9/17/07 FCST: MN/SD/IA

[Skip Talbot]

Based off Saturday's 12z WRF run, portions of southwest MN, into SD, and IA look like they have some potential with a northwest to southeast warmfront draped across the area and an area of low pressure on the MN/IA/SD corner. The area right on the warmfront has strongly backed surface winds, CAPE forecast over 3000 J/Kg and strong helicity on and ahead of the front. Another impressive feature is the strong low level jet. With 1km shear forecast between 20-30 knots and LCL's between 800-1200, I imagine there is a moderate tornado potential with this setup.

The one thing that concerns me are the thermodynamics verifying. The WRF is indicating 70+ dewpoints pooling along the warm front. That is quite a recovery seeing that dewpoints are in the 30's in Oklahoma right now. The GFS is more modest but still showing dewpoints over 65 over much of MN. I guess we'll have to see how well that LLJ advects moisture over the next couple of days, but are instability will be cut down if it doesn't.

The cap might also be trouble, as it looks pretty stout except for a portion of eastern SD into western MN, eroding around 0z and then filling back in. The WRF is showing precip and vertical velocities right on the SD/MN border though. Right now I'm focusing on a line from north of Marshall, MN where the cap is weaker and the effective shear stronger, to south of Mankato, where the helicity and CAPE are better. Making the long haul more temping: the region could get a second round on Tuesday with the LLJ strengthening and warmfront sticking around... but that's another thread.

 

[Bill Schintler]

Chase target for Monday, September 17

Chase target:
Vesta, MN (Between Marshall and Redwood Falls).

Timing and storm mode:
Elevated convection in west central MN will expand to the southeast and become surface-based through 4 PM CDT. A supercell or two appears likely early in the period before a transition to a linear storm complex takes place. An isolated tornado or two cannot be ruled out with any rotating updrafts.

Synopsis:
Satellite imagery indicated a classic baroclinic leaf over the NRN Rockies and into SD. A digging WRN CONUS trough will become increasingly negatively tilted throughout day-2. The H85 WF was located from ERN IA into WCNTRL MN and to the S of this boundary moisture advection is underway courtesy of a 40kt LLJ. However, the richest LLVL moisture (TOP and SGF soundings) remains S of I-70 in KS ATTM. The LLJ should also aid in the maintenance of the ongoing SWRN NEB MCS, along and N of the SFC WF, as it tracks E through SRN NEB overnight. These storms will weaken after midnight as the LLJ veers to the E. Overall model performance with ongoing convective trends and mesoscale forcing features is poor, and sampling may further be compromised by the missing OAX 00Z sounding.

Discussion:
This is a difficult forecast due to uncertain location and effects of early convection as well as the strength and location of forcing provided by several small but intense ULVL impulses that will overspread the area, and inhibited insolation with a CI plume associated with the seasonably strong synoptic-scale forcing. Two areas of elevated convection should be ongoing early in the period. An MCS; associated with WAA, isentropic lift in the 900-800mb layer, and forcing provided by the first of several compact but intense H5 S/WVs; will be exiting ERN IA and SERN MN during the late morning hours. Outflow from this feature will serve to enhance backing and convergence along a developing SFC WF along a Marshall to Rochester line during the afternoon. Morning convection will also develop along the SD/ND border into WCNTRL MN as the strongest convergence and forcing develops with a second H5 wave, and this convection will later expand to the S and become SFC-based during the afternoon hours. A CI shield will overspread all but the NRN-most areas of MN during the day, limiting SFC heating. Later in the period, however, clearing will take place in SRN and SWRN MN as the SRN edge of the mid- and high cloudiness pushes N of this area.

Moisture will increase during the afternoon hours with H85 dewpoints AOA 15C at the nose of a 40kt LLJ. SFC dewpoints AOB 70F coupled with modest mid-level lapse rates of 6.5C/km will contribute towards MLCAPE’s of 1500J/kg and LI’s to -4C in SRN MN. Moderate LLVL hodograph curvature should exist as left-exit of SSWRLY LLJ pushes over backing SFC flow. Cloud base heights of 800-1000m AGL may support an isolated tornado threat along and just N of the WF.

- bill
9:40 PM CDT 09/16/07