Bill Schintler
EF4
Chase target:
Wendell, MN (15 miles south of Fergus Falls)
Timing and storm mode:
Storm initiation: 3:30 PM CDT. Expect a few supercells early in the convective evolution, with a transition to a linear storm complex by early evening. Large hail to golf-ball size will be the primary severe threat. This complex should increase in aerial coverage and move to the east during the evening hours, reaching the Twin Cities between 10 PM and 11 PM, while providing an impressive lightning show there.
Synopsis:
Synoptic pattern is dominated by a NRN Hudson Bay low, with low-amplitude cyclonic flow increasing over time from W to E over the NRN tier of states in the Upper-Midwest. A look at the H5 chart and the WV loop shows a number of disturbances embedded within the stronger flow, with large-scale assent associated with a lead vorticity Ribbon spreading though ND ATTM. At the SFC, moisture has been increasing ahead of a CF extended SW from a low centered near Portage La Prairie, MB; and a broken line of supercells have developed along this feature in CNTRL/NERN ND. OUN and BIS soundings both indicate deep moisture while upstream soundings in SD/ND, and CO show dry-adiabatic mid-level lapse rates. Also, a surge of 20C H85 moisture was present over ERN NEB/SD. Overnight, a 40kt LLJ along with forcing with a second wave now over WRN ND/ERN MT, as indicated on WV imagery, should aid in the maintenance of two storm complexes as they track to the E though NRN MN. The second MCS should develop in WRN ND during the late evening hours. During the morning hours Monday, strong mid-level warming will commence in wake of the ND S/WV’s and a 16C H7 thermal ridge will nose into SWRN MN by 18Z Mon.
Discussion:
Of primary concern will be the location and timing of renewed convection during the afternoon hours, and parameters suggest a severe outbreak over a limited area following SFC-based convection. The morning MCS should push an OFB S to a Wahpeton, SD to Brainerd, MN line by 18Z, with backed and stable flow N of this feature. Models continue to advertise a compact but potent H5 S/WV that will enter WRN MN during the early afternoon hours as a band of attendant H7 SW/NE-oriented frontogenetic forcing forms across the area along with a cooling of the mid-levels by several degrees. The WRF is an outlier as it is faster and further S with the progression of this wave and SFC reflection then are the GFS, NGM, and UKMET; the latter three of which appear to have a better handle of the current situation. In any event, the strongest forcing from this feature should be east of the favored SFC features by the time of convective initiation, however convection should still take place as mid-levels cool and convergence is locally enhanced at the intersection of boundaries. Meanwhile, strong capping should inhibit SFC-based based convection further S and W in SWRN/SRN MN into SD. To the W, in SD, an area of broken CI, which signals the arrival of strongest H5 assent, will persist through most of the peak heating hours; this cloud shield should remain W of the SD/MN border through 20Z. Further N, the intersection of the primary CF and the OFB from morning convection should provide the best focus for storm initiation in WCNTRL MN. LLVL cloudiness and lingering elevated convection should persist N of US-10 through early afternoon, enhancing differential heating along the boundary. Additionally, backed SFC flow beneath the NWRN periphery of a 35 Kt SWRLY LLJ should enhance hodograph curvatures along/just N of the OFB; in a region with otherwise veering and unidirectional SFC-H8 flow. Strong instability will develop by late afternoon in response to WAA along the LLJ nose in addition to seasonally impressive evapotranspiration. A 150mb-deep moist layer should exist below the inversion, while mid-level lapse rates in excess of 8.8C/km in the EML sounding will contribute towards MLCAPE’s AOA 4500J/kg. Steep lapse rates in the H7-H5 layer also suggest a threat of large hail.
- bill
9:22 PM, 08/26/07
Wendell, MN (15 miles south of Fergus Falls)
Timing and storm mode:
Storm initiation: 3:30 PM CDT. Expect a few supercells early in the convective evolution, with a transition to a linear storm complex by early evening. Large hail to golf-ball size will be the primary severe threat. This complex should increase in aerial coverage and move to the east during the evening hours, reaching the Twin Cities between 10 PM and 11 PM, while providing an impressive lightning show there.
Synopsis:
Synoptic pattern is dominated by a NRN Hudson Bay low, with low-amplitude cyclonic flow increasing over time from W to E over the NRN tier of states in the Upper-Midwest. A look at the H5 chart and the WV loop shows a number of disturbances embedded within the stronger flow, with large-scale assent associated with a lead vorticity Ribbon spreading though ND ATTM. At the SFC, moisture has been increasing ahead of a CF extended SW from a low centered near Portage La Prairie, MB; and a broken line of supercells have developed along this feature in CNTRL/NERN ND. OUN and BIS soundings both indicate deep moisture while upstream soundings in SD/ND, and CO show dry-adiabatic mid-level lapse rates. Also, a surge of 20C H85 moisture was present over ERN NEB/SD. Overnight, a 40kt LLJ along with forcing with a second wave now over WRN ND/ERN MT, as indicated on WV imagery, should aid in the maintenance of two storm complexes as they track to the E though NRN MN. The second MCS should develop in WRN ND during the late evening hours. During the morning hours Monday, strong mid-level warming will commence in wake of the ND S/WV’s and a 16C H7 thermal ridge will nose into SWRN MN by 18Z Mon.
Discussion:
Of primary concern will be the location and timing of renewed convection during the afternoon hours, and parameters suggest a severe outbreak over a limited area following SFC-based convection. The morning MCS should push an OFB S to a Wahpeton, SD to Brainerd, MN line by 18Z, with backed and stable flow N of this feature. Models continue to advertise a compact but potent H5 S/WV that will enter WRN MN during the early afternoon hours as a band of attendant H7 SW/NE-oriented frontogenetic forcing forms across the area along with a cooling of the mid-levels by several degrees. The WRF is an outlier as it is faster and further S with the progression of this wave and SFC reflection then are the GFS, NGM, and UKMET; the latter three of which appear to have a better handle of the current situation. In any event, the strongest forcing from this feature should be east of the favored SFC features by the time of convective initiation, however convection should still take place as mid-levels cool and convergence is locally enhanced at the intersection of boundaries. Meanwhile, strong capping should inhibit SFC-based based convection further S and W in SWRN/SRN MN into SD. To the W, in SD, an area of broken CI, which signals the arrival of strongest H5 assent, will persist through most of the peak heating hours; this cloud shield should remain W of the SD/MN border through 20Z. Further N, the intersection of the primary CF and the OFB from morning convection should provide the best focus for storm initiation in WCNTRL MN. LLVL cloudiness and lingering elevated convection should persist N of US-10 through early afternoon, enhancing differential heating along the boundary. Additionally, backed SFC flow beneath the NWRN periphery of a 35 Kt SWRLY LLJ should enhance hodograph curvatures along/just N of the OFB; in a region with otherwise veering and unidirectional SFC-H8 flow. Strong instability will develop by late afternoon in response to WAA along the LLJ nose in addition to seasonally impressive evapotranspiration. A 150mb-deep moist layer should exist below the inversion, while mid-level lapse rates in excess of 8.8C/km in the EML sounding will contribute towards MLCAPE’s AOA 4500J/kg. Steep lapse rates in the H7-H5 layer also suggest a threat of large hail.
- bill
9:22 PM, 08/26/07
