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2017-05-17 EVENT: NE/KS/IA/MO/IL/WI/MN

Jesse Risley

Staff member
The ECM and both American models are consistent in propagating a stout H5 perturbation across the mid-Missouri valley on Wednesday, with H5 wind speeds AOA 90 kts. There are some slight differences in regards to the position and timing of the shortwave, but the models are coming into better alignment. It is worth noting that guidance has previously differed in regards to the timing of this wave in conjunction with the associated surface features, ongoing convection or lack thereof, and resultant destabilization in the warm sector.

Today's NAM looked particularly intriguing with regards to favorable instability in western IA, northern MO and eastward into parts of western IL, though the GFS continues to depict miserly instability profiles. Nevertheless, this is one to monitor, as a surface cyclone looks poised to orient itself near KSUX and deepen slightly, before moving east across norther Iowa.

If the warm sector destabilizes with greater preponderance than that which has been depicted, shear profiles are ambient for a damaging wind event, as signaled by some CAMs, including higher res NAM products. Notwithstanding, I do still see a brief window of opportunity for a yet to be determined tornadic threat, of at least an hour or two, ahead of the main forcing and across the western portions of the warm sector. Embryonic convective development may remain discrete across portions of far NE KS, far SE NE, NW MO and SW IA, taking advantage of a seasonably modest instability profile, favorable 0-1 km shear profiles, and the presence of the boundary somewhere INVO I-80 to US 20. However, overall, that looks to be largely propitiated by otherwise unidirectional wind profiles, which suggests the possibility of an enhanced wind damage threat, depending on the evolution and degree of instability, juxtaposed with otherwise ambient bulk shear profiles.

I suspect this will vacillate between now and Wednesday, with the biggest unknown being overnight and morning convection and the resultant impact on the warm sector, as well as the evolution of wind profiles to a more or less favorable degree to enhance tornadogenesis potential.
I'm actually quite interested in tomorrows potential, which is primarily dependent on how quickly morning convection lifts out, if clearing can occur, and how much destabilization is realized. Forecast soundings show fairly impressive shear profiles with a 500mb 90kt speed max over much of IA and NW IL by 00z. Surface winds will be out of the SSE and combined with a 50kt southerly LLJ spreading northeastward, 0-3km SRH values should be in the 250-350m2s2 range. However, while speed shear is abundant, I am noticing that directional shear has decided it doesn't really want to show up to this party with relatively southerly(SWly) winds throughout most of the VWP. Despite this, I still think a few tornadoes are certainly possible across much of IA tomorrow evening, especially if aforementioned clearing and destabilization are able to occur. However, based on speed shear, damaging wind is probably going to be the highlight of the day. Tornado threat is pretty dependent on how quickly storms line out and congeal into bowing segments, which would yield straight line wind gusts. Potential caveats here include morning convection and associated cloud cover(which could be somewhat mitigated by fairly strong dynamics), quick upscale growth and a veer-back signature. All things considered, could be a solid day across IA and IL.

(Sounding from Cedar County, IA)

Western to Northwestern IL is looking fairly interesting ahead of the frontal convection as the day progresses. Assuming something can initiate in the warm sector in that area, the parameters that will be in place are quite significant, especially with CAPE looking like it's going to be quite a bit higher than was originally forecast. Due to distance limitations because of not being able to leave work until mid to late afternoon, I will be playing anything that can get going in NW IL myself. The LLJ in IL will be quite intense as 00z rolls around.
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