Bill Schintler
EF4
11:20 PM CDT, 07/25/07
Chase target:
New Ulm, MN.
Timing and storm mode:
Storm initiation: 3 PM CDT. A few supercells will be likely early in storm evolution while a transition to multicell and linear segments will take place by 6 PM.
Synopsis:
Little change in the synoptic pattern as the strongest ULVL flow remains well to the N in CAN. ULVL circulation over the ERN Great Lakes is showing a slow weakening trend as it shifts slowly to the W. Weak H5 trough was noted over WRN WY with slight ULVL cooling noted over SLC, with associated widespread convection. Slow H7 height falls were noted over WY as a 15C thermal ridge shifted slowly to the E towards NEB and SD. A second, weaker H7 trough was noted over WRN MN. The highest H85 moisture was lifting towards the N into SRN MN and NRN IA as indicated by 15C dewpoints in this area. This was coupled with an axis of 70+ SFC dewpoints. At the SFC, a weak CF has provided the focus for strong convection over NERN ND as well as earlier convection further SW along this feature which later dissipated with the loss of daytime heating. This has served to push a strong OFB towards FAR. Isolated convection also fired earlier near Webster, SD in association with a WWRD moving gravity wave. Upstream soundings (ABR, BIS) where characterized by steep lapse rates below 700mb along with modest mid-level lapse rates. Both the GFS and NAM initialized too low with SFC moisture over IA and SRN MN, however given the shallow nature of the deepest moisture this is probably represents an accurate representation for a near SFC-based parcel.
Discussion:
Models have a poor handle on the evolution and motion of an NRN MN MCS which should travel further S then MDL’s indicate. The weakening convective complex will be 50 miles N of St. Cloud, MN by 12Z. This is supported by 17C dewpoints transported on a veering 20kt LLJ along with MUCAPE’s of 1000J/kG. This MCS in turn will push an OFB to a Willmar to Monticello line by 18Z which should be the focus for renewed afternoon convection. LLV flow just N of this OFB will be more backed then MDL’s indicate. Deep layer shear will be enhanced by one or more compact pieces of energy diving to the S through MN and IA during the afternoon hours. By 00Z, a lead H5 wave axis will be located over SWRN IA while a more significant wave will dive S through CNTRL MN while locally increasing SFC-6km shear to 30-40 kts. This moderate shear will couple with MLCAPE’s AOB 2000J/kg to produce some supercells during the first hours following initiation. The Great Lakes low will prevent EWRD movement in the aforementioned H7 thermal ridge with temperatures holding near 10C.
[FONT="]- bill[/FONT]
Chase target:
New Ulm, MN.
Timing and storm mode:
Storm initiation: 3 PM CDT. A few supercells will be likely early in storm evolution while a transition to multicell and linear segments will take place by 6 PM.
Synopsis:
Little change in the synoptic pattern as the strongest ULVL flow remains well to the N in CAN. ULVL circulation over the ERN Great Lakes is showing a slow weakening trend as it shifts slowly to the W. Weak H5 trough was noted over WRN WY with slight ULVL cooling noted over SLC, with associated widespread convection. Slow H7 height falls were noted over WY as a 15C thermal ridge shifted slowly to the E towards NEB and SD. A second, weaker H7 trough was noted over WRN MN. The highest H85 moisture was lifting towards the N into SRN MN and NRN IA as indicated by 15C dewpoints in this area. This was coupled with an axis of 70+ SFC dewpoints. At the SFC, a weak CF has provided the focus for strong convection over NERN ND as well as earlier convection further SW along this feature which later dissipated with the loss of daytime heating. This has served to push a strong OFB towards FAR. Isolated convection also fired earlier near Webster, SD in association with a WWRD moving gravity wave. Upstream soundings (ABR, BIS) where characterized by steep lapse rates below 700mb along with modest mid-level lapse rates. Both the GFS and NAM initialized too low with SFC moisture over IA and SRN MN, however given the shallow nature of the deepest moisture this is probably represents an accurate representation for a near SFC-based parcel.
Discussion:
Models have a poor handle on the evolution and motion of an NRN MN MCS which should travel further S then MDL’s indicate. The weakening convective complex will be 50 miles N of St. Cloud, MN by 12Z. This is supported by 17C dewpoints transported on a veering 20kt LLJ along with MUCAPE’s of 1000J/kG. This MCS in turn will push an OFB to a Willmar to Monticello line by 18Z which should be the focus for renewed afternoon convection. LLV flow just N of this OFB will be more backed then MDL’s indicate. Deep layer shear will be enhanced by one or more compact pieces of energy diving to the S through MN and IA during the afternoon hours. By 00Z, a lead H5 wave axis will be located over SWRN IA while a more significant wave will dive S through CNTRL MN while locally increasing SFC-6km shear to 30-40 kts. This moderate shear will couple with MLCAPE’s AOB 2000J/kg to produce some supercells during the first hours following initiation. The Great Lakes low will prevent EWRD movement in the aforementioned H7 thermal ridge with temperatures holding near 10C.
[FONT="]- bill[/FONT]
