2019-04-13 EVENT: TX/AR/LA/MS/AL

Jesse Risley

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Apr 12, 2006
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Since no one else started a discussion yet, I'll post some brief thoughts after perusing the previous few model runs.

A fairly classic, potent SVR weather episode looks to be shaping up for Saturday for the ARKLATEX region, in particular. By early Saturday, models are indicating a system taking on a more negative tilt as a stalwart mid-level perturbation begins to overspread the western Gulf region. A surface low is progged to deepen, and while not a substantially compact cyclone per se, as it tracks proximal to the Interstate 30 corridor, ample moisture this time around should yield more than adequate surface instability AOA 2000 J/KG across the warm sector ahead of the approach of large scale forcing for ascent. Short-range models are yielding particularly favorable surface flow and ample directional shear for a heightened tornadic risk. CIN begins to erode across the open warm sector after 15z, as the main surface forcing begins to move into EC Texas as the surface low peregrinates NNE.

While there were and may still be some early concerns about CIN eroding too quickly given ample WAA and ongoing ascent, allowing too much convection, models seem to be indicating some convective initiation in the late morning and early afternoon across far E TX, WC LA and far S AR in areas of greatest destabilization with increasingly favorable 0-6km shear values. Ongoing convection in the warm sector, as depicted by most CAMs for several days, may also molest the northward extent of the surface warm frontal placement, perhaps further south than indicated by the 12z/12 HRRR and NAM runs. Several bands of convection appear likely, including a squall line INVO of the main surface forcing, following initial, possibly discrete supercellular activity. Mid-level lapse rates and ambient shear values will also support an attendant threat of some large hail and damaging winds with this activity, and there is also another, secondary interest of tornadic potential with initial, discrete storms that interact with the warm front itself, including some initial development indicated on the HRRR across far W Mississippi, as well as embedded mesovortices within the main line given the ample 0-1 km SRH values that increase with time as the LLJ strengthens. The only other caveat I noted on the recent two runs of the short-range models, particularly the NAM, was a limited scope of more widespread, favorable low and mid-level lapse rates, though it would appear there will at least be a narrow corridor where ingredients across the warm sector do want to align for heightened tornadic potential, particularly east of US 59, south of I-30 (or wherever the surface WF chooses to lay sprawled), and west of the Mississippi River.
 
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Kyle Whitley

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Jul 26, 2018
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I know some of the models have been iffy this year, but I feel good enough to drive to LA after work tonight from Eastern NC and see what's going to happen. I too was concerned about the lack of widespread Lapse Rates. Based on what I've drawn up for the last few days on CAM runs I'm shooting for the Ruston LA area to target, which I narrowed down by the 7C lapse rate boundaries. I'd like to start there anyhow as I think discrete imitation will start just South and West of there and will give me a good opportunity to watch and see what cell(s) I want to get on.

I'd love to have other's input on if they think I'm in the ballpark or not area-wise. I'm beginning to wonder if I should be initially positioning myself further South than that??
 
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Dan Robinson

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Powerful and classic-looking spring storm system on Saturday in the South that would normally be "yes" for chaseworthiness if not for its location. The issues with this setup:

1.) some of the parameters (lapse rates being a big one) are shown peaking out early in the afternoon and waning toward evening, as they sometimes do with early season Dixie events.

2.) CAMs are showing rapid upscale growth or at least enough to have storms close together with seeding/HP issues. Despite the favorable shear, very strong forcing will work against storms staying isolated.

3.) Northern Louisiana has atrocious terrain and trees, close to the worst you'll see in the South. Satellite images show very few clearings. I did some Street View surveying - this is representative of the secondary roads:


Summary: no question that this is a strong tornado setup, but looks very challenging in terms of chaseability.
 
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Kyle Whitley

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Jul 26, 2018
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The only saving grace (for this South East Boy) is that once you break East of Monroe, there seems to be a good 100 mile stretch of pretty open country in the Mississippi Basin region, all the way to Yazoo City MS. I-20 looks clear(ish) and is a good E-W route but no real good N-S option until Monroe.

I've only chased in the South East US and never in LA - W Alabama is it, so I'm fairly worried about the topography there if I'm honest. I don't know that I can pass up a day like this though. Thank you for your insight Dan.
 
Jun 1, 2008
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Two escape routes is an absolute must in the Delta. One does not want to get backed up on the Mississippi River only to find the south route is flooded. Chasing near a Mississippi River crossing is pretty much necessary.

Coming from the east, will target the flat mostly tree-free Delta. Initial target is Greenville, Miss. Will evaluate on the way in TUS or JAN stop whether to shift with WF. Will err on the north side. Easier to work south than rush north with fast NNE storm motions.

