7AM Day 1 Convective Outlook for Thursday, June 26. THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON ACROSS FAR SOUTHEAST MN NORTHERN IA AND SOUTHERN WI
SUMMARY
A few tornadoes and isolated wind damage will be possible this afternoon from northern Iowa and southwest Minnesota into southwest Wisconsin. Isolated wind damage will be possible across a broad area of the eastern Plains, Ohio Valley, Mid-Atlantic, the Southeast and Montana.
Southern MN/IA/Southern WI
Early morning radar and satellite imagery show a well-defined MCV moving northeastward into eastern NE. Surface analysis reveals a low over eastern NE, which is associated with this vorticity maximum as well. A stationary boundary extends northeastward from this low through northwestern IA to another low in the MN/IA/WI border intersection vicinity. This boundary, which separates a moist airmass characterized by 70s dewpoints from a more continental airmass with low 60s dewpoints, could act as a favored corridor for northeastward progress of the surface low throughout the day. At the same time, strengthening southerly low-level flow will increase the low-level moisture advection south of this boundary, resulting in some tightening of this boundary throughout the day. Some guidance suggests this boundary could move slightly northward as a warm front as well. This low-level moisture advection will also help offset surface dewpoint reductions due to mixing, with dewpoints likely remaining in the 70s along and south of the boundary. Strong heating of this moist airmass will bring temperatures in the upper 70s/low 80s, fostering airmass destabilization and strong buoyancy by the early afternoon.
Thunderstorm development is expected as the approaching vorticity maximum and associated surface low interact with this unstable airmass, with initiation occurring both near the surface low as well as farther east amid the convergence along the warm front. The development near the low should becoming linear quickly while the activity along the front should remain more cellular longer. The initial/more discrete storms could be supercells with the potential to produce a few tornadoes within the moderately sheared and vorticity rich environment along the front. Damaging gusts will be possible with the linear segments as well, particularly in close vicinity to the surface low where mesoscale circulations could augment updrafts and downdrafts.
Central/Eastern KS into Western/Central OK
Recent surface analysis reveals weak surface troughing to the south of a low over eastern NE. This troughing will likely shift eastward as its parent low progresses northeastward, with convergence along this boundary contributing to thunderstorms across central and eastern KS. The character of the boundary will become more diffuse with southern extent, but modest low-level confluence should still be enough to support afternoon thunderstorm development. Vertical shear will be weak through this entire corridor, but thermodynamic profiles will favor strong downdrafts and the potential for isolated strong-severe outflow gusts for a few hours this afternoon/evening.
Southeast
Overnight convection has resulted in extensive cloudiness across much of the region this morning. Even so, ample low-level moisture remains in place and the airmass is expected to destabilize by the early afternoon amid strong heating. Initial development appears most likely near the westward progressing outflow moving across MS, with additional development later near a convectively generated vorticity maximum moving across central GA. Widespread storms are also expected across FL as an upper low drifts slowly west-northwestward. Shear will be weak areawide and a largely disorganized storm mode is anticipated. Surging outflow is possible with the strongest storms, fostering the potential for damaging gusts and also contributing to additional storm development. Several forward-propagating clusters are possible although their location, strength, and longevity are uncertain, precluding any increased severe probabilities with this outlook.
MT this afternoon/evening
A subtle midlevel trough and associated/diffuse front will move across the northern Rockies this afternoon into tonight. Deep mixing and inverted-V profiles will favor some potential for strong-severe outflow gusts with scattered high-based thunderstorms along and ahead of the diffuse front this afternoon into this evening.
OH Valley into the Mid-Atlantic this afternoon
Scattered thunderstorms are expected again this afternoon as the airmass destabilizes amid strong daytime heating and ample low-level moisture. Residual boundaries from prior convection, and terrain circulations will act as favored areas for initiation. Vertical shear will be weak, but steep low-level lapse rates and precipitation loading could support isolated wind damage with downbursts from mid afternoon into the evening.