04/29/05 FCST: OK/MO/AR/IL/KY/TN

[Todd Beal]

As of 10:15, cloud cover is socked in big time here in Memphis. I dont know if we will see any clearing here today. I am growing a little pessimistic now, but alot can happen b/w now and later this evening.

 

[Bill Schintler]

Chase target for today, April 29

Chase target:
Don’t chase today. But if you do, Little Rock, AR.

Timing:
Renewed convection developing in SWRN AR by 1PM CDT, and tracking NE towards the target by 3 PM.

Storm type:
HP supercells with some tornadoes likely. Lack of warm sector surface heating will temper the amount of instability today. Low cloud bases and rapid storm movement of 30 mi to the E will result in poor chasing conditions. By late afternoon, storms will evolve into a convective complex and track towards Memphis and Jackson.

Discussion:
Complicated SFC pattern at 15Z: SFC low centered just N of DFW with synoptic WF extending NE along a MLC/JSV/RUE/SRC/NQA line. Meanwhile, ongoing convection has pushed a few outflow boundaries S of this feature. The first, moving SE at 10mph, is located along a MWT/RUE line; while a second nearly stationary outflow boundary is located along a RUE/LIT/SGT line. The area is blanketed by extensive low cloudiness, while a CI shield also covers the area roughly N of I-20. This will serve to reduce heating today, with warm sector temperatures currently near 70F. Warm sector dewpoints between 60-65F will keep LCL levels around 600m. Despite cloud cover limiting insolation today, impressive shear with 0-6km shear to 70kts and 0-3km SRH’s locally to 500m2/s2 in the region of the outflow boundaries will result in supercells.

- bill

 

[Chris Allington]

I agree with Bill in the fact that very limited surface heating will take place...looking at the vis sat imagery shows an extensive cloud deck streaching across the area and back to the SW...only a few breaks were present over Northern LA... still think that severe thunderstorms/ supercells will fire but the instability should be held down somewhat by the cloudiness... good luck to all who chase today!

 

[Andrew Revering]

Obviously the cloud cover and instability have become an issue. I agree with Amos's last post on the topic. This is partially why I am more interested farther south... closer to Texarkana and Shreveport.

However, I don't think the threat has diminished any. I think we're still looking at a solid moderate risk event.

I'm new to this Target Area forum, and I read the rules about not posting images, but are links that eventually will be outdate alright? I apologize ahead of time if this is incorrect, although I did see some link posts in this thread already... here is what I'm thinking in my mind:
http://www.aprweather.com/severe.jpg

..and a bit of a morning forecast...

As feared, instability is going to be the concern for today's storms in the Lower Mississippi River Valley. Widespread cloudcover this morning is going to prevent good insolation to build up instability. Although this still looks like a moderate risk day, the lower instability levels will likely prevent this event from getting horribly out of hand. Still yet, damaging tornadoes and straight-line winds and large hail will have to be expected. Already this morning there have been hail reports across Northern Arkansas.

Severe storms are already underway, but only a couple cells in Northern Arkansas. They are pretty well put-together and are not lining out. This may be an indication of things to come.

Initial cells this morning will be developing above a strong cap, keeping storms elevated. This should mean that initial tornado threats would be low, until later on when the cap can start to slowly erode and storms can become more surface based.

Where the instability can build up the most and cap erode the fastest is where storms will probably initiate with some greater organization. I think this may be near Mount Pleasant, TX to the Texarkana area where a nose of clearing skies from the southwest may help to speed up this process by around Noon central. The surface based storms and threat for tornadoes should begin by around 2 p.m. in Northeast Texas.

Storms firing in this areas should move ESE at a pretty good clip, while at the same time backbuilding to the south. Therefore the focus of the activity should be near Henderson, TX by 4 p.m.

Between 4 p.m. and 7 p.m. the area of greatest severe weather should move due east to near Minden, LA as areas of subsidence, capping, and poor tilting shear start to surround the focus area, causing it to become much more focused into a line of severe storms in this area by 7 p.m. This will probably be the southern edge of the line as storms extend all the way up to Northern Arkansas and Northern Mississippi by this time.

Severe storms may also be firing as far east as Atlanta, GA and as far north as Lexington, KY.

