Bill Schintler
EF4
Chase target:
Wendell, MN (16 miles S of Fergus Falls).
Timing and storm mode:
An elevated storm complex ongoing in southeastern SD will become increasingly surface-based through 6 PM CDT. A supercell or two will likely be embedded in the leading edge of the MCS as it pushes to the SE.
Synopsis:
Evening analysis indicated an H5 circulation along the NRN CA coast with strongest height falls to the S. Downstream, a vorticity max embedded within weak cyclonic flow along with isentropic lift in the 300-310 K layers has touched off an area of convection along the ERN SD/ND border. Overnight, a strengthening LLJ should assist in the maintenance of this convection. Trough in NRN CA is forecast to amplify while translating SWD and further closing off from the main flow. Meanwhile, the NRN stream trough will deepen as a 60kt H5 max translates through MT and ND by Thursday evening. SFC low-pressure in ERN WY will weaken and then redevelop in NCNTRL WY while a WF develops E of this feature along the ND/SD border. During the day, a DL will mix towards the E into CNTRL SD. The most probable location for SFC-based convection should be near the intersection of the WF and DL near the SD/ND/MN border.
Discussion:
Three main FCST challenges to consider: 1. available LLVL moisture, 2. cap strength, and 3. location of SFC boundaries and possible OFB’s. A complex of elevated storms and rain, maintained by WAA along the nose of a 50 kt LLJ, will be ongoing Thursday morning over CNTRL ND and into NERN SD. This convection should persist throughout the day and move E towards NWRN MN as the LLJ veers to the NE. The primary severe risk with this convection, in the cool sector to the N of the WF, will be large hail. An area of ST will cover ERN SD until early afternoon, after which it will lift off to the NE while allowing for full insolation to take place there.
H7 temperatures between 5-10C will lift to the N and E throughout the period, and capping of SFC-based convection is a concern. However, forcing associated with a vorticity ribbon in SD should be sufficient for initiation in the 01Z timeframe. As often has been the case with situations similar to this where strong moisture advection is forecast to take place in an initially dry SFC environment, the NAM appears overly optimistic with LLVL moisture and resulting CAPE/CIN. During the early evening hours, convection should initiate along the outflow-enhanced WF and become SFC-based as an axis of 55-60F dewpoints lifts into NERN SD. Forecast soundings indicate a favorable combination of moderate instability and strong low- and deep-layer shear which may support a full spectrum of severe weather. Steep upstream lapse rates (RIW RAOB) will overspread the area and somewhat compensate for the modest moisture. MLCAPE’s in the range of 1000-1500J/kg should be juxtaposed with impressive hodograph curvatures (SFC-2km) as SWRLY 40kt H85 flow surges over weak and backed SFC winds. Additionally, deep-layer shear will increase to nearly 60 kts by 00Z as the aforementioned H5 trough digs slowly to the SE.
- bill
[FONT="]9:53 PM, 09/19/07[/FONT]
Wendell, MN (16 miles S of Fergus Falls).
Timing and storm mode:
An elevated storm complex ongoing in southeastern SD will become increasingly surface-based through 6 PM CDT. A supercell or two will likely be embedded in the leading edge of the MCS as it pushes to the SE.
Synopsis:
Evening analysis indicated an H5 circulation along the NRN CA coast with strongest height falls to the S. Downstream, a vorticity max embedded within weak cyclonic flow along with isentropic lift in the 300-310 K layers has touched off an area of convection along the ERN SD/ND border. Overnight, a strengthening LLJ should assist in the maintenance of this convection. Trough in NRN CA is forecast to amplify while translating SWD and further closing off from the main flow. Meanwhile, the NRN stream trough will deepen as a 60kt H5 max translates through MT and ND by Thursday evening. SFC low-pressure in ERN WY will weaken and then redevelop in NCNTRL WY while a WF develops E of this feature along the ND/SD border. During the day, a DL will mix towards the E into CNTRL SD. The most probable location for SFC-based convection should be near the intersection of the WF and DL near the SD/ND/MN border.
Discussion:
Three main FCST challenges to consider: 1. available LLVL moisture, 2. cap strength, and 3. location of SFC boundaries and possible OFB’s. A complex of elevated storms and rain, maintained by WAA along the nose of a 50 kt LLJ, will be ongoing Thursday morning over CNTRL ND and into NERN SD. This convection should persist throughout the day and move E towards NWRN MN as the LLJ veers to the NE. The primary severe risk with this convection, in the cool sector to the N of the WF, will be large hail. An area of ST will cover ERN SD until early afternoon, after which it will lift off to the NE while allowing for full insolation to take place there.
H7 temperatures between 5-10C will lift to the N and E throughout the period, and capping of SFC-based convection is a concern. However, forcing associated with a vorticity ribbon in SD should be sufficient for initiation in the 01Z timeframe. As often has been the case with situations similar to this where strong moisture advection is forecast to take place in an initially dry SFC environment, the NAM appears overly optimistic with LLVL moisture and resulting CAPE/CIN. During the early evening hours, convection should initiate along the outflow-enhanced WF and become SFC-based as an axis of 55-60F dewpoints lifts into NERN SD. Forecast soundings indicate a favorable combination of moderate instability and strong low- and deep-layer shear which may support a full spectrum of severe weather. Steep upstream lapse rates (RIW RAOB) will overspread the area and somewhat compensate for the modest moisture. MLCAPE’s in the range of 1000-1500J/kg should be juxtaposed with impressive hodograph curvatures (SFC-2km) as SWRLY 40kt H85 flow surges over weak and backed SFC winds. Additionally, deep-layer shear will increase to nearly 60 kts by 00Z as the aforementioned H5 trough digs slowly to the SE.
- bill
[FONT="]9:53 PM, 09/19/07[/FONT]
