...IA/MN vicinity... A Slight risk has been introduced across northwest IA into southern MN. Most guidance shows an embedded shortwave impulse or possible MCV migrating across the area on Sunday. This appears plausible given expected convection over the central Plains in the Day 1 period. A weak warm front will also be draped across the region from near the Sioux Falls area southeastward toward the Quad Cities IA vicinity. Strong heating and upper 60s F surface dewpoints will result in strong destabilization and any storms that develop will have the potential for damaging wind gusts and hail. Additionally, given the warm front draped across the area, a few tornadoes are also possible due to enhanced SRH and mean mixing ratios near 16 g/kg. Some guidance suggests capping could limit coverage of storms, but most guidance does develop at least isolated storms. Given the thermodynamic and kinematic parameter space supportive of severe supercells/bowing segments, an upgrade to Slight appears warranted.

The RAP is forecasting MLCAPE to peak above 5500 J/kg in eastern Nebraska with a tight instability gradient across western Iowa. Low-level convergence is forecast to become maximized in far northeast Nebraska by 00Z/Monday which will help with convective initiation. Thunderstorms are expected to develop during the early evening and move eastward across central and northern Iowa. In addition to the extreme instability, RAP forecast soundings in western Iowa at 00Z have 0-3 km shear near 30 kt with some directional shear in the low-levels. This environment should be favorable for severe storms as cell coverage increases during the early to mid evening. Wind damage, isolated large hail, and perhaps a tornado or two will be possible with supercells and organized multicells.

AS A LOW-LEVEL JET STRENGTHENS, WARM ADVECTION ON THE SOUTHWEST FLANK OF THE MCV (ROOTED IN THE RICHER MOISTURE/STEEPER LAPSE RATES ACROSS EASTERN NE) WILL INCREASE AND BECOME SUPPORTIVE OF THUNDERSTORM INITIATION. DAYTIME HEATING AND BOUNDARY-LAYER DEWPOINTS OF 70-72 F, BENEATH MIDLEVEL LAPSE RATES GREATER THAN 8 C/KM, WILL RESULT IN MLCAPE NEAR 4000 J/KG, WHICH WILL SUPPORT INTENSE UPDRAFTS. A MESOSCALE REGION OF ENHANCED LOW-LEVEL SHEAR/HODOGRAPH CURVATURE AND DEEP-LAYER SHEAR IN THAT ZONE OF WARM ADVECTION WILL INITIALLY FAVOR SUPERCELLS CAPABLE OF PRODUCING VERY LARGE HAIL AND A COUPLE OF TORNADOES. THE THREAT FOR OCCASIONAL LARGE HAIL AND DAMAGING WINDS WILL PERSIST OVERNIGHT AS CONVECTION GROWS UPSCALE INTO AN MCS.

Visible imagery confirms ample moisture is in place with substantial CU fields developing. As the cap is completely eroded over the next few hours, subtle lift will cause storms to form from north to south within the surface trough/front. Lapse rates are steep, and will favor rapid development. While SRH is not particularly high, substantial low-level stretching and slow storm motions within the enhanced vertical vorticity zone may allow a broken line of storms to produce tornadoes prior to merging. In addition, large hail is possible with the initial strong cores.