Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
606 ACUS02 KWNS 120602 SWODY2 SPC AC 120600 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 0100 AM CDT Fri Jul 12 2024 Valid 131200Z - 141200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN PLAINS INTO THE UPPER MIDWEST... ...SUMMARY... Multiple clusters of strong to severe thunderstorms are possible across portions of the Northern Plains into the Upper Mississippi Valley on Saturday. ...Synopsis... Upper ridging is forecast to be centered over the Four Corners early Saturday morning, remaining largely in place for the remainder of the period. An upper trough is expected to progress through north-central Canada, contributing to moderate westerly mid-level flow between this trough and the Four Corners ridging. A shortwave trough will likely move through this westerly flow, progressing across the northern Plains/southern Canadian Prairie Provinces into the Upper Midwest. Primary surface low associated with this wave will likely move just ahead of it from central Manitoba into far northwestern Ontario. A secondary surface low may develop at the triple point, with this low likely moving from central ND eastward across northern MN. Farther east, a shortwave trough is forecast to progress eastward from southern Quebec/western NY across the Northeast while subtropical ridging persists over the Southeast. ...Eastern Dakotas into the Upper Midwest... A very moist airmass will likely be in place from the eastern Dakotas into the Upper Midwest on Saturday morning, preceding the shortwave trough and associated surface features mentioned in the synopsis. Some isolated thunderstorms may be ongoing across MN early Saturday morning, supported by warm-air advection throughout the warm sector. Evolution of this early morning convection is uncertain, but it will likely shift gradually southeastward throughout the morning, potentially persisting long enough to become surface-based in the northeast IA/southern WI/northern IL vicinity. If this scenario is realized, the primary severe threat would be damaging gusts. Afternoon thunderstorm development also appears probable across western MN as the surface low interacts with the warm and moist airmass expected to be in place. Surface temperatures in the mid to upper 80s/low 90s amid upper 60s/low 70 dewpoints will support strong to very strong buoyancy, with MLCAPE over 3000 J/kg probable. Moderate vertical shear is anticipated across the region, but precipitation-laden updrafts are still expected to promote a predominantly outflow-dominant storm mode. Some outflow amalgamation is possible, with the resulting convective line then progressing southeastward into west-central WI. Primary severe threat is damaging gusts, but isolated large hail is possible with early cellular development. Outflow-dominant storm mode should keep the tornado threat low, but more southerly flow is possible near the surface low, with surface vorticity greater in this region as well. This could result in some low-probability tornado potential. ...Northeast MT into northern ND... Combination of low-level convergence along the lee trough and moderate low-level moisture (likely via post-frontal upslope flow) may result in convective initiation over northeast MT. Easterly low-level flow veering to westerly aloft will support strong vertical shear capable of maintaining a supercellular storm mode. Large hail and/or damaging gusts are the primary risks with these storms as the move eastward into northwest/northern ND. ...Western/Central SD and the NE Panhandle... Isolated thunderstorm development is possible Saturday afternoon amid the low-level convergence across western SD and the NE Panhandle. High cloud bases and moderate northwesterly flow aloft could support a few strong downbursts with any more robust convection. ...Lower MO Valley/Ozark Plateau into the Lower OH Valley... Thunderstorms may be ongoing over the Lower MO Valley/Ozark Plateau early Saturday morning, supported by warm-air advection associated with the low-level jet. These storms will likely be sub-severe, but an associated convectively induced vorticity maximum may drift eastward throughout the day, leading to additional storm development during the afternoon across the Lower OH Valley. Current expectation is that modest vertical shear should limit the severe potential within these storms. ..Mosier.. 07/12/2024 $$