3/26/2011 FCST: MS,AL,GA,TN

[Zack Hargrove]

I'm surprised no one has started a forecast thread for Saturday. It seems to me that there is a possible real destructive severe threat. A warm front appears to be setting up in Northern MS and into Northern AL. Models seem to show that impressive helicity values (350+ m2/s2) could be present all along the warm front as well as isolated CAPE areas from 1500-2000 J/kg. SPC significant tornado parameters via the NAM also seem to be extremely high in the area of the warm front. Temperature gradients from this system could be insane with 20-30 degree gradients over very short distances. Supercells that fire may be more of a classic nature as well and not quite as HP as they usually are in this area. And since this is mainly a warm front tornado threat, supercells could end up training over the same areas.

I am still relatively new at forecasting, and I'm afraid I'm missing something, but to me this setup looks very promising and it has the potential makings of a very dangerous day in MS and AL particularly. The threat could also nudge its way into Southern TN if the warm front is a little further North. The severe threat may extend into Georgia during the night, but the dynamics will probably start dying off as the system gets that far East.

Thoughts from you more experienced Mets out there?

EHI parameters from the 51-hour NAM, valid 21Z Saturday. These values are pretty scary:
http://i168.photobucket.com/albums/u163/nexrad45/eta_ehi_51.gif

*Update, the SPC now has the area in a Day 2 30% hatched zone. This could easily turn into a MOD by the next update.

 

[dmckemy]

There is definitly potential for severe weather that day, though I'm not quite sold on the tornado potential. Most of the 00z NAM soundings in the warm sector show uni-directional flow (mainly SW or WSW), and the hodographs in those areas don't have a lot of curvature in the low levels of the atmosphere. I think the best tornado potential would be along that warm front (which is quite strong as you mentioned earlier!), especially if you could get an isolated supercell to ride along or near the front, but I wouldn't expect a tornado outbreak from this type of setup. Shear is definitly favorable for supercells, but I'm thinking the bigger threat may be hail and wind damage rather than tornadoes...just my two cents...
BTW, I've chased in the TN/AL/GA area before, and the terrain is pretty rough for storm spotting! Good luck to those who do go out and chase!

 

[Zack Hargrove]

I agree with most of what you're saying as I am not completely sold on the big tornado threat yet, either. This is mainly because the AFDs and the SPC isn't jumping on the bandwagon quite yet. My young forecasting experience, however, is looking at the models and wondering if it may not be something quite significant. As you said, this would not be a big tornado day at all for the general warm sector because the spin is just not present. The warm front is turning into the story for me, however. Some of the low level hodographs near the warm front are seeming to show some nice curve in them to me. Also, whenever 2000+ J CAPE is coupled with 300+ m2/s2 helicity (forecast from the models), then you have the chance for some major rotating updrafts. This much instability could make the supercell threat much more than isolated along the warm front. They could simply just fire along the boundary and train all day long. Helicity looks much weaker as you move south of the warm front, so I believe that is all going to be hail and wind with an isolated tornado possibly involved. The warm front is unique in its strength and its relatively straight orientation for much of the day Saturday. As I stated before, if super cells begin to form here, they will keep moving over the same areas. Again, this is just what I'm seeing and could be off base.

I have chased this area as well (Yazoo City tornado day on April 24) and the terrain is tricky if you haven't been there before. I like MS much better. Some areas in northern Alabama are chaseable, but I'd rather just hang out in MS I believe and then chase home if myself and my team decide to go out. As of now, we would leave tonight and head towards the Tupelo, MS area and re-evaluate from there. Earlier in the afternoon, we may need to get further south as the warm front is moving north. At this point I am waiting on the next Day 2 update and some new model runs to make my final decision.

 

[dmckemy]

The latest 18zNAM still shows a severe weather threat in the areas previously mentioned, although the warm front isn't nearly as strong as it was in previous runs. Tornado potential near the warm front looks good though, and storm motions would carry most of the storms along the warm front. I took a quick look at the 12z High-Res WRF, and interestingly it doesn't initiate a lot of convection during the day (http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/hiresw/12/images/hiresw_ref_033l.gif ...this image is at 21z on Saturday). It's not until 06z Sunday (http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/hiresw/12/images/hiresw_ref_042l.gif) that the action really gets going over TN/AL/MS. Both the 18z NAM and 12z GFS develop showers and thunderstorms during the day though, so the High-Res WRF seems to be an outlier.

If I was chasing and had to pick a target, I'd probably set up somewhere near the northern MS/AL border along the warm front. A quick look at the Tulepo, MS, sounding from the 18zNAM at 00z on Sunday shows nearly 400 0-1km SRH and a nicely clockwise curved hodograph, though by 00z the boundary layer is already beginning to stablize so storms may become elevated in nature.

 

[Jeremy Gilchrist]

I am trying to decide on if I am going after this or not as well. The drive would be an intense one and I am waiting to see if my chase partner wants to go. We would definitely position along the warm front without question if we did. I too anxiously await the next SPC update. Hodos and instability etc. definitely seem to be there in my view for a supercell mode on that boundary. What kind of terrain can I expect in N. AL?

 

[Jeremy Gilchrist]

Somebody can correct me if I am wrong but I am seeing storm motions of 20-25 knots which is very much chaseable. Doesn't seem these would be the 50 knot, one stoplight and you're done type.

 

[dmckemy]

Yup, that's what the 18z NAM is showing...the 12z GFS shear profiles are slightly weaker than the NAM's, so possibly a little slower if you average the two, but I'd say anywhere between 20-25 knots.

 

[Drew.Gardonia]

Should make for an excellent chase day. I'm heading out in the morning for my target area of about 20 miles NW of Tupelo, MS on Route 78 to get into position, and plan to chase the storms into Huntsville. Anyone wants to tag along let me know.

The Energy Helicity Index, leads me to believe that somewhere in the area of Tupelo, may be a good starting point for good initiation of supercells, so that's why I picked that area as well as the SPC's consensus that Nrthrn Central and N'Eastern MS will be active. (i don't know enough yet about forecasting to be more accurate/detailed, but this is my best effort at a WAG (wild ass guess).

 

[Erik Perozo]

To be honest I'm not that impressed with tomorrow's set up. If I were to pick a spot to start I would choose Kosciusko,MS. The winds at 850 could be backed more but if you can get something rooted on the warm front then you have a chance. What I do like is the models showing drier air over that area around 18-0Z. Forecast sounding for KJAN showing CAPE around 2500J/kg...not bad. Just a rough forecast. I'll definitely look at things further in the am and make my final decision to chase. I might chase anyway because it's going to be in the lower 50's here and raining. Let's see cold,miserable and raining or basking in the 70's south of the front and doing something I love. Hmmmm.

 

[RMacDonald]

Right now my latest analysis is tracking on Greenville/Greenwood/Louisville, MS areas. My model painted a this area with some Moderate instability. Also seeing the NAM pushing CAPE values around 2500 (j/kg), MLCAPE running around 2200 (j/kg) coupled with 35-40kts of 0-3KM Shear, should be a favorable environment across this area. Any thoughts for these areas.