sufficiently destabilize. Right now, the consensus seems to be for a messy setup over Tennessee (clouds and ongoing convection) with higher potential for moderate buoyancy being realize over central/northern Alabama.

Comparing this setup to some previous events, I think that drier air in the mid-levels could be a positive, allowing for more cooling aloft (larger buoyancy) and less potential for a convective mess to ruin the event early on. Dixie has had plenty of messy severe events with moist thermodynamic columns, but the difference between an outbreak and a more run-of-the-mill March event will come down to lapse rates and how much clearing is able to take place in the warm sector. I still can't help but double-take at the NAM lapse rates, even if they are almost certainly overdone.

Let's keep an eye on this setup and see how it evolves. Unless there is a complete model failure, severe thunderstorms will occur in Dixie on Monday. If the models trend a bit slower (as they have a bit over the past 24-36 hours), you may see the threat zone focus more on the Mississippi Valley region. While this would be better for chase prospects, I don't think we'll see much of a slower/westward trend, as time is running out for large shifts and the models appear to be converging. Likewise, if the system speeds up and shifts east, it could move into even worse terrain/metro areas, or see less overlap between favorable kinematics and thermodynamics. I think the models are honing in on a solution and the northern half of Alabama is my main focus, possibly beginning by midday/early afternoon as Ben eluded to, over northern/eastern Mississippi. Tennessee is more of a wild card. The synoptic setup is considerably different than 2/2/16 (which was also about six weeks earlier in the year [thermodynamics]), but that event was hyped up in Tennessee, however convective junk/lack of northward extent with the warm sector essentially resulted in a non-event there, while a localized tornado outbreak affected parts of AL/MS.

I'd also reference 3/13/16 for what a compact shortwave trough can do with respect to significant hail and tornadoes over Dixie, even though that event was farther west (Arkansas) and had a somewhat less impressive warm sector than what is being modeled for Monday.

 

[Quincy Vagell]

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.

 

[Mike Johnston]

I like Russellville AL as a target. Best combination of instability and wind shear. Plus, it looks to be right at the nose of the theta-e axis. A lot of precip looks to break out across the whole region, though. That is my main concern with the overall setup.

 

[wfrogge]

This system is just east of my back yard and I know the area quite well. Target area for me will be a compromise of models and terrain. Tupelo south to Meridian and east into far west Alabama is extremely flat, lots of farmland, and not too many trees. The better dynamics right now appear over NW Alabama and terrain here can be hit and miss. Florence AL is not that bad go 20-30 miles either direction and you can be in trees (or the Tennessee river). Will not decide until 6 AM Monday on a final location.

 

[Ethan Schisler]

I'm planning on chasing this event as of right now unless something changes logistically in the next 8-12 hours. Probably target somewhere around the TN/AL border toward mid-late afternoon. I've chased this area before in the last several years and its obviously not the best chase terrain, but if you stay close to the interstate you'll have decent cell data and you can intercept the storms as they move toward you. East of there and its pretty dicey in terms of cell coverage/terrain. Northeast Mississippi isn't much better either, there are spots further SW that are decent for chasing, but that will probably be out of the coverage area for this event. I would say my biggest concern is keeping cell service, I chased Louisiana a couple times last year and it was awful.....never again.

 

[Jeff House]

I recommend targeting North Alabama, but not northeast AL, based on both terrain and meteorology today. As some have noted northeast Mississippi is not great terrain except the narrow Corinth to Tupelo corridor. On the other side Huntsville east begins hills and trees. North-central Alabama offers flat agricultural areas near the Tennessee River and decent roads. Just plan for the River.

Middle Tennessee has a tornado risk too, but likely the low contrast sloppy variety. Clearing will happen later and visibility should remain lower than in Alabama. Middle Tennessee has a couple swaths of less awful terrain through Murfeesboro and Manchester, but they are divided by hills in between.

In contrast with Tenn, the warm sector should be more firmly established in Alabama. Visibility should improve by afternoon. Close to the warm front supercells will have plenty of SRH. The first 2-3 cells south of the WF should as well, but I'd target the first one south of the WF. Morning rain is exciting my target area, north-central Alabama. Outflow OFB should remain, even if the synoptic WF jumps into Tenn. I still like north-central Alabama with decent flat areas and roads. Plus storm motion is forecast nearly west to east, which due to terrain orientation favors North Alabama over Middle Tenn.

Again in North Alabama plan for the Tennessee River. Both US-72 and Alt-72 are nice west-east roads; however, they are on opposite sides of the river with few crossings. Just plan with the meteorology and OFB position.

 

[Mike Johnston]

There we go. A large and dangerous tornado reported near Russelville

 

[Dan Robinson]

LLJ kicking this event into overdrive after dark. Jacksonville, Alabama has apparently been hit by two significant tornadoes within minutes of each other:

https://twitter.com/stormhighway/status/975913518624014336

 

[Jamie G Cox]

LLJ kicking this event into overdrive after dark. Jacksonville, Alabama has apparently been hit by two significant tornadoes within minutes of each other:

https://twitter.com/stormhighway/status/975913518624014336

As for the link, the 1964(?) Palm Sunday Outbreak had two F4 tornadoes go through several counties, including Hillsdale County, Michigan, where I am from. They were on almost the same path and within 30 minutes of each other, if I remember correctly. Both F4, although I think many dispute that they may have been larger. They were the same tornadoes that started in Indiana and caused the Elkhart double tornado and they went all the way to almost Lake Erie.