The area of greatest concern throughout the day is going to be in area including and around Texarkana, TX, Longview, TX, Henderson, TX, Nacogdoches, TX, Shreveport, LA, Rustin, LA, Monroe, LA, Minden, LA, Magnolia, AR & Hampton, AR.

Tornadoes are certainly a concern with supercells, once the cap erodes.

Here's a graphic of my blabbering above: http://www.aprweather.com/severe.jpg

 

[Glen Romine]

Looking things over late this morning makes me glad I'm at work and not in Arkansas right now (a decision made last night based on the 00Z NAM trends). As Bill just mentioned, the surface pattern is indeed complex. Weak ridge along the gulf coast combined with a weak trough extending from roughly just south of Durant along the Red River, where it appears to merge with outflow boundaries aligned roughly along I-30 to just south of Little Rock. Veered surface flw is common between these two features, with weak but backed flow north of the boundary. Concerns of forest in this area seem legitimate, note coverage by county according to this site:

http://www.uaex.edu/cross/images/county_pr...files/fig54.gif

Anyhow, another weak surface low was even further back along the front, and looks to be back around Waco TX, which could merge in with the inflection in the surface boundary from the formerly mentioned convergence near Durant. Cloud cover is indeed overly abundant, and weak, elevated convection appears to be redeveloping over southeast OK which will not help at all in allowing some recovery to occur. However, the cirrus appears to be thining along the western edge which will help, and heating is effective across LA this morning. Morning soundings showed substantial cap in place to surface based convection south of the boundary, especially at Shreveport, and model guidance calls for only a weak shortwave to emerge, with best forcing well north of the surface boundary. RUC forecasts for instability are pathetic, but probably underdone as pockets of better instability are almost certain. But, with a strong cap, weak insolation and apparently weak forcing along the southern boundary, it is unclear if surface-based convection will be able to develop even by early evening. Best bet still looks to be around the Hot Springs to Little Rock area, where forests are abundant. Maybe something mesoscale will become more obvious later to allow another potential initiation point further east, but right now the Texarkana area is the "hot spot".

Just read the updated SPC outlook, and I really just don't see the potential for tornadic supercells as being that great across srn AR - but do agree with the evolution to an extensive squall line this evening with widespread damaging wind potential.

Glen

 

[Tony Laubach]

Looking at the latest visible satellite image and animation, Arkansas is socked in pretty good. A hole trying to develop in the clouds over extreme Northwest Arkansas, near Fayetteville, may be a sign of things to come, but it appears as if is well behind the frontal boundry. Definately not a large space to work with and definately on the wrong side of the front, but hopefully those clouds can start to burn away down south and give way to some sunlight; otherwise anything south of a Fort Smith to Searcy to Memphis line is pretty well caked in and looks like it will be for a bit longer. SPC does have an MD out for the area south of that above mentioned line, but they seem to concern themselves with severe potential rather than tornadoes. The two storms in the area are riding right along that line I mentioned at a pretty decent clip. However, they are kicking out some outflow boundries to the south of Little Rock at this time, so if some clearing can work its way in there, it may be worth sticking close to one of those boundries where the low level shear oughta be greatly enhanced.

This is a tough call, and since I haven't looked to deeply at the forecasts previous to now, its hard to judge things. I'm walking blindly into a complicated setup, so I'm really not going to reach out for answers with this one. I still think potential is there, but I certainly would not be very excited being blanketed in like that, either. SPC's latest Day 1 has a hatched 15% area much further south and east than previously thought, extending from Southeast Arkansas eastward into Northern Mississippi, Northwestern Alabama, and extreme Southwestern Tennessee. Their text still has very strong wording and doesn't really harp too much on the cloud cover over the area right now. It'll be interesting to see how all this pans out over the next few hours. They're still thinking supercells with tornadoes are likely and who am I to argue... I don't think I'm quite as excited about the prospects, but hopefully I am proved wrong.

