7/17/06 FCST: IA / WI / MI / IL

Jul 23, 2004
Iowa City, IA
Chase target:
Waterloo, IA.

12 noon CDT.

Ongoing convection in north central IA will expand to the east and southeast through the late morning. Additional supercells will develop ahead of ongoing storms during the midday hours. By mid-afternoon, this area will evolve into a forward-propagating MCS which will produce very strong winds to 80 mph.

A SFC CF along with a few disturbances imbedded within weak ULVL ridging will provide the focus for convective development today. Very high instability will couple with decent deep layer unidirectional shear to 30kts along with locally impressive hodograph curvatures through the first few km above the cloud bases. SFC dewpoints in the mid-70’s F along with deep moisture depth – the 12Z DVN sounding already shows a solid moist layer of 150 mb in depth, along with a SFC dewpoint of 70F; resulting in a mixed layer CAPE of 2500J/kg along with nearly dry-adiabatic mid-level lapse rates and a MLCIN of only -200 J/kg. Of particular note is a large theta-E gradient in the 900mb through 700mb layer, suggesting that a rear-inflow jet within the mature storm complex may mix down to the SFC resulting in very high winds.

Model performance (12Z RUC, 12Z WRF – which actually verified too low on SFC moisture, and 00Z GFS) all develop strong QPF signals in ERN IA my 00Z, along with dewpoints in the mid-70’s and temperatures to near 100F. Verification will result in extreme MLCAPE’s to 6000J/kg. While SFC flow will be weakly veering, strong NWRLY storm outflow a few kft above the SFC will result in locally impressive hodograph curvatures, with SFC-3km SRH’s to 400m2/s2. This may aid in development of imbedded mesoscale vortices along the leading edge of the developing storm complex, further enhancing damaging wind potential.

- bill