Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
890 ACUS02 KWNS 121721 SWODY2 SPC AC 121720 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1220 PM CDT Wed Jun 12 2024 Valid 131200Z - 141200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS IN SOUTHERN IA...NORTHWEST IL...AND NORTHERN MO... ...SUMMARY... Severe thunderstorms are most likely across southern Iowa into northern portions of Missouri and Illinois during the late afternoon to early evening on Thursday. Very large hail, destructive wind gusts, and a couple tornadoes are possible. ...Central Great Plains to central Great Lakes... Increasing concern exists for a potentially intense severe weather episode, centered on the late afternoon to early evening across parts of the Midwest. A highly favorable thermodynamic and kinematic space should exist for significant severe hail and wind threats with supercells likely growing upscale into southeast-moving clusters/MCSs. Spatial confidence is only about average for the D2 time frame, rendering moderate uncertainty on the swaths of greatest threat. For now, have upgraded to level 3-ENH risk across the most likely corridor and broadened lower probabilities with added sig severe highlights surrounding it from eastern KS to northwest IN. A fast upper-level jet streak will become centered across parts of the Upper Midwest/Great Lakes by Thursday afternoon, as a series of embedded shortwave impulses track along the international border into parts of ON/QC. A surface cyclone attendant to the leading impulse should move across eastern ON into QC. A cold front will trail to the southwest, likely centered on southern WI to the KS/NE border area at 21Z. A secondary thermal low should form over the central Great Plains portion of the front with hot temperatures from 100-105 F west of the dryline. A plume of large to extreme buoyancy should develop amid very steep mid-level lapse rates and a corridor of enhanced boundary-layer moisture ahead of the impinging front. Guidance does differ somewhat on the spatiotemporal aspects of convective development near the frontal zone by late Thursday afternoon. 12Z CAMs such as the HRRR are farther east-northeast with their centroid of development along the Mid-MS Valley, while the NSSL-ARW would suggest a centroid farther west over the Mid to Lower MO Valley. Low-level convergence along the front should be fairly similar across both regions, suggesting either corridor may sustain intense storm development. Strong speed shear above 700 mb amid west-northwest flow will support an elongated hodograph. This coupled with ample instability should yield initial splitting supercells capable of very large hail. Orientation of the shear vector semi to nearly paralleling the front should yield upscale growth by early evening, especially with western extent where effective bulk shear is relatively weaker. Given the big MLCAPE, potential for intense downdrafts will exist with significant severe wind gusts of 75-95 mph possible. One or two convective wind swaths may surge southeastward briefly through about dusk. The eastern lobe of the pronounced elevated mixed layer should foster increasing MLCIN after dusk, suggesting that severe wind gust potential should wane Thursday night. ..Grams.. 06/12/2024 $$