 

[Bill Tabor]

Well I just thought I'd mention that so far the day has not lived up to the apocalyptic hype. It
appears amongst other things that cirrus has shut down heating and convection most of the day. I
think this has led to lack of strengthening of the low and proper strengthening / backing of the
surface and 850mb winds to support tornadoes (so far). Additionally as you all may have noticed the
cape is not developing and positioning as forecast by the models a couple of days ago. Then it was
centering it in AR, MS, TN. Today it has mainly been in TX, LA, and now starting to be southern AR
finally. Additionally the strong area of vorticity forecast for this area at mid levels appears to
be going more north of the area. 300mb winds also place much of the area in subsident flow.

Now I'm not saying there won't be supercells and tornadoes in these areas; however I think it is
much more diminished from what it was showing previously. Looking at SPC Mesoscale Analysis you see
that AR is now starting to bubble and CINH is mostly gone. Supercell parameter is high but sig tor
is still low (at 1). With elongated surface low and cold front and stronger 850mb winds from the se
I fear that the primary mode will quickly be squall. That of course would be good news for those
living in Little Rock and Memphis. I don't think it will get going all that well until almost 0Z
after that it will soon be dark so not much chase time. Perhaps MS will be able to pull out some
torns yet though.

Hmm looks like the hoped for cells are starting to pop nearby home. I don't expect much but better
than no storms at all. I may go check them out. Hmm cell E0 already showing as a meso.

 

[Glen Romine]

Originally posted by Bill Tabor
Well I just thought I'd mention that so far the day has not lived up to the apocalyptic hype.

I don't know that anyone here was openly awarding this event with that much distinction, but it is certainly worth noting that the models failed to ever converge on a solution - showing a progressively weaker and slower system with time. This was well pointed out in several discussions - where early model forecasts showed what would have been too much forcing to what later became too little in reality, but the trend was well shown with progressive weakening in each run. Further, last night Rich Thompson (and you can't as for much better insight than straight from an SPC forecaster) noted how the wave was entering a confluent region of flow - which acts to elongate and weaken short waves - in this case leading to a lack of low-level convergence and improved surface wind fields so agressively shown in early model runs. Further - as mentioned, there are inherent limitations to model forecasts, and it is really only rough guidance in the most favorable sense, a view at what *might* happen. After review of morning observations, it was pretty evident at that stage that things would struggle to come together today - and that was noted by several regular contributors who took a closer look. While in this case the model tended to overdo the real threat, the opposite can and sometimes does occur. However, those who follow regularly practice forecasting are aware that in general models display an overly optimistic view of what might happen - particularly at long range. I think the SPC outlook for today, aside perhaps from the torn probabilities, is pretty darn good under the circumstances, and I fully expect the number of storm reports to increase dramatically by later tonight as the LLJ ramps up and with the arrival of the primary upper wave.

Glen

 

[Bill Tabor]

Originally posted by Glen Romine
I don't know that anyone here was openly awarding this event with that much distinction

To clarify: actually that was an off the cuff remark regarding a post by Andy Revering referring to his Tornado Index a couple of days ago. It was showing 'Apocalypse' for the area near TN, AR, MS for today. Actually your reference to the quality of the models for over or under interpreting an event is what I was referring to also. In this case Andy's index - as are most others that I know of are based on model input primarily. Therein lies it's weakness for as models wax and wane so does the output of the index for strength, location, and type of event. Probably what his index does is pretty good, but it's weakness is it relies on the models. Other weaknesses with it and other indexes are lack of being able to model cirrus clouds, outflow boundaries, frontal locations, other mesoscale boundaries, mesolows, and effects of earlier convection. I expect the next level of indices should / would take these things into account.

My reference was only in regard to today through 0Z as a chase forecast. I don't always chase at night.

 

[Andrew Revering]

To clarify: actually that was an off the cuff remark regarding a post by Andy Revering referring to his Tornado Index a couple of days ago. [...] In this case Andy's index - as are most others that I know of are based on model input primarily.

Exactly. That output was entirely driven by ETA/NAM model guidance. As we got closer to the event as NAM got increasingly unfavorable (weaker low, slower moving, etc.) the index calmed down considerably.

Still yet in the hours before the supposed event, the index was at Moderate risk. I think the large amount of cloud cover, fact that the low was almost non-existent and didn't hardly move, and location of the boundaries in relationship to what instability was accessible was the cause to the demise of this setup.

Andy