2011-04-19 FCST: MO/OK/IL/IN/AR

[Joe Pudlik]

Tuesday continues to show potential, especially across MO/IL.

Both the GFS and NAM are advertising a nice flow aloft. The only possible issue may be somewhat veered winds above 700mb, which the GFS shows more than the NAM. Otherwise it looks like a nice setup. Moisture will not be a problem with td's in the mid-upper 60's on the GFS to the low 70's on the NAM(likely overdone). Cape values on both models are also well over 2000 j/kg. Winds at the surface are generally S/SSE, with some locations backed to the SE. So besides aforementioned veering issues above 700mb, the wind profiles are nice...along with curved hodo's in many locations. Capping will likely prevent development until 21-0z.

 

[Joel Wright]

Yeah this setup has the usual capping issues we have all grown to love lol. Other than that the setup looks pretty favorable for discrete supercell development from central Missouri into central Illinois late Tuesday afternoon/early evening. Tremendous surface baroclinicy should make things even more interesting. The NAM shows upper 30s over far northwest IL while at the same time temps in the lower 80s are found south of the warm front near Springfield IL. The low-level jet is forecast to strengthen over central IL early Tue evening which should help enhance low-level shear profiles. The 00z NAM forecasts 3km EHI values of over 10 in eastern Missouri as well.

 

[Bob Hartig]

Latest NAM runs have been dropping the warm front steadily southward. Yesterday the 12Z NAM was in good agreement with the GFS in stretching the WF from Moberly, MO, northeast toward south of Gary, IN, by 00Z. Today's 6Z now drapes that boundary roughly from Jefferson City to Terre Haute. The GFS is following this southerly migration as well, though less aggressively. Since I live in Michigan, I'd naturally like to see this trend reverse itself.

 

[Matt Hunt]

I agree Bob, I'd rather see it trend back to the north. South of I-70 is not good chasing terrain in any of the three states. Ideally I'd like to see it just to the north of Lafayette where the wind farm is. Plenty of open space up there, on into IL. We're still a few days out, however, so I wouldn't be surprised if it moves again. The SPC doesn't seem to be too concerned with the strong cap, and they expect the shortwave to fire storms in the evening.

 

[Bob Hartig]

Oh, well then...there's nothing to worry about, eh, Rob? Guess I'll just park it in the lawn chair right here in Caledonia and let the action come to me. ;-)

Okay, now how can I add some value to my wisecracking so I don't cross protocol? How's about, once again it looks like some good parameters are going to be challenged by the cap. But I keep coming back to the area just north of St. Louis and east of the river for initiation later in the day. SPC is more optimistic than I am about an earlier, afternoon initiation. I tip my hat to their knowledge and hope they're right.

ADDENDUM: Matt, you're right, the SPC evidently thinks the cap may be weaker than the forecast models are showing. Seems like that's happened before recently, so I won't be surprised if it happens again here.

 

[John Farley]

Looking at this morning's model runs (first real close look I've taken, so no comment on the trends from earlier runs), the NAM seems faster than the GFS with this system, as the latter holds the low back in far eastern KS. Thus the NAM has the warm front farther north, up around the I-74 corridor, while the GFS holds it back between I-70 and I-64. The NAM seems to break out precip mostly ahead of the cold front, suggesting the cap will hold farther east along the warm front, while the GFS does break out some precip along the warm front. Let's hope SPC is right about the cap breaking along the warm front during daylight. If the cap does break and we get daytime convection along the warm front, it could be a big day in Illinois - certainly this is the type of setup that generally leads to the best chase days in IL - assuming storms do break out along the warm front.

 

[Jeff Duda]

Keeping my input to the MO/IL part of the situation: looks like pretty anemic shear above about 2 km over much of MO/IL, although a strong LLJ is resulting in great low-level shear. Also I noticed that winds in the warm sector appear somewhat veered - S to SSW. The NAM also holds a beastly cap in IL throughout the day. This suggests to me that IL may see no warm sector convection during the day, and if it does, the organizational mode would be linear.

 

[Doug Drace]

Being based in southcentral Oklahoma, im really keeping an eye on the southern target in the SC/SE OK area once again. With a 60 knt 500 mb flow rounding the base of the trough by 0z over SC OK, surface based cape on GFS as high as 4500-5000 j/kg, NAM around 3800, really high Theta-e and a pretty sharp dyline very near the I-35 cooridor by 0z. NAM fires some precip by 18z in NE OK, GFS by 0z all the way into SE OK...but more near 69/75. My biggest concerns right now with this are... how strong will the cap be along the DL, convergence, how far east will storms fire? East of 69/75 in SE OK is virtually unchaseable. Shear isnt spectacular in S OK like 4/14 either. Better overall shear will definately be up north near the WF, but decent enough south to make it worth a look. I dont expect this to be a huge day for OK, especially with veered 850 winds, but there are enough positive features that its worth chasing if storms can initiate close to I-35. Might be a nice structure show, and maybe a tornado or 2 before they end up in the jungles...

