Bill Schintler
EF4
Storm Chasing Targets for Friday, August 7
Spencer, IA (east).
Bullhead, SD (west)
Timing and storm mode (east):
Storms should fire after 6 PM CDT, with all modes of severe weather likely. A transition into a derecho is possible during the evening, with widespread wind damage possible as the system tracks through IA.
Timing and storm mode (west):
Storms will develop after 7 PM CDT.
Synopsis:
Temperatures were again mild in the Upper-Midwest with high pressure centered over eastern IA into IL and sunshine filtered from CI blow off from numerous convective complexes over NEB and SD, which resulted from WAA. Looking ahead, the closed upper-level low currently over the PAC NW will slowly edge southeastward. One or more disturbances will eject from the trough base, round the top of the northern Plaines ridge, and then translate SE into the Upper-Mississippi valley. By Friday morning, a SFC boundary should be aligned parallel to and north of I-90 in SD and MN. At the lower-levels, southerly flow will transport higher dewpoints into the western Dakotas; with the LLJ then veering towards the NE into MN over the next 24-48 hours. This will bring a gradual moistening of the BL over the northern Plains and Upper-Midwest through day-2. Meanwhile, high-pressure over MN into IA will shift eastward as a lee low-pressure organizes over the western NEB panhandle into western SD, with a WF extending northeast of this feature, extending into CNTRL MN.
Discussion – western Dakotas and Nebraska:
Tonight, storms will continue track E through NEB. Several elevated convective complexes will be ongoing during the first part of day-2. Outflow boundaries in the wake of these storms will provide the focus for surface-based convection later in the day. Moderately strong capping will be in place with an inversion around 750 mb owing to the EML. This cap will weaken after 22Z as the exit region of a compact shortwave overspreads the area, with initiation by 00Z over northwestern SD in a zone of easterly flow north of the SFC boundary. Steep mid-level lapse rates coupled with generous LLVL moisture will result in a very unstable AMS. Moderate shear parameters will contribute towards storm organization, with deep later shear AOA 50 kts associated with the aforementioned shortwave. Modest low-level directional shear will slowly increase after 03Z as the BL decouples and a LLJ backs back towards the west.
Discussion – Upper-Midwest - Tonight:
Two convective complexes should affect the area as a 50-60kt LLJ feeds an extremely high PW AMS into the area. One elevated convective complex is ongoing in association with the H85 WF in SRN MN and NRN IA. This activity should increase in coverage and intensity overnight as the LLJ veers into the area along with steepening of mid-level lapse rates along the IA/MN border. Corfidi vectors takes this activity E and later SE into ERN IA, especially areas N of US-30. A second bulls-eye for convective genesis is southwestern IA after 3 AM along the SFC WF. This activity may eventually merge with the complex further to the north over eastern IA. Heavy rainfall will be the primary hazard with this activity with large amounts of moisture feeding into it.
Tomorrow:
Guidance initialization is better then average for this time of year with the UKMET and ECMWF similar with forcing trends, with the GFS also seeming to have a decent handle on these fields, while the NAM is the worse and does not depict the ongoing MCS activity in southwestern MN.
A large convective complex should be ongoing in the NERN half of IA during the AM hours, with some degree of back building during the morning hours. This activity shifts east through mid-afternoon, with attention turning towards northern IA for renewed severe development Friday evening. Clearing will slowly take place from W to E, with strong instability developing over IA. Capping will also be strong with H7 temperatures of 12-13C nosing into the area. Initiation should take place along the NRN edge of the CAP along the IA/MN border after 00Z, aided by assent from another shortwave. An outflow boundary from earlier convection should also increase convergence. Instability and shear parameters will support all modes of severe weather. SFC-3km SRH will increase to 300m2/s2 as a veering LLJ strengthens to 50 kts, while deep-layer shear increases to 50 kts aided by the mid-level jet streak. This activity should track through northeastern IA in areas N of US-20, and later into WI and NRN IL.
