SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1237 AM CDT Tue Jul 27 2021 Valid 271200Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHEAST AND UPPER MISSISSIPPI VALLEY... ...SUMMARY... Isolated thunderstorms are possible late Tuesday across the upper Mississippi Valley. A conditional risk for large hail and damaging gusts will accompany the stronger storms. Strong to locally severe gusts will be possible mainly this afternoon and evening for parts of the Northeast. ...Northeast... Upper trough is expected to shift east across the lower Great Lakes region into eastern NY by 28/00z. Currently this feature is located over the UP of MI/WI along with an associated MCS. This activity should propagate across northern lower MI very early in the period and likely weaken considerably after sunrise. Remnants of this MCS should move over southern ON into far western NY by early afternoon. Latest model guidance suggests strong boundary-layer heating will occur ahead of the short-wave trough and renewed convective development is anticipated by 17-18z. 500mb flow will also increase across the lower Great Lakes, and forecast soundings suggest adequate surface-6km bulk shear for potential storm organization. Deep westerly flow should encourage line segments with potentially damaging wind as this activity spreads east-southeast. ...Upper MS Valley... In the wake of the early-day short-wave trough that ejects toward the lower Great Lakes, weak mid-level height rises are forecast across the upper MS Valley. Latest thinking is a surface boundary will orient itself across southern WI (aided by the early MCS)-southern MN-SD. Models generate very strong instability along this frontal zone by afternoon, as surface temperatures warm through the 90s to near 100F as far east as the southwestern corner of MN. Inhibition will weaken along the western portion of the boundary, and there is some indication that a weak disturbance will top the ridge over western ND early in the period, then track southeast toward southern MN by late afternoon. Timing of this feature suggests isolated supercells should develop near the frontal zone then propagate southeast. Large hail is the primary risk along with some threat for damaging winds. It's not entirely clear how far this activity may spread after dark, but sustained low-level warm advection across the eastern Dakotas may contribute to additional isolated elevated convection overnight. ..Darrow/Bentley.. 07/27/2021
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