Quincy

Last check, low-level lapse rates and 0-3km instability are still fairly marginal. I think the most intense discrete action won’t be until 21-00z.

Was just about to say that. Looks like this could be one of the pre-frontal bands developing. Satellite shows filtered sunshine near/south of I-20 in MS. Models are inconsistent, but if any convection can initiate in south-central/southeastern MS over the next few hours, that would pose a conditional risk for a long-tracker.

Still moderate risk, but it was expanded NE a bit into parts of Tennessee.

It’s one potential limiting factor. All it takes is one lone storm in the open warm sector away from other convection to go on to produce an intense long-lived supercell.

That’s effectively the warm front. As that surges north, so does the moist, unstable air mass. So, I don’t think it will play much of a deleterious role.

Granted the storms started a bit earlier than 11 a.m., but I think most of us were/are more focused on the warm sector, where long-lived, intense/violent tornadoes will be possible.

5.1 C/km 0-3km lapse rate... jives with mesoanalysis. A good example of why you don’t necessarily need steep low-level lapse rates when you have low LCLs and substantial low-level shear.

Here’s a closer look of that image, but note that QLCS tornadoes are likely west and northwest of this highlighted area.

I’d say a localized outbreak of tornadic supercells is most probable from central to northeastern MS and northern AL. The ongoing convective activity will probably temper the threat a bit farther NW, but even there you’ll likely see a few QLCS tornadoes and maybe a few embedded supercells. That environment advecting NE from Louisiana toward MS is going to be nasty once you get just a few hours of daytime heating.

Every run of the HRRR since 22z has gradually been backing off. Right now, there’s very little convection at all. FWIW, the HRRR also shows gradually less convective blobbing along the warm front tomorrow morning. My guess is the tornado action could start as early as late morning on the SW/S flank of early day storms, probably in Louisiana, but possibly as far west as extreme eastern Texas. Arkansas (possibly eastern Oklahoma?) is interesting with funky wind profiles that could result in hybrid tornadic supercells. MS/W AL is still a wildcard area with many possible scenarios. Overnight tomorrow still really concerns me across a broad area from MS to GA, whether it be a broken line of supercells or a massive squall line with numerous QLCS tornadoes Overall, given the background environment, any convection near or south of the warm front needs to be watched as early as 16-17z. Even with messy storm modes, you can get strong tornadoes with so much low-level shear.

It has some utility, but not always. It gives you a good idea of where the background environment would favor tornadoes, but it doesn’t really distinguish between storm mode (supercell vs. QLCS for example). Usually it exaggerates the threat between 06-12z due to the LCL and 0-1km terms skewing during that time frame, however I think this situation is a outlier, where nighttime and morning tornadoes could be fairly widespread.

Sometimes for continuity they use the same text and only tweak some wording details. I’ve noticed it a bit more lately.

Let’s not forget that meteorology > modelology. Given the environment around warm sector convection, I wouldn’t take NAM radar simulation as gospel, especially 51-54 hours out. It’s still interesting...

At first it was the global models shifting west and the NAM lagging behind. Maybe it’s just now catching up.

IF the 18z NAM 3/12k suites are to be believed, the westward trend might be continuing.

Aside from model noise, the signal for a significant and potentially higher end event remains. There was a slight westward tick yesterday, but I don’t think the threat area will change all that much going forward.

Not sure I agree with this. Even from 21-00z, when the mid to upper level wind profiles become rather unidirectional, low level shear remains highly favorable for tornadic supercells. Point forecast soundings over most of MS and vicinity exhibit a classic, large, sickle shaped hodograph in the lowest 1-2km. Pre-frontal/warm sector wind profiles out ahead of the shortwave maintain even larger hodographs longer.

There are failure modes for sure. If there’s too much convection too early, that would also throw things off. To get a major outbreak, the needle really has to be threaded just right. Low-level instability is one limiting factor. I’m mostly concerned because global and mesoscale models alike show CI in the unstable/highly sheared warm sector, which is what you’d expect to see during an outbreak. How high will the ceiling be? We’ll see. I’ve been burned for being too bullish on occasion. All it takes is one... Let’s hope that for the sake of the local residents, this does perform below expectation.

6.8 C/km is the average 0-3km lapse rate for significant tornadoes: https://www.spc.noaa.gov/publications/mosier/2018-JOM1.pdf Taken verbatim, the NAM shows 6-6.5 C/km lapse rates with around 100 J/kg 3CAPE through much of the risk area. Given the wind profiles, that’s definitely supportive of strong, long-track tornadoes. Maybe if the low-level lapse rates were steeper we’d be talking about a historic event. I would not go there yet, but I think to argue against a tornado outbreak at this stage is a stretch.

Uh, that’s a sounding from hour 59 in NW Mississippi. I see plenty of warm sector CI by 19-20z. (Including cells directly over the 33.82, -88.65 data point at 23z if you look closely. I would normally advise against detailed sounding analysis of the 59hr 3km NAM, but FWIW:

It’s a convectively contaminated sounding at 23z that’s near a squall line associated with a cold front. The NAM shows multiple rounds and areas of warm sector storms, to the east and several hours earlier. I don’t think low-level lapse rates are a big issue here. The models show a relatively uncontaminated warm sector around 18-21z. You can get QLCS tornadoes even with the surging cold front, but I’d be more focused on the warm sector and any pre-frontal trough.

CIPS analogs show a relatively high correlation with tornadic events based on the 00z GFS. 4/27/11 is #2 and yes, some other analogs are much more tame. Percent of analogs with at least one long-track tornado:

I think it’s more about placement. To be clear, I’m not seeing any significant trends with respect to speed and orientation of trough amplification. It’s more about a gradual westward tick. Hopefully that makes sense. I’m trying to make sure I don’t get too hung up on specifics either.

Want to try to not get too specific on details this far out, but the synoptic signal is there for a potentially higher-end event. There are some key differences, but 4/27/11 is ranking as a relatively close analog at this stage. Speaking of details/trends, the GFS/Euro have been trending a bit slower and to the west. That’ll be something to keep an eye on, which may put AR/LA more into the action, as opposed to AL and areas farther east. If the system slows a bit and remains more neutrally tilted as opposed to negatively tilted, that’s setting off more alarm bells. The shortwave is looking somewhat more progressive and compact than some historical major events. 4/27/11 and 4/28/14, for example, were more amplified with upper level lows cutting off. If the progs are correct, a northwesterly 100kt 500mb jet will spell trouble. Remember your MS vicinity climo. Mean MLCAPE for tornadoes is only ~500 to 1000 J/kg and 1000 to 2000 for significant tornadoes. Global models and longer range NAM get you into the latter camp. Wind profiles speak for themselves, very impressive at face value. Global models showing a string-of-pearls QPF signal ahead of the cold front is alarming, especially if there is a pre-frontal trough. I try to not look at forecast soundings too much, but a warm nose/cap around 850-750mb during the first half of the day would suppress precipitation and with cooler temperatures aloft penetrating, that’s why you’re seeing seasonably large instability profiles. Summary: Don’t get hung up on details. The synoptic signal is alarming. Watch for a slower/westward trend.