SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1225 PM CDT Tue Mar 18 2025 Valid 191200Z - 201200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF EASTERN ILLINOIS INTO WESTERN INDIANA... ...SUMMARY... Scattered severe thunderstorms are possible on Wednesday across parts of eastern Illinois and western Indiana from mid-afternoon to early evening. Severe gusts and hail will be the primary threats, though a couple of tornadoes are also possible. ...Synopsis... A potent mid-level trough will traverse the Plains and overspread the MS Valley, encouraging an intense surface cyclone to track from the Mid-MO Valley to the Great Lakes on Wednesday. Despite the intensity of the surface low and accompanying upper support from the mid-level trough, buoyancy is expected to remain scant within the warm sector given very limited low-level moisture. Nonetheless, strong isallobaric surface flow and the presence of a low-level jet ahead of the surface cyclone will allow for the northward advection of some moisture (however marginal) to support low-topped storms amid a highly sheared airmass. Some of these storms may become strong to occasionally severe, particularly in the Midwest toward the OH Valley. ...Midwest to OH Valley... Modest clearing and associated insolation ahead of the surface low will promote modest boundary-layer mixing and destabilization amid marginal moisture. Surface dewpoints of at least 50 F with the aforementioned heating, beneath 7-8 C/km mid-level lapse rates, will support 250-750 J/kg SBCAPE by mid to late afternoon. During this time frame, surface winds are expected to be backed from the southeast, with a 50+ kt southwesterly low-level jet contributing to substantial veering/strengthening of the low-level wind profile. Somewhat curved and elongated hodographs will contribute to over 300 m2/s2 of sfc-3km SRH. Given marginal buoyancy, an arcing band of low-topped supercells will precede the surface low, accompanied by a damaging gust/hail threat. Furthermore, the strong low-level shear, and increasing low-level vertical vorticity (as the surface low approaches) will also foster a risk for at least an isolated tornado. There are some discrepancies among guidance members regarding the degree of low-level destabilization across portions of the OH Valley ahead of the surface low. RAP forecast soundings show mid 50s F surface dewpoints beneath 8+ C/km mid-level rates, supporting a relatively higher tornado threat compared to NAM, which shows low 50s F dewpoints overspread by 6.5-7.5 C/km lapse rates. Higher tornado probabilities may be needed in future outlooks if guidance consensus depicts more boundary-layer instability. ...TN Valley into the Southeast... At least isolated thunderstorms are expected to develop by late afternoon into evening along the surface cold front, which will sweep across the TN Valley/Southeast areas. These storms are expected to be low-topped in nature, but highly sheared (given a 40+ kt southwesterly low-level jet, beneath 80+ kts of southwesterly 500 mb flow, contributing to 50-70 kts of effective bulk shear). In addition to marginal low-level moisture, modest warming in the 850-700 mb layer will also limit thunderstorm intensity and coverage. Given strong low-level and deep-layer directional and speed shear, any storms that can become established and sustained may become supercellular, posing mainly a risk for a few instances of damaging gusts/hail. ..Squitieri.. 03/18/2025