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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.

Friday, June 18, 2021

SPC Jun 18, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0302 PM CDT Fri Jun 18 2021 Valid 182000Z - 191200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM OHIO WESTWARD ACROSS INDIANA AND ILLINOIS AND INTO SOUTHERN IOWA... ...SUMMARY... Scattered severe storms will pose a threat of damaging wind, large hail and perhaps a couple tornadoes this afternoon and tonight over parts of the Ohio Valley region and mid Mississippi Valley. ...Discussion... The substantial change this outlook update was to increase tornado probabilities from near the IN/OH border eastward into parts of central OH through the early evening. The airmass continues to destabilize north of a residual outflow boundary/eastward-advancing warm front located over east-central IN as of 20 UTC. Model forecast soundings show enlarged low-level hodographs (effective SRH 200-300 m2/s2) over west-central OH late this afternoon. It appears the initial supercell development over east-central IN will track into west-central OH and probably become more favorable for strong mesocyclone development and a supercell-tornado risk. Therefore, have increased the tornado probabilities to a 10-percent significant area to account for this shorter-term forecast. Have lowered tornado probabilities farther west over central IN where the surface winds have veered to southwesterly. ..Smith.. 06/18/2021 .PREV DISCUSSION... /ISSUED 1137 AM CDT Fri Jun 18 2021/ ...Ohio Valley through Middle Mississippi Valley... Only modest adjustments have been made to previous forecast in this area. Ongoing MCS from Lake Erie through OH should continue weakening, and in the wake of this feature the atmosphere is in the process of recovering as a southwesterly low-level jet transports rich low-level moisture through the pre-frontal warm sector. A plume of steeper (7-7.5 C/km) mid level lapse rates have overspread this region, and areas of strong diabatic heating will further destabilize the atmosphere with MLCAPE from 2500-3000 J/kg likely by late afternoon. Thunderstorms are expected to redevelop ahead of a southeast-advancing cold front, especially from OH westward through central and southern IN and spread southeast. Vertical wind profiles with 30-40 kt effective bulk shear will support some supercells, but tendency will probably be for storms to grow upscale into lines/clusters with time. Large hail and damaging wind will be the primary threats, though a couple of tornadoes will also be possible given sufficient 0-1 km hodograph size for low-level mesocyclones. Potential will also exist for a few significant wind gusts, especially if activity evolves into a more organized MCS. ...Upper MS Valley... Trailing portion of the front will stall across northern IL through southern IA. The atmosphere in this region will become strongly unstable (3000-3500 J/kg MUCAPE) supported by steep lapse rates associated with an EML plume that resides above rich low-level moisture. Potential will exist for a few storms to develop along this boundary this afternoon, but storms should become more numerous into the evening as a strengthening southwesterly low-level jet augments mesoscale forcing within the frontal zone. Supercells with large to very large hail will be the initial primary threat before activity evolves into an MCS with an increasing threat for damaging wind. ...Central High Plains... Isolated to widely scattered thunderstorms should develop this afternoon and last into part of the evening, moving eastward across the outlook area. Isolated severe wind and hail will be the main concerns. After the frontal passage, northeasterly flow will veer gradually to easterly, imparting a substantial near-surface upslope component that will aid in enhancing deep shear, as well as in moist advection and storm-relative winds in the inflow layer. Associated lift, along with diabatic heating of elevated terrain, will promote deepening afternoon convection beneath steep midlevel lapse rates, with MLCAPE to near 1000 J/kg, atop a well-mixed and deep subcloud layer suitable for strong to locally severe gusts. 35-45-kt effective-shear vectors will support mostly multicells, but with at least transient supercell characteristics possible. ...Gulf Coast... A tropical storm is forecast to develop from an area of low pressure now over the west-central Gulf, its center making landfall on the south-central to southeastern LA coastline late tonight. [See the latest NHC advisories for specific details on forecast track and intensity, as well as all tropical-related watches and warnings.] A couple of tornadoes cannot be ruled out as this system moves inland overnight. Read more LIVE: