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

[Ben Prusia]

Ben here at the hospital waiting for the baby! But while I am waiting, I am doing some more glancing at the system for Friday:

6Z NAM is showing a much more powerful upper level storm system (991mb) that will move through the Ozarks during the day on Friday. By the NAM, the storm system should be centered near Cape Girardeau, Missouri at 0Z Saturday. At 18Z Friday, the storm system should be centered in northwest Arkansas near Eureka Springs. I feel that the best chances for severe weather will develop along and to the east of the surface Low Pressure, which will give the best chances to southcentral and southeastern Missouri, along with northcentral and northeastern Arkansas. The cities of Memphis, TN, Poplar Bluff, MO, Jonesboro, AR, and Cape Girardeau, MO are all in the path of the storm system. With dewpoints reaching into the upper 60's in the area close to the Mississippi River, along with instability values of up to 2500 J/Kg, this area could see some dangerous thunderstorms developing. With helicity values highest nearest and to the east of the surface low, there could be a good chance for tornadoes. Only problems I can see with this storm system is that with the upper level winds, this storm system may end up producing a squall line (after a little time with discrete cells) instead of the isolated discrete supercell situations. This may turn into a derecho event with extremely high winds. Most likely, I may be chasing this system, but I will not chase through the forests and hills of southcentral Missouri, rather, I will be targetting the Poplar Bluff area (eastern edge of the Ozark Mountains) and head east from there as storms develop. Given the fast storm speed, the flatlands of extreme SE Missouri might be my best bet. I agree with Andrew on every point he made about the forecast, I am just hoping there isn't much early convection to hinder any chances for good development.

Graphic for this forecast can be found at the "My Chase Forecasts" link on my signature.

 

[Alex Lamers]

I was becoming increasingly concerned as I was doing a national outlook last night, for an significantly severe event on a moderately sized scale (150-300 reports) from Arkansas and Southern Missouri through the Ohio/Tennessee Valleys.

I would readily agree with all who said primary convective mode will be supercells although given mean flow sfc:400 millibars being roughly parallel to the warm front and excellent large-scale ascent traveling along the boundary, I would watch for the low-topped supercells over MO/S IL/W KY to eventually evolve into a damaging bow echo(es). Meanwhile across SC KY into N MS/N AL, looks like the mode should be supercells through much of the duration of the event, perhaps evolving into a squall line with numerous embedded tornadoes.

The parameters look almost deceivingly good, and therefore I am apprehensive about blowing this out of proportion. However if things keep looking the same over the next 2-3 model runs I would say we could have a considerable number of tornadoes to chase on Friday.

...Alex Lamers...

 

[Jeff Snyder]

Well, this event certainly has the potential, assuming the NAM/GFS at least mostly verify, to be a signficant severe weather outbreak... Rich Gulf moisture (aside: what's Gulf moisture? I seem to remember it from last year, but I'd be darned if I've seen it this year... :roll: ) will stream northward into the area of concern ahead of an initial low progged to weak Thursday evening and overnight. NAM is forecasting CAPEs in the 1500-2500 j/kg range across a large portion of the forecast area. LCLs should be sufficiently low, with T-Td deficits in the 10-15 degree range (upper 70s / near 80 + mid-upper 60 Tds). Low-level shear will be ridiculous 35-60kts at 850mb and 45-65kts at 700mb with a gradually veering profile. The cold front / dryline takes on a NE-SW orientation by evening, which could lead to a transition from supercell mode to squall line as the NAM is forecasting 0-6km deep-layer shear vector to be out of the SW (parallel to the front) along and immediately behind the front, though shear orientation is more normal to the front (westerly) a tad farther east.

Right now, the only down side that I can see is veered surface winds. With the expected boundary orientation, it appears that winds will veer to the SSW or SW immediately ahead of the front across AR, western MS, LA, and western TN. This seems a little odd since we should see a little more isollabaric contribution from the slowly deepening surface low(998mb at 12z, 997mb at 18z, 995mb a 0z, 991mb at 6z), which may result in a slightly more backed surface flow pattern. As of now, that's really the only main thing that I can find that would limit tornado potential Friday (well, that and storm mode). Only time will tell I suppose...

