Bill Schintler
EF4
07/19/08 FCST: MN/IA/SD/NE
Chase target:
Jackson, MN.
Timing and storm mode:
A large but elevated convective complex will affect much of southern MN during the morning hours. Hail will be the primary severe threat with these storms. By 5 PM CDT, surface-based storms will develop along an outflow boundary. The primary storm mode will be multicell clusters with embedded supercells, especially along the southern periphery of the storm complex.
Synopsis:
The current zonal pattern will persist for the next 48 hours, resulting in a continuation of the active WX, after which a transition to an increasingly NWRLY pattern takes place over the upper Midwest. A number of disturbances will traverse this flow, most notable if which will be a potent vort max advertised to track through the Dakotas and into MN during day-2. Looking at model guidance, the GFS is further S with the features of this system then is the WRF. The GFS is having considerable problems with feedback, and a compromise of the WRF, GFS, and SREF will be used; siding with the SREF. Regarding LLVL moisture in MN, SFC observations are verifying several degrees higher at 03Z then depicted by either the WRF or GFS. Closer inspection of area soundings indicates that this moisture is skin deep and is shallower then the vertical resolution of the models. That said, the models probably have a good handle on things when considering a mixed parcel.
Discussion:
The large MCS ongoing in NRN and NWRN SD will track E and SE overnight, reaching the SD/MN border by 11Z while forming into an MVC. Once in MN, it will outrun LLJ support and will be in a weakening state. An OFB resulting from this convective complex will be aligned along US-18 in NWRN IA by 20Z, and will provide the focus for renewed, SFC-based convection by 22Z. In the few hours prior to initiation, an area of enhanced CU should form in NWRN IA along and S of the OFB. Strong instability should be in place as SFC dewpoints climb to about 70 F beneath H85 dewpoints of 17C, owing to both evapotranspirational processes and LLJ moisture transport. MLCAPEs should rise to 2000J/kg and surface-based LI’s to -6C with moderate mid-level lapse rates of 7C/km. Strong deep layer shear and increasing lift will exist in the left-exit region of a 40kt H5 jet max. LLVL directional shear will also increase on a strengthening 30kt WSWRLY LLJ over locally backed SFC flow along the OFB, with SFC-3km SRH of 200m2/s2 indicated. The SPC significant tornado parameter (effective layer) should increase to 1.5-2, suggesting an increased probability of tornadic supercells.
- bill
10:45 CDT 07/18/08
Chase target:
Jackson, MN.
Timing and storm mode:
A large but elevated convective complex will affect much of southern MN during the morning hours. Hail will be the primary severe threat with these storms. By 5 PM CDT, surface-based storms will develop along an outflow boundary. The primary storm mode will be multicell clusters with embedded supercells, especially along the southern periphery of the storm complex.
Synopsis:
The current zonal pattern will persist for the next 48 hours, resulting in a continuation of the active WX, after which a transition to an increasingly NWRLY pattern takes place over the upper Midwest. A number of disturbances will traverse this flow, most notable if which will be a potent vort max advertised to track through the Dakotas and into MN during day-2. Looking at model guidance, the GFS is further S with the features of this system then is the WRF. The GFS is having considerable problems with feedback, and a compromise of the WRF, GFS, and SREF will be used; siding with the SREF. Regarding LLVL moisture in MN, SFC observations are verifying several degrees higher at 03Z then depicted by either the WRF or GFS. Closer inspection of area soundings indicates that this moisture is skin deep and is shallower then the vertical resolution of the models. That said, the models probably have a good handle on things when considering a mixed parcel.
Discussion:
The large MCS ongoing in NRN and NWRN SD will track E and SE overnight, reaching the SD/MN border by 11Z while forming into an MVC. Once in MN, it will outrun LLJ support and will be in a weakening state. An OFB resulting from this convective complex will be aligned along US-18 in NWRN IA by 20Z, and will provide the focus for renewed, SFC-based convection by 22Z. In the few hours prior to initiation, an area of enhanced CU should form in NWRN IA along and S of the OFB. Strong instability should be in place as SFC dewpoints climb to about 70 F beneath H85 dewpoints of 17C, owing to both evapotranspirational processes and LLJ moisture transport. MLCAPEs should rise to 2000J/kg and surface-based LI’s to -6C with moderate mid-level lapse rates of 7C/km. Strong deep layer shear and increasing lift will exist in the left-exit region of a 40kt H5 jet max. LLVL directional shear will also increase on a strengthening 30kt WSWRLY LLJ over locally backed SFC flow along the OFB, with SFC-3km SRH of 200m2/s2 indicated. The SPC significant tornado parameter (effective layer) should increase to 1.5-2, suggesting an increased probability of tornadic supercells.
- bill
10:45 CDT 07/18/08
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