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2018-05-09 EVENT: MO/IL/IN/MI

Jesse Risley

Staff member
Wednesday offers a marginal local setup for the Corn Belt region, namely far eastern MO, much of IL, and perhaps parts of W IN. A 500 mb impulse will nose into the Upper Mississippi Valley by late in the period. Overnight tonight an associated disturbance should create some convection that continues early into the 09/12z period, before moving out by late morning, which, if models are correct, allows ample clearing across the area through mid to late afternoon.

As associated area of low pressure should move into the Upper Mississippi Valley Wednesday afternoon, with an attendant cold front approaching the axis of greatest instability across the risk area. If the atmosphere destabilizes as forecast, MUCAPE value AOA 2500 J/KG should lie across parts of SE MO/IL/W IN, partly owed to a modest return of moisture overnight, with LI values AOB -7 across the region. Overall bulk shear profiles are rather anemic, precluding a more substantial severe risk (even the best skew-T profiles on the NAM across EC IL barely approach classic supercell threshold), though a broken line of mostly multi-cell to possible transient supercell structures should evolve by 20z-21z proximal to the Illinois River, as more convection ignites along the area of greatest forcing and propagates east through early evening.

Though possibly a nice gentleman's chasing opportunity, this one has the possibility of producing one or two diamonds in the rough, depending on the evolution and extent of the progged overnight convection. Though surface flow is otherwise modest and directional shear is mostly unfavorable for tornadic activity, historically speaking, remnant, localized boundaries from overnight or early morning convection co-located with ample instability may be enough to enhance low-level rotation and get a few of these storms to produce brief, low-end tornadoes. Given the overall synoptic setup, the usual threats of marginally severe hail and some damaging wind reports would be the primary threats.
IMO the NAM models are really highlighting Indiana so far. Better shear and wind profiles. I am probably chasing this one, but the question is how far south do I want to wander. I'd prefer to stay local, but current guidance points towards Centralish Indiana

NAMMW_con_scp_036 (1).png 2018050812_NAM_036_41.09,-87.17_severe_ml.png
Afternoon cells should start in eastern Illinois, but may very well mature in western Indiana. Terrain is fine both places north of I-70, especially north of I-72 IL and I-74 IN.

I would look for the intersection of the outflow boundary OFB (from morning rain) with a pre-frontal trough (well ahead of cold front). Current NWP has it in eastern IL tracking into western IN. Too early to look for favored cells on CAMs but the HRRR-para does appear to show a beefy one on the OFB/trough intersection. As always, I favor real-time satellite and surface over CAMs for target boundary intersection tracking.

Should the OFB sink into the jungle it would be tough to justify much travel. Maybe local chasers could try. Right now appears it'll be just barely north enough in decent terrain. On the east-west question, LLJ does appear stronger Indiana side.
Deep layer shear does not appear to be particularly favorable, with most model progs on the order of 20-30 knots of 0-6km shear.

Data suggests that storms developing along the cold front will probably be multicellular, but one cannot rule out brief supercellular structures, especially if moderate buoyancy coincides with areas of relatively stronger upper level flow in central/southern Illinois to west-central Indiana.

I would be most interested in any pre-frontal activity during early to mid-afternoon in Indiana. Since the warm front will probably lift into Michigan, I don't think that boundary would be in play. Focus then shifts toward the open warm sector, perhaps along a pre-frontal trough, where near-surface winds remain more backed. Even here, deep layer shear appears marginal at best, suggesting any robust cells should tend to transition into a linear mode fairly quickly.

I agree that this looks like a local chase, especially since mesoscale details may not be fully resolved until Wednesday morning. Assuming early day activity is not too disruptive, look for outflow boundaries as others have mentioned. I've seen this happen several times in the Ohio Valley, where open warm sector activity can be sneaky and go on to produce severe in somewhat marginal setups. The main limiting factor in this case will be deep layer shear, suggesting updraft organization will be quite limited. Even low-level shear appears relatively weak. With that said, low-level hodograph size should increase with eastward extent, suggesting central to eastern Indiana might have the best conditional threat at something going up before the cold front approaches.
NAM and RAP forecast soundings show an improvement in the wind profiles and attendant 0-3km helicities in the southern IL/IN portion of this event late this afternoon, though overall flow at all levels remains weak (less than 20 knots).
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I'm likely going to play with whatever decides to go up near the central IL/IN border as it's fairly local. My hopes aren't high obviously but it'll give me a chance to fine tune some settings on my camera in a semi-chase environment.