For what it's worth, the Jackson MS morning AFD reads like a page out of the forecasters diary... LOL. http://kamala.cod.edu/offs/KJAN/0504270930.fxus64.html

 

[Amos Magliocco]

I agree with others that this is a powerful setup with the potential for numerous reports of wind damage and tornadoes. I'm curious how others evaluate the rapidly veering low level wind fields, as the surface and lower-level low pressure centers (sfc-850) get caught up in the Great Canadian Pinwheel and zip off to the Ohio River. By 0z Saturday, the sfc low is practically in London...lol.

At 18z, it looks like supercells to me, and along the warm front in N AR/MO bootheel, I think storm motions are even such (~35-40 knots) that you could actually "chase" these for a small period of time. Anything faster (in that terrain), and the strategy is nothing but pick a spot with a view and wait for them to parade on by. People have seen tornadoes that way too. But even the 18z cells won't play nice for long as they head for the Ohio River Valley.

My sense is that around 21-22z, the mode will rapidly transition to linear convection, albeit with a continued threat of large hail, damaging winds, and isolated tornadoes from embedded sups in the line.

 

[Mikey Gribble]

If I decide to go chasing on Friday I would be setting up in the NE corner of Arkansas or along the far West side of the Missouri bootheel depending on the exact position of the surface low and associated warm front. CAPE should be around 2000-2500J/KG. Helicity is forecast to increase throughout the afternoon with values forecasted to be greater than 400m2/s2. Deep layer shear will be good at 60kts. Low level shear will be very good. Dewpoints of 60-65 can be expected throughout the target area. LCL and LFC heights will be low enough to get the job done.
The only concerns I have are early morning precip./cloud cover and the amount of time supercells will remain discrete. Storm motion has come down a bit over the last couple of runs, but >40kts is a sure thing. With good road networks this would be very challenging. With poor road networks your'e screwed. If the models verify, I would expect to see several strong tornadoes and possibly one or two significant tornadoes, assuming the storms remain somewhat discrete for several hours. I think the best chance for strong long-track tornadoes will be along the warm front where backed surface winds will increase tornado potential. I would love to get out chasing for this, but I am flirting with disaster in a couple classes if I don't stay home and do homework this weekend. If things are still looking really good tomorrow though, I will probably buckle.

edit- I forgot to mention that the storm motion greater than 40kts is kind of deceiving considering they should slow down once they turn right. I am sure 99% of you already knew that, but I just figured I had better throw it in.

 

[Alex Lamers]

The setup for Friday is still looking very healthy per the latest model solutions and I'm beginning to think that a severe MCS or bow echo evolving from low-topped supercells is likely up the Ohio River. The setup is pretty classic and the +75 knot MLJ will help. The only thing that is concerning with this event is poor environmental lapse rates on the NAM which have ruined many-a-good event thus far this year, primarily when concerning potential tornado outbreak situations.

...Alex Lamers...

 

[Ryan McGinnis]

One interesting feature of the NAM is that it places the jet streak maxima with diffluence over the lower MS valley at around 0Z. This could boost storms in that area -- but would also place the AR/TN/AL area in the rightfront converging quadrant of the jet. This could help keep storms that pop up in this region from going linear.

Another to note if you're chasing on Friday: (bunkers) storm motion is progged to be VERY FAST. >50 knots to the ENE.

 

[Jeff Snyder]

The 0z NAM output continues to look relatively similar on the large-scale, with very strong shear and moderate instability across the region of concern. It does appear that the veered surface flow invof the front will likely limit tornado threat to the south of the surface low, as seen by the fact that model forecast helicities are stronger farther east across central and eastern MS, well removed from the instability axis, which, at 0z, is progged to be near the MS/AR border area.