 

[John Farley]

This evening the NAM and GFS are much closer together; both have the surface low at 0z a little northeast of COU and northwest of STL. The NAM shows stronger capping than the GFS along the warm front in IL. I would FAR rather chase a warm front in IL than a cold front in MO, but if it looks like the NAM is verifying, the cold front will probably be the better play. Thankfully, even on the cold front the directional shear is pretty good, and the EHI is off the chart on the NAM in eastern MO. (Of course, very high EHI is not unusual on the NAM this far ahead; undoubtedly it will come down, but it likely will still be good.) For me, it all comes down to the issue of the cap - if it looks like it will break on the warm front in IL before dark, that will be the place to be; if not, somewhere in eastern MO, like around Warrenton, will be better.

I don't think that shear will be an issue on the warm front - the 0z hodograph for around the Taylorville area looks great. If decent storms fire in that area, they will quickly go supercellular with a likelihood of tornadoes - the issue is whether they will fire before dark in that area.

 

[Aaron Shaffer]

I am really starting to like how the NAM pulls in some better directional shear by 00Z as the low moves NE. By 00Z the NAM is starting to erode some of the CIN on the western edge of the WF. Now... with how these wonderful computer models have been performing lately, the accuracy of my NAM-based thoughts remains to be seen. I did notice, however, that the first few frames of the 00Z GFS are starting to shift the WF back north a bit again.

 

[Matt Hunt]

I had a long response typed out, and then accidentally clicked on a bookmark, and it erased the whole thing! Anyway, we might see two rounds of storms, especially here in central IN where there won't be a significant cap in place in the afternoon hours. It's just a matter of whether or not the warm front will be strong enough to force convection this far west. If it does happen, I may hang around here and make a play on those cells, unless they are in southern IN, in which case I'll pass. Both the 06z NAM & GFS are trending towards a weaker cap by 21-00z. Given the steep temperature gradient on that warm front in IL, I wouldn't be surprised to see some isolated cells break out a couple hours before sunset. Like John, I'd much rather stay on the warm front than get involved with what will likely be linear storms on the cold front.

 

[Bob Hartig]

This is another tricky scenario that seems pretty borderline for chasing. Matt, I agree with you about Indiana--NAM shows windows of CINH erosion fairly close to the WF, and a couple of hodographs east of Terre Haute (roughly speaking) are the best I've seen with the 6Z run. Farther west, in MO, the NAM keeps wanting to break a narrow strip of cap, and with this run the CINH erosion has broadened from previous runs. Problem is, most of that lies south or southwest of the surface low and the WF, and the surface winds veer pretty quickly. However, the extreme northeast edge of the weakened CINH shows promise, and as the evening progresses a narrow lobe of breakable cap extends northeast into IL.

Regardless of whether you're in northeast MO, IL, or IN, upper-level winds look crappy per the NAM, in the order of 45-50 knots. Mid-level winds equal or outstrip them. The SREF paints a somewhat better picture, but right now the question seems to be, how far do you want to travel in order to chase HP storms at sundown? I'm crossing my fingers and hoping for better news with subsequent runs.

 

[Jim Parsons]

It looks like the cap will rapidly degrade prior to 0z, on the warm front in the area of St Louis and back west into central MO...

 

[Bob Hartig]

Yeah, I take back what I said about cap erosion in MO being out of place. It looks like it's smack along the WF. Sometimes I get too hung up on models--in this case, hodographs--and overlook what's staring me in the face. In this case, that's the likelihood of backing winds. The warm front actually looks pretty good.

UPDATE: Per F5 Data, the 12Z NAM is a big improvement! Model sloshing--don't you love it?

 

[John Farley]

Looking at this morning's model runs, the GFS is now slightly faster with the surface low than the NAM, though both have a similar track and take it just northwest of the STL metro. The bigger difference between the models is that the NAM predicts both better instability (CAPE up to 4000 in the warm sector) and better directional shear due to less veered surface winds. I am thinking the place to be at the time of initiation would be about due east of the surface low, which would allow a play either on the initial storms ahead of the cold front (while they are still cellular, before they evolve into a line) or any storms that might initiate on the warm front just northeast of the low (a better play if such storms do break the cap). A problem with this strategy is the nearness of the surface low to the St. Louis metro area and the Mississippi, Illinois, and Missouri rivers - getting across any of the above could become a real issue in trying to chase storms in this area or re-position. Additionally, I am concerned about the potential human impacts of this system in the STL metro area and eventually eastward into southern IL, where much of the greatest threat may come after dark.