SUMMARY...Sustained strong supercells appear increasingly likely
through early evening. Some probably will pose a risk for producing
tornadoes, particularly around the Storm Lake and Fort Dodge IA
areas by 7-9 PM CDT, where a strong tornado or two is possible.
Additional, a severe thunderstorm watch will probably be issued soon
to the north of the current watch, where severe hail may become an
increasing concern this evening.
DISCUSSION...Thunderstorms are now initiating along the warm frontal
zone, north through east of a deep surface cyclone slowly migrating
east-northeast of the Sioux City area. This is being supported by
lift driven by low-level warm advection, and perhaps a subtle
mid-level perturbation forecast to migrate north-northeastward
across the mid Missouri Valley around Sioux City through early
evening.
The strongest thunderstorms probably will tend to evolve along the
front to the east-southeast of the warm front/dryline intersection,
now generally east of the Missouri River, where a narrow tongue of
modest boundary-layer moistening wrapping toward the surface low
center may be contributing to mixed-layer CAPE as high as 3000 J/kg
to the southwest of Fort Dodge. Into the the 00-02Z time frame,
southerly 850 mb flow across the narrow warm sector, into and across
the warm frontal zone is forecast to strengthen to 50-60+ kt,
contributing to very large, clockwise curved low-level hodographs.
As this occurs, the warm frontal zone is expected to become the
focus for intensifying supercells, including the evolution of strong
low-level mesocyclones, potentially capable of producing tornadoes.
The warm front is rather sharp, and the air to the north of the
front rather cool and stable. However, the front is slowly
advancing northward, and model forecast soundings indicate low-level
thermodynamic profiles will destabilize across much of northwestern
through north central Iowa into early evening. It appears possible
that this will become sufficient for tornado development, with
highest probabilities for sustained/longer track tornadic supercell
development around the Storm Lake/Fort Dodge Iowa vicinities.
As storms progress north of the warm frontal zone, into the colder
air, stronger cells could continue to pose a risk for severe hail
while the tornado threat diminishes.