- bill
09:45 PM CDT, 08/06/09
Spencer, IA (east).
Bullhead, SD (west)
Timing and storm mode (east):
Storms should fire after 6 PM CDT, with all modes of severe weather likely. A transition into a derecho is possible during the evening, with widespread wind damage possible as the system tracks through IA.
Timing and storm mode (west):
Storms will develop after 7 PM CDT.
Synopsis:
Temperatures were again mild in the Upper-Midwest with high pressure centered over eastern IA into IL and sunshine filtered from CI blow off from numerous convective complexes over NEB and SD, which resulted from WAA. Looking ahead, the closed upper-level low currently over the PAC NW will slowly edge southeastward. One or more disturbances will eject from the trough base, round the top of the northern Plaines ridge, and then translate SE into the Upper-Mississippi valley. By Friday morning, a SFC boundary should be aligned parallel to and north of I-90 in SD and MN. At the lower-levels, southerly flow will transport higher dewpoints into the western Dakotas; with the LLJ then veering towards the NE into MN over the next 24-48 hours. This will bring a gradual moistening of the BL over the northern Plains and Upper-Midwest through day-2. Meanwhile, high-pressure over MN into IA will shift eastward as a lee low-pressure organizes over the western NEB panhandle into western SD, with a WF extending northeast of this feature, extending into CNTRL MN.
Discussion – western Dakotas and Nebraska:
Tonight, storms will continue track E through NEB. Several elevated convective complexes will be ongoing during the first part of day-2. Outflow boundaries in the wake of these storms will provide the focus for surface-based convection later in the day. Moderately strong capping will be in place with an inversion around 750 mb owing to the EML. This cap will weaken after 22Z as the exit region of a compact shortwave overspreads the area, with initiation by 00Z over northwestern SD in a zone of easterly flow north of the SFC boundary. Steep mid-level lapse rates coupled with generous LLVL moisture will result in a very unstable AMS. Moderate shear parameters will contribute towards storm organization, with deep later shear AOA 50 kts associated with the aforementioned shortwave. Modest low-level directional shear will slowly increase after 03Z as the BL decouples and a LLJ backs back towards the west.
Discussion – Upper-Midwest - Tonight:
Two convective complexes should affect the area as a 50-60kt LLJ feeds an extremely high PW AMS into the area. One elevated convective complex is ongoing in association with the H85 WF in SRN MN and NRN IA. This activity should increase in coverage and intensity overnight as the LLJ veers into the area along with steepening of mid-level lapse rates along the IA/MN border. Corfidi vectors takes this activity E and later SE into ERN IA, especially areas N of US-30. A second bulls-eye for convective genesis is southwestern IA after 3 AM along the SFC WF. This activity may eventually merge with the complex further to the north over eastern IA. Heavy rainfall will be the primary hazard with this activity with large amounts of moisture feeding into it.
Tomorrow:
Guidance initialization is better then average for this time of year with the UKMET and ECMWF similar with forcing trends, with the GFS also seeming to have a decent handle on these fields, while the NAM is the worse and does not depict the ongoing MCS activity in southwestern MN.
A large convective complex should be ongoing in the NERN half of IA during the AM hours, with some degree of back building during the morning hours. This activity shifts east through mid-afternoon, with attention turning towards northern IA for renewed severe development Friday evening. Clearing will slowly take place from W to E, with strong instability developing over IA. Capping will also be strong with H7 temperatures of 12-13C nosing into the area. Initiation should take place along the NRN edge of the CAP along the IA/MN border after 00Z, aided by assent from another shortwave. An outflow boundary from earlier convection should also increase convergence. Instability and shear parameters will support all modes of severe weather. SFC-3km SRH will increase to 300m2/s2 as a veering LLJ strengthens to 50 kts, while deep-layer shear increases to 50 kts aided by the mid-level jet streak. This activity should track through northeastern IA in areas N of US-20, and later into WI and NRN IL.
- bill
09:45 PM CDT, 08/06/09
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