I'm quite surprised to see only a SLT risk on the new Day 2 outlook. The reason stated is that there is still much question regarding storm mode (linear vs. discrete). Regardless, I think the relatively widespread coverage of severe weather expected supports a MDT risk, even if we "only" end up with a fast-moving, intense squall line. I was thinking that the linear vs. discrete argument may be most valid in the debate between MDT and HIGH, though the lack of real intense instability will probably preclude the issuance of such a grand risk. At any rate, I'd be incredibly surprised if we don't see a MDT risk out by tomorrow... The magnitude of the flow and the moderate instability will at least support a risk of widespread, very damaging winds...

 

[George Tincher]

Jeff, agreed. Regardless of the mode, there will still be a significant severe weather threat. With dynamics as strong as they are, this would warrant a moderate IMHO even if it was all expected to be a wind threat. That is some very, very strong shear. Some of that is bound to be transported down to the sfc with these storms. However, even if it quickly goes linear, I would expect a little bit of everything in the way of severe weather. There will likely be bowing segments with rotating comma heads, embedded sups, etc. So this stuff IMO will maintain an isolated tornado threat well into the evening.

I would expect them to likely go MOD by 1730. More so than the storm mode, something tells me they are a little less confident in cloud cover and precip over part of the area. Edwards made mention to the possibility that there may be a rather weak or non-existent cap over some of the area, allowing stuff to fire very early. This may have been the main reason to avoid a MOD for the time being.

Still, with a 25% hatched area, an upgrade seems highly likely by 1730z unless there are drastic changes (which is always a possibility).

 

[Andrew Revering]

Explosive initiation near Danville & Mount Ida, AR around Noon Central time. Storms will explode along the boundary and to the east of there, but the biggest storms look to develop from the initiation point and track ENE at about 50 MPH to Little Rock and Conway, AR and on right through Memphis. If this forecast holds, the major metro areas of Little Rock and Memphis may have some very nasty weather they need to be concerned about on Friday. Right now I think Moderate risk is a given, potentially a high risk. However, instability could be better for a high risk, so I'm not entirely sold on that just yet. On the other hand, the insane low level jet and great winds aloft along with the vertical motion may be more than enough to compensate and bring this risk to what one might consider a high risk category. Another factor to consider is mode of storms. I think initially they will be supercells with tornadoes, however, by 6-7 PM they should start to line out in Arkansas and begin to bow. The risk from that point on will be a high wind event from the squall line. The fact that two major metro areas are in the direct path of the nastiest part of this system (from what I can tell) I think too should be a consideration in the risk given. We'll see what the pros at the SPC do with this.

My target for initiation is Mount Ida, AR... with Little Rock and Memphis both in the direct path of the worst storms farther downstream.

 

[Mickey Ptak]

This is per last nights 00Z run but via the Bufkit NAM run things could start supercellular in Little Rock by 19Z Friday providing initiation starts that early.

www.mesomick.com/Temp/KLZK20052904_00ZRUN.jpg

Forecasted hodograph doesn’t look all that good though IMO.

www.mesomick.com/Temp/HODO.bmp

Still unsure if I will go though, but I need to decide tonight… Hmmmmm

Mick

 

[Glen Romine]

Quick review of the 12Z NAM forecast run. Several changes this cycle for the event on Friday, some good and some bad. I'm sure most everyone has read the SPC discussion from this morning, where Roger Edward's primary concerns were related to the convective mode. This may be somewhat relaxed by the new run showing a much slower upper wave - which does not slam into the front as early now giving peak heating a chance to try and compete. Associated with the slower arrival of the upper wave is weaker deep layer shear (still plenty but now slower storm motions during the day) and the surface cyclone lags well to the southwest of the former forecast position at 00Z waiting for the main upper wave.

Also new is a better defined E-W oriented trough/warm front, which could serve as a better focusing mechanism than the stronger thermal boundary to the northeast, along with a corridor of reasonably backed winds (this is not a day to look for tornadic storms in the warm sector where winds are strongly veered). Forecast cap strength stronger this cycle as well - which may limit convection except along the secondary warm front and very near the surface low. If strong heating breaks out in this forecasted middle corridor - things could get very interesting. NAM breaks out clouds and precip late morning across the MO-AR state line. If this verifies, the day becomes more drab, but if it doesn't ..... WRF run similar, but had this precip area breaking out during the pre-dawn hours, which would be preferable - but that run is based on last night's faster NAM solution.