Parameters are higher west of my target but obviously it's not chasable. Looking for supercell development on a prefrontal trough lifting from Louisiana into Miss. Now appears likely the Delta will have supercells during daylight hours.

Neatness remains a question with the 500/200 mb wind forecast. Otherwise it's a good hodo with excellent low level shear. Instability is actually pretty robust for that far south, usually lower than in the Plains. Latter two could overcome upper hodo.
 

Jesse Risley

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As an aside, after having a better look at some of the recent runs, I've noticed some of the forecast skew-T profiles on the NAM runs aren't showing as much turning between H85 and H5 further into the open warm sector in LA. It would be nice to see a bit more turning there, though there are definitely much better directional shear profiles right proximal to the warm front.
 
Jun 16, 2015
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My take is that this event will probably feature several tornadoes, including at least a few strong to intense ones, but there are a few limiting factors that may prevent this from being a high-end, violent tornado outbreak.

I'll start by saying that the subtle kinematic issues between 750-500mb are not deal breakers by any stretch of the imagination. Look back to previous major events in Dixie, including 4/28/14 (JAN 18z sounding and BMX 00z sounding), and subtle VBV signatures are not game-ending. Where you want to look is in the lowest 1-2km. The forecast soundings tomorrow afternoon across Louisiana show most of the SRH is contained in the lowest 1km with enlarged low-level hodographs. The hodographs overall look quite favorable for tornadoes, including some EF-2+. SW winds at 500mb will get it done, especially when there should be SSE to SE surface winds across the warm sector.

@Dan Robinson eluded to it. As is often the case with Dixie events, convective inhibition (CIN) should be negligible by early afternoon, allowing for rapid convective development across the risk area. This suggests the potential for mixed/messy storm modes and storm mergers, which by itself will probably prevent this from being an event with numerous discrete, long-track supercells. You can get supercells and a few could be long-lived, but note that deep shear vectors are closer to parallel than perpendicular to the initiating boundary (focused mainly on a pre-frontal trough between 20-23z), so this may be more of a messy outbreak than an outbreak with distinctly discrete storms staying isolated for long periods of time.

Moisture-wise, profiles look relatively saturated during this event as well, so HP storms should make this messy from a chasing standpoint. LCLs are low, which is usually a given in Dixie, plus there's the issue with terrain and forests making chasing extra challenging.

Instability and lapse rates are favorable, but don't expect 700-500mb lapse rates to be on the high end of the spectrum. I'll use 4/28/14 as the example again, as in this case, expect such lapse rates to average in the 6-7 C/km range, as opposed to 7-8 C/km for 4/28/14.

I use the 2014 event as a point of reference, since that was a moderate risk event that did not get upgraded to high risk until the 20z update.

Farther west, wind profiles become more problematic with time over East Texas. Behind the initial pre-frontal trough, look for less veering with height, limiting hodograph size. A few tornadoes can still occur here, but expect them to be shorter-lived and probably more via QLCS than discrete supercells.

To the north into Arkansas, storm modes will be messy and a mass of convection will probably temper the supercell threat, but a few tornadoes cannot be ruled out.

Areas to the east are a wildcard. Forecast soundings show a favorable environment for tornadoes by 20z from eastern Louisiana into western Mississippi. Here, wind profiles show classic turning with height and being displaced ahead of the main system, profiles appear somewhat less saturated. Should any convection develop in the open warm sector and become sustained, you could have a few rogue, long-track supercell/tornado events well ahead of the pre-frontal trough. Again, this is conditional, but should be left open as a possibility.
 
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Jul 5, 2009
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It will be a dangerous day in the region for sure, but I don’t think it will be favorable from a chasing perspective. As others have said, terrain, storm modes, lack of visibility in general from low clouds and storms everywhere, fast storm motions, etc. If I lived there, I would certainly take my chances and see what I could see, but I would never make a long drive out there for this.

Not saying the hodos don’t still look good, but it does seem as if the strongest winds at 500 and above are now slightly more west of the Louisiana warm sector at 21Z than they were yesterday.
 
Aug 9, 2012
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If I were chasing today (which I’m not), I’d probably target Southeast AR/Northeast LA/Weatern MS as Devin stated above in the delta region. Appears as though this area will have the best shot at something going up prefrontal with good overlap of shear and CAPE later on this evening. I think we are going to have to wait until 22z and after to realize the potential in this region as the strongest forcing for ascent spreads eastward. My only concern is the amount of convection going up at once and low level lapse rates. 12z sounding from Shreveport samples nearly moist adiabtic lapse rates from the surface to 3km....however above that isn’t had with around ~7c/km. Otherwise I think things look good for tornadoes, potentially strong, just not sure how visible they will be. Chasing in the delta would probably be the only hope if I had gone. I chased 4-2-17 and that area is downright horrid (NW LA), almost no cell service either. So keep that in mind if targeting further west (I have Verizon).
 
