Chase Target:
Between Storm Lake, IA; and Marshall, MN.
Timing:
Storm initaition 3 PM
Severe Mode:
Supercells and tornadoes before 00Z. By early evening, storms will transition into a linear system by 00Z with the severe threat transitioning into a wind event.
Discussion:
A complicated pattern will affect the plains and upper-Midwest on Friday. A long-wave trough dominates the western US, with the main vorticity lobe approaching the Missouri around 00Z. At the surface, the dryline will mix eastward to the MO river by 00Z, with the CF moving SE from SD. To the W of the dryline, veering BL winds and resulting mixing should allow temperatures there to soar.
The 0Z ETA has come into reasonable agreement with the 12Z UKMET and GFS, and all are representative of the 0Z analysis. Apparently the FGZ (Flagstaff) observation did not make it into the 0Z ETA, which is an important observation for this WX setup.
Significant moisture advection is already underway – the 0Z OUN sounding indicates a well-mixed 100 MB-deep moisture layer, with a 66F Td at the surface. The TOP and OAX soundings similarly indicate approximately 100 MB-deep moist layers, with a nearly 65F Td at TOP. A 30 KT moisture conveyer belt has set up from OK through MN. Related to the generous moisture return is a concern that excessive cloudiness will be in place over IA most of the day. First, it appears as though a CI shield will be in place through 0Z Sat along and E of a SPW-FNB-GAG line, based on ETA progged 250 MB RH. Indeed, the CI appearing in the current IR satellite concurs with the 6Z ETA. The lower layers show some drying from W to E between 18Z and 0Z; the 700MB and 500MB layers should be cloud-free over the NW half of IA, while a dry push at 850MB will move into SW IA by 18Z.
Initiation should occur along the dryline between Marshall, MN and Storm Lake, IA; by 3 PM. Significant severe is a possibility, with excellent wind shear in the 0-2KM layer as well as moderate instability. Storms in this area should become surface-based in the saturated BL.
bill