8/20/07 FCST: IA/IL/MO/NE/IN/OH

[Brandon Sullivan]

A large stationary front should be laid out in an east west oriented fashion across the slight risk area tommorow. While pockets of destabilization are in question, it appears that an area that covers most of IA, will be able to effeciently destabilize with forecast CAPE to reach 3500+ especially across Western IA. Also, remnants of Erin should help boost shear/SRH values thorugh out the area. I think any areas that can clear out and recieve decent insolation during the day should be able to initiate deep convection by late afternoon, especially along the stationary boundary.

School starts for me tommorow. So my ability to chase is limited.

 

[Derek Weston]

Could have some nice severe cells, though, not seeing much of a supercell threat. Tuesday looks more interesting to me across NW Iowa, SW Minnesota.

 

[Bill Schintler]

Chase target for Monday, August 20

Chase target:
Emerson, NE (20 miles southwest of Sioux City, IA).

Timing and storm mode:
Storms will redevelop along the southern periphery of a weakening MCS in SD and become surface-based near Yankton, SD though 3 PM CDT. Storms should reach severe limits as they approach the Sioux City area at 5:30 PM. While severe coverage should be limited in aerial extent, a full spectrum of severe weather appears possible near the target area, including small risk of an isolated tornado.

Synopsis:
Over the next 36 hrs, the H5 trough in the PAC NW will become increasingly positively tilted and then shear off as a 60kt speed max punches towards the Upper Midwest and a zonal pattern develops. A mid-level circulation from T.D. Erin is still evident over ERN KS, and this feature will open up and lift off towards the Great Lakes. Moisture from remnants of Erin has pushed into the Upper Midwest. Several areas of convection in IA and MN are underway; some of these are in areas that have experienced enormous rainfall totals over the last few days. Precipitation along the IA/MN border appears to be associated with isentropic lift along the H925/H85 WF. Further S and E, a line of strong storms approaching ERN IA initiated along an OFB in CNTRL IA during mid-afternoon. Finally, in SERN IA and WRN IL, storms are starting to form at the nose of a strengthening LLJ. The latest model runs have trended towards a slower EWRD movement of the ULVL features along with a weakening of the LLJ on Monday.

Discussion:
Challenging FCST as details of the mesoscale forcing as well as location of OFB’s resulting from earlier convection will be important in determining the exact location and timing of convection on Monday. A series of compact H5 SH/W’s will push ahead of a broader area of stronger flow, and a large area of elevated and weakening convection will be ongoing over SD during the morning hours in association with the stronger ULVL forcing. By noon, an OFB-enhanced boundary will lie along US-20 in NW IA. This boundary will intersect a N/S oriented SFC trough near Yankton. A plume of CI will move from W to E during the morning hours, but this should push E into IA by early afternoon while having little effect of insolation. Strong heating will occur in NERN NEB to the S of the SD convection.

Large amounts of LLVL moisture will pool near the intersection of these boundaries, with SFC dewpoints pushing 80F in some areas. This moisture layer should also extend thorugh 5000ft AGL. When combined with mid-level lapse rates of 7.5-8 C/km and LI’s of -6C, MLCAPE’s in excess of 3000J/kG will be realized. Shear parameters likewise should be favorable for storm organization and supercells. SFC-6km deep layer shear should reach 40 kts as the stronger ULVL flow arrives, while hodograph curvatures will be enhanced N of the OFB/WF although LLVL flow will be AOB 10 kts. Additionally, LCL’s of 1000-1200m AGL just north of this boundary may support a tornado risk. During the evening hours, the MCS should propagate rapidly to the SE into SWRN IA as a cold pool is established.

- bill
[FONT=&quot]10:30 PM CDT, 08/19/07[/FONT]

 

[Daniel Christianson]

Im going for a Stanton, Wayne, Cuming NE line, very nice parameters are showing up for that area, i think if we can get something going it will be a good chase, i know the further south theres a capping inversion as usual, good luck to all who chase today.

 

[Brandon Sullivan]

WOW!!! 21z MSAS shows CAPE at over 8000 J/KG in Central/E Central NE!

 

[Dustin Wilcox]

Definitely extreme instability, storm near Albion is finally really coming up, Sean McMullen and I will be intercepting on hwy 91 shortly.