...IA/MO/IL/IN...
Scattered thunderstorms are expected to form near the front and
retreating outflow boundary late this afternoon into this evening
over IA. Initial mode should include supercells, one or two of
which may be persistent and cyclic enough for a threat of tornadoes
(some possibly strong) and/or significant, damaging hail.
An ongoing area of clouds/precip, and embedded precip/thunderstorms
was evident in radar composites from central IL to portions of
central/eastern MO. This activity is expected to continue weakening
through the remainder of this morning, while its trailing outflow
boundary begins to retreat/reposition northward across northern MO
in the face of low-level WAA and diurnal heating. Some uncertainty
remains as to how far north the favorable airmass near the boundary
will retreat and surface cold front. However, given the latest
convective trends compared to assorted CAM guidance since 00Z, and
likelihood of diurnal heating to its north, the general idea remains
on track for strong surface-based destabilization to occur this
afternoon well into southern and perhaps part of central/eastern IA.
That destabilization should continue into the evening ahead of
convection that develops over IA as well, supporting its maintenance
and potential for upscale growth this evening.
The environment south of the front and near the retreating boundary
(which will broaden and become more shallow/diffuse with time) is
reasonably progged to be favorable for both early-stage supercells,
followed by upscale clustering/MCS development tonight. Forecast
soundings suggest steep low/middle-level lapse rates atop upper 60s
to low 70s surface dewpoints, yielding MLCAPE commonly in the 2000-
3500 J/kg range south of the front. Mass response and large-scale
ascent ahead of the approaching shortwave trough will strengthen
vertical shear in tandem with the front and boundary, such that
effective-shear magnitudes of 45-60 kt may be realized, along with
effective SRH in the 200-400 J/kg range.
A small, unconditional significant-tornado area is being added
within the "enhanced" risk, and may need to be adjusted some as
mesoscale trends warrant. The already supercell-favorable ambient
environment will be augmented in the vorticity-maximized
remnant-boundary corridor, where the largest hodographs and very
rich boundary-layer moisture should be maintained. Any relatively
discrete storm remaining in this environment during maturity -- and
especially into the time window when the LLJ enlarges hodographs
still more -- could become an intense and cyclic (but perhaps also
heavy-precip) supercell. A damaging-wind and sporadic large-hail
threat may persist through much of the night over parts of
central/northern IL into IN, both with the MCS and any convection to
its northeast in the strongest WAA/moisture-transport regime.