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Jan 18, 2009
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For those chasing today, be advised that over an inch of rain has already fallen across some parts of Southern AR. This area has received over a foot of rain in the last 30 days, and part of a state highway was just placed underwater. May I suggest downloading the iDRiveArkansas app from either Apple or Google Play or go to the IDriveArkansas website for the latest road conditions. All update in real time. Be safe.
 
Mar 3, 2012
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The first PDS Tornado Warning of the day recently went up for Robertson County in Texas. Confirmed large tornado on ground. The radar shows some pretty intense rotation and some debris. It looks like it is probably rain wrapped pretty badly. I'm guessing that might be a common theme today.
 
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Kyle Whitley

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Jul 26, 2018
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I tried to remain optimistic about being able to see something today, but that's going to be challenging I think. In Tallulah LA sitting in the Delta waiting for storms to come through here and the cloudbases look like I could touch them they are so low. The Delta is nice and clear of trees but I have no idea how anyone could see these anywhere else. At least Sim reflectivity on the HRRR is consistent, if messy.
 

Drew Wales

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Aug 16, 2017
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The only saving grace (for this South East Boy) is that once you break East of Monroe, there seems to be a good 100 mile stretch of pretty open country in the Mississippi Basin region, all the way to Yazoo City MS. I-20 looks clear(ish) and is a good E-W route but no real good N-S option until Monroe.

I've only chased in the South East US and never in LA - W Alabama is it, so I'm fairly worried about the topography there if I'm honest. I don't know that I can pass up a day like this though. Thank you for your insight Dan.
Welcome to Northeast Louisiana! We’re so awesome we drive 2.5 hours through MS to drive to New Orleans :)
 
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Jesse Risley

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So far it's looking a bit messier than anticipated, due to storms interacting with one another and being more linear. I was thinking perhaps the convection further east near the Mississippi River was going to be more discrete, which may have offered the benefit of both higher tornadic potential in higher visibility environment. As of right now, it might become more a matter of positioning oneself downstream of embedded convection in the lines. Given progged SRH values ahead of the ongoing convection, I'd expect some of those cells to start exhibiting more substantial low level rotation with time.
 
Jun 1, 2008
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The Delta southwest of Yazoo City was indeed more of a swamp. It seemed like a mess as far north as Greenwood too. We thought the WF might get hung up on I-20. However, per rules with fast moving storms, we hedged north to start. That's how we got the north-south tour of the Delta from US-45 to I-20. Nothing to report, so will keep in Event thread.

We cautiously approached the Vicksburg cell on US-49E until Yazoo City. We could have made it to Flora; but, elected to bail east on Hwy-16 before approaching Flora from the east on Hwy-22. Going direct seemed risky. While storm motion indicated an easy win, we don't race HP supercells if there is even 1% doubt. My main point is more discussion than report; hence, I write below in Event.

Few years back I had a close call racing a supercell in OK. Got caught in the hail core, just north of the rotation. Despite 20 yrs chase experience, fully aware of 5-10 min. old radar images, I elected to race it in a 50/50 situation. When I returned home from the trip my young daughter gave me the biggest bear hug to-date. She does not know the story, but she was happy I was home.

Back to present: She gave me a particularly tight hug before departing this chase trip. Again, too young to understand Dixie Alley, but I remembered it while we rolled down US-49. Race it, or go cautious? The choice was clear to be cautious.

There will be other opportunities, in the Plains. May is coming!
 
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Oct 10, 2004
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There will be other opportunities, in the Plains. May is coming!
Yeah, that's what we all said last year...

Your analysis in the other thread about the lackluster storms for a 15% hatched MDT being due to the lack of turning above about somewhere between 850-700mb and 500mb (poor low-level lapse rates and too-numerous storms didn't help) seems spot-on to me. I was noticing that on the forecast soundings in the days leading up to the event and mentioned it as a caveat on another forum where many people were talking up doomsday tornado outbreak.

SPC seems to have finally caught on to this regime we're in this decade where if the atmosphere can find a failure mode for an insane parameter space, it will; and rightly held off on the high risk/PDS watches. Heck, if I was them based on the weakness in the wind profile and the pretty strong CAM signal for cluttered/closely spaced storms, I might have held off on the MDT altogether and gone with a 10 hatched Enhanced risk.
 
Jun 1, 2008
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Holding off on high was great. Saturday had potential so I liked MDT. Warm front and/or prefrontal trough could have provided low level turning if the instability was higher.

I found the 70/60 watch interesting. Seems SPC knew it's all or nothing Delta. HRRR had Vicksburg and Starkville. Otherwise the NAM was right to crap on the setup.
 
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