Other main concern Roger mentioned was the lack of a well defined elevated mixed layer (a.k.a. decent mid-level lapse rates). Quick tour of area soundings suggests there are at least decent lapse rates around today - and the NAM holds on to a nose of 7 C/km 850-500 LRs through at least 18Z. That should be good enough.

Given the complex convective evolution forecast to occur between now and this event - I'll probably have to wait and see what morning brings in terms of locations of precip/clouds and morning soundings to decide if this is worth chasing or not. Tonight's NAM should also have the upper level system well sampled - and should offer a reasonably accurate forecast.

Glen

 

[Brett Adair]

Well....going through things this mid afternoon with the SPC upgrading to MDT, I think they have pretty good reason to. Looking at some of the meso-ETA forecast soundings from MEM for tomorrow evening showing a pretty largely curved hodo with a few indicies I will list below.

LI : -7.7
CAPE : 3978
LCL : 892mb
SWEAT : 552
EHI : 7.5
0-2 KM SRH : 303 m2/s2
DCAPE 1145
Cap : 0.0°C
Supercell Percentage : 96.6

Looks to me like SW TN/NW MS is a great target area for around 00z when the best diffluence (kissing jets) and congealed spead maximums will come across the region. As 700mb dry punch erupts, I see several long track tornadoes possible tomorrow with the 65° isodrotherm slamming northward in the midst of that 50+kt LLJ axis. As things spread east and ascent and forcing punches through the region, the LLJ/MLJ look to intensify more as they head in the general direction of WRN AL. This will pose a rather damaging bow echo/derecho threat as we head into the night time hours. SRH and vertical shear will still support tornadic activity within the line, but any supercell that can form in the maritime tropical air ahead of the linear structure, I would NOT rule out a large tornado through the morning hours Saturday with a powerhouse storm system of this proportion.

 

[Justin Turcotte]

Looks to me like SW TN/NW MS is a great target area for around 00z

Agree. 00z NAM place the warm front from Little Rock to just south of Memphis. Forecast soundings in this area indicate some nice looking horse shoe hodographs. 0-6 km bulk layer shear of at least 60 knots, impressive divergence above 300 mb. Agree that significant tornadoes are possible along the front. The quad from Little Rock to Pine Bluff to Batesville to Memphis looks very ripe for warm front chasers.

 

[Amos Magliocco]

I think we have instability questions in Arkansas this morning. Obviously there's lots of clouds between the blowoff from the convection north of I-40 and the lighter coverage in the southern half of the state which does not appear to be dissolving and might be increasing ahead of the lead shortwave. Warm front is still well south which isn't unexpected, but so are the dewpoints, as a result of the upper energy being weaker and slower, and 850 mb being very lackluster and veered for the last several hours. All that can change in a hurry of course, but the boundary is sagging south and I'm thinking it will gain speed from the cold pool generated by the extensive convection up there.

All this serves to push our warm sector further and further south, so that now I'm hoping we can develop something in the area from LIT to Helena to Arkansas City. I don't know jack about that terrain, but I have heard there are pine forests somewhere down there in southern Arkansas.

Hopefully this surface low will get moving. We need it to put some vigor in our southerly flow, move the front, and make some kind of stand against the sagging cold air.

It's very early in the day, obviously, so a lot can change, but I feel like the tornado potential has decreased and the possibility of the cap holding for anywhere other than directly on the boundary is higher. With such a sharp temperature gradient, I'm less certain how beneficial that front will be if storms move immediately north into significantly cooler air. Hopefully it will stall and we'll see more diffusion there.

I'm still in Forrest City and will remain here a little while, conserving gas and making the most of a high speed connection. By 10:00 AM or so, I might cruise down 40 towards LIT and maybe drop south to Stuttgart, which looks to have some